H. J. CECCALDI
INTRODUCTION
The idea of comparing the function of nutrition in fish and in crustaceans seems at first impossible, as these two animals groups are so dissimilar. what can a sardine and a edible crab have in common ? Skeleton, way of life, type of food, way of feeding, way of growth and this is only taking into account the principal aspects of their morphology or of their biology. However, these two animal groups which are so dissimilar possess two important common characteristics: they are both poikilothermic animals and most of them are aquatic species, adapted to living in fresh or sea-water. This report will deal especially with marine species.
LARVAL NUTRITION
After having given special attention to several homologous segments of the biological cycle, similarities can be clearly remarked between these two aquatic animal groups. First, it must be remembered that marine larval species coexist together in the plankton. They are submitted to the same conditions of environmental factors salinity temperature, light, oxygen, nitrate and phosphate tenors. They are also submitted to the same variations from the external environment. They find the same phytoplankton species. they eat the same food particles, the same consumable protists.
In each case, the chemical composition and the biochemical characteristics of these food particles depend greatly on their origin, and these on the species to which they belong. The small size species will not have the same composition as large size species. Thus, when the larvae eat, they will obligatory select the small size particles, which mostly consist in phytoplankton cells.
The larger food particles are mostly of animal origin: large protozoa, small invertebrates, eggs or larval stages of other species. Thus the selection, according to size imposed by the dimension of the mouth on one hand, and by the grabbing capacity of these prey on the other hand, leads obligatory to the choice of food being made, the composition of which has been already defined. It must then be underlined that only species whose larvae have their nutritional requirements covered by the food available can survive in the pelagic environment.
QUALITATIVE NUTRITIONAL REQUIREMENTS
Apart from this similarity in composition when considered at a same period of time and during a same season, we must also take into consideration that the composition of these phytoplanktonic organisms varies with time especially concerning the sterol fatty acid tenors (the length of the chains are more or less long, the gross insaturation of which is more or less high) or the tenor in carotenoid pigments We have only taken into account a few examples, taken from compounds of marine vegetable matter which have been studied correctly, although more often in a preliminary way. The small size animals which consume the phytoplankton see their own composition greatly modified. They themselves become prey for the larger larvae or post-larvae of fish and crustaceans. Thus is it a great similarity which exists between all plankton waters -most fish and crustaceans- as their food has the same basic composition and this composition evolutes with time in a similar way as for all consumers.
The ecological studies concerning the transfers in the trophic chains, rarely take into account this fact, although fundamental: the chemical composition of the elements which make up each of the segments of a trophic chain varies according to the different months of the year.
It is very probable that the larvae of different benthic species or the adult invertebrates of small size such as rotifers or copepoda have very delicate mechanisms for the sorting out, grabbing and selection of food particles, whether live or dead, animal or vegetable, having or not a bacterial covering. These mechanisms must be studied and explained in detail, thus enabling to quantify the influence of the composition of food ingested by the larvae, in other words, the particles available, for survival and vitality, the composition and growth of the animals. Survival and the good physiological state of crustaceans and fish at adult stage -until they reach the age for reproduction- will depend on the tenor in essential fatty acids or the indispensable amino-acids. It is most remarkable to observe that the requirements in long whain polyunsaturated fatty acids are very much alike for both fish larvae and crustacean larvae -at least for those which have been properly studied-.
The requirements in long chain polyunsaturated fatty acids, especially the linolenic series such as C 18: w 3, C 20: 5 w 3, c 22: 6 w 3 correspond to well defined physiological functions, which have been recently identified and require more research.
These particular fatty acids, whose freezing point is situated at a much lower temperature than their saturated homologues, permit the membranes of diverse tissues to remain flexible and functional, even at very low temperatures. Marine animals must indeed permanently adapt themselves to the water whose salinity and temperature vary constanly. It has now been almost established that without a sufficient tenor in long chain polyunsaturated fatty acids, the survival of larvae along with the vitality of adults are greatly reduced. The mechanism and the method of action of these fatty acids have not yet been clearly established, although due to their presence in the membranes, the characteristics of the physical adaptation and control, at least partially, of the exchanges of the different ions between the external medium and the blood on one hand, and between the blood and the intercellular medium on the other hand.
This means how important the distribution of lipids of defined quality in the food is, for the survival of fish and crustaceans larvae, which are essential elements of marine trophic chains.
On the contrary, the requirements in sterols are quite different for crustaceans and for fish. The former are incapable of synthetizing the carbonaceous structure of sterols, although they have an essential requirement, both to synthetize their moulting hormones, ecdysones, and to preserve the integrity of their membranes. Without ecdysones, no exuvia nor growth take place. On the contrary, fish like all vertebrates, synthetize sterols and there is no need to add them to the compound food with which they are reared, as is necessary for crustaceans.
3. DIGESTIVE ENZYMS
An other fundamental aspect of marine animal larval nutrition is entailed by the evolution of their digestive enzyms during their larval growth.
The digestive enzyms -can be found in individuals even at egg stage- which start digesting the vitellus while the digestive tract is not yet distinguishable. It is remarkable to observe that it is the enzyms digesting the glucids and polysaccharides which appear first in the digestive tracts of marine animals.
Laminarises and amylases correspond in the mechanisms of larval feeding, to the composition of phytoplankton and particles which are available. These enzyms digesting glucides will progressively decrease and proteases of different nature, the list of which is but in beginning phase, appear progressively during growth.
A global index can be employed: For the report between the amylasic and proteasic activities which develop during growth and which present certain similarities in crustaceans and fish.
4. NUTRITION AND DIGESTIVE ENZYMS
Proteases exist in fish as soon as they hatch, even when their mouth is still closed. Their activity, feeble in the beginning, increases during their larval devolopment. With herbivorous fish, for example carp, which live in fresh water, the amylasic activity maintains a remarkable level all through larval development. With carnivorous fish, the proteases/amylases ratio increases with age during larval and juvenile development. This ratio can constitute an index of the food diet. It reflects the digestive physiological potentialities of the species under study. Thus, we can differenciate the greatest and smallest degree of herbivoricity or carnivoricity of the different species belonging to a same family, Mugil or Penaeus for example. But this index is submitted itself to variations as the digestive enzymatic activities evolute themselves according to the quality of the food available and its composition.
Thus, when an adult crustacean eats food lacking proteins, its proteasic enzymatic activities are feeble. There is a better activity remarked when animals of the same species are food protein-rich food. But when the protein tenor is too strong and when it reaches more than 45 to 50 %, the proteasic activities decrease, more so when there is a strong protein content in the food.
To our knowledge, there exists no detailed studies which has as objective the comparison of the molecule structures of the proteases in crustaceans and fish. Authors agree on the existence of trypsins in both zoological groups. Chymotrypsin has a very feeble activity in crustaceans and in decapoda, there exists a very feeble molecule weight protease, 11 000 daltons.
NUTRITION AND ENERGETIC BALANCE
There are many utilities for food, especially concerning the energetic expenditure: fish and crustaceans are confronted with the same biological problems: to ensure their growth, to fight the invasion of salts and thus maintain a compatible osmotic pressure with their physiology, to ensure their excretion to permit the development of their gonads and their reproduction, to permit compatible movements with their ethiology.
With fish as with crustaceans, it seems that proteins play a very important role not only for the supply of essential, the amino-acids employed for the biosynthesis of proteins, but also from a energetic viewpoint. In both cases, and due to the fact that they live in an aquatic environment, the catabolism of proteins is more economic on energy than for terrestrial animals, as 60 to 80 % of the nitrogen excreted is found in the form of ammoniac which is directly rejected through the gills into the environment.
There is a better growth return for fish than for crustaceans which suffer material losses through exuvia at each growth stage. These losses do not exist for fish, which make much better use of the food ingested, permitting much quicker growth and better conversion rate.
Locomotion is also better for fish, hydrodynamic animals generally do not employ a lot of energy to swim and have more flexible and efficient movements than crustaceans do. The latter have an articulated carapace, of rigid segments, which, only permits limited mechanical movements and sometimes permits the lighter ones to swim in a slightly similar way as fish. Except in the case of small size species like copepoda, or krill, the benthic character of crustaceans is ineffaceable. The complete opposite applies for fish which are less sedentary, and the species staying at the bottom, like pleuronects, living exclusively on benthic or soil-dwelling prey, are less numerous.
THE INTAKE OF FOOD
Filterer fish, such as sardines or herrings, in comparison to the numerous species that exist, are more scarce in proportion to filterer crustaceans, copepoda, euphausiacea and other pelagic crustaceans.
Due to the presence of multiple filters in the stomach of decapoda crustaceans, they can be considered as intense filterers, as they crunch their food into very fine particles. Only the fine particles have no access to the digestive tubules of the hepatopancreas, and are selected out by the filters, brushes, teeth and needles located in the posterior part of the pyloric stomach of this type of species.
Although lobsters have impressive sized claws and often agressive reactions they can not be compared to sharks whose jaws are of legendary ferocity.
Crustaceans can only swallow small size prey after they have masticated and dilacerated them by means of their anterior appendixes. On the contrary, certain fish can be very ravenous and extremly voracious and their eating habits depend greatly on the teeth which surround the anterior part of their digestive system.
The teeth of fish can very considerably. From pointed teeth located in the buccal cavity of primitive fish such as lamprey, to the complete disparition of teeth in cyprinids, carps and gudgeons, there exists great differences in the masticating systems of fish. The group with the most efficient and rightly named “frightening” teeth is without doubt those of the shark, who changes its teeth periodically. Ray have their teeth set in rows with which they crunch the carapaces of sea-urchins and the shells of molluscs. Sea-bream have very hard teeth located on the palate with which the crunch the shells of lamellibranch-molluscs.
Blennys have very varied types of teeth and are heterodonta, as are sargos, borgues and oblades as like most omnivorous fish.
Most fish have similar teeth and are homodonta. These teeth can be found implanted not only in the maxillary arch but also in the roof of the mouth and even in the tongue as is seen in the pike.
Tetradons must adapt to their particuliar situation, diodons and parrot fish have four and sometimes only two teeth with which they browse on the coral reefs and catch prey which have found refuge in the deep cavities of the madrepores. Mullet have a wide open mouth which allows them to swallow debris and organic matter lying on the surfaces of the sediment. They can even suck in the particles which float on the surface of the water.
Among the other remarkable adaptations which fish present so to ensure the capture of food, we can quote the sea-bream and cat-fish barbels, the anglerfish traps, the apparition of denticles and asperities on the pharyngeal bones which play the role of tritural teeth for cyprinidae, the beaks of coral fish such as chaetodons the blockated snouts of the hyppocampus, of Centriscus or oyster catchers, and of syngnathidae or of the fine and disproportioned teeth of numerous species of abyssal fish.
This list could be much longer. It would given an idea of the great diversity of fish teeth, which is quite the opposite to the simplicity and homogeneity of the means which crustaceans employs to collect their food.
DIGESTIVE TRACT
The digestive tract of crustaceans is more often very simple and rectilinear. Mastication takes places mostly place mostly in the stomach and absorption takes place in the tubules of the hepatopancreas. The digestive tract has not the same capacity of absorption as in fish and this is even more evident in mammalia as this function takes place in the tubules of the hepatopancreas.
In fish, the degree of differenciation of the digestive tract is very variable, starting with fish having a straight digestive tract and no stomach and arriving at fishing having sections which are very different morphologically, especially those with a stomach having chloride cells and areas for the secretion of pepsin. In this listing, we can thus find digestive systems which are quite similar to those of crustaceans and other which are closely related to the so-called superior vertebrates. During larval period, fish have simple digestive tracts. The digestion of proteins seems to be ensured by trypsin and chymotrypsin as is the case of crustaceans.
In most aquatic animals, salivary glands are missing. In most cases, the pyloric appendixes join up the digestive tract of the pylorus. They vary in number from one species to another. Most sharks, rays and sturgeons have their absorbing surface extended by the presence in the mid-section of the intestine of a spiral valve which is not found in crustaceans.
Most fish families have a dorsal evagination of the digestive tract which is filled with gas so as to ensure a correct buoyancy and which permits the animal species to float between two waters. The crustacean does not possess an equivalent mechanism. Some, such as copepoda small in size, can contain, especially when they accumulate reserves, lipid drops, which enables them to float. But these lipids have no direct connection with the digestive tract.
NUTRITION AND TROPHIC MIGRATIONS
Both fish and crustaceans accumulate reserves at the end of a good season, when food is abundant. The reserves are accumulated in the form of lipids, principally triacylglycerols and glycogen. In fish, fatty tissue appears between the organs in the conjunctive tissue which supports the viscera. This is particularly remarked in sardines which can be very fat after the active consumption season of food. Glycogen can be stored in the liver and muscles. When an effort is made, it is the muscular glycogen which will be the first consumed. Migrating fish, especially salmon, store their reserves, after they leave the river where they were born, when they start their growth phase and acquire active feeding habits in the sea. When they begin their migration linked with spawning, they completely stop eating, so while in a completely fasting state, they swim up the estuary and water courses where they were born in the search of an original spot to spawn. After the emission of the gametes, becoming thin and fatigued, they die, We see how important the active feeding period is, it completely conditions the whole reproduction phase. Trophic migrations have been observed for numerous species. They take place without changing the environment, at sea like for herrings, sprat, anchovy, mackerels and again sardines, cod, albacores and tuna fish. On the contrary, eel actively feed in fresh water and go to the sea to spawn, for example european eel go to the Sargasso Sea to spawn.
The similarities with crustaceans are very limited. Generally having very limited swimming capacities, migrations are not quite so important. The migrations of tropical lobsters have not yet been clearly explained. Indeed, the active feeding phase are very limited at stage C of the intermoulting cycle, which corresponds to a phase of active consumption of food for superior crustaceans and especially decapoda. If the quantity of food required has as consequence the accumulation of enough reserves, growth will be good during the following exuviation. If on the contrary there is no accumulation of reserves growth will be average or non-existant.
With small planktonic crustaceans, active consumption phase depends especially on vertical migrations and the food available during the circadian movements in the different water masses which they cross through.
NUTRITION AND REPRODUCTION
One of the ultimate objectives of the nutritional activity linked with the reproduction of each animal consists in accumulating reserves in its ovocytes. Each female must consent to making this effort without realizing that the survival of the species depends on this. It is necessary that each one enable the genetic programme to create an embryo, the latter must have at its disposition all the reserves necessary, both in quantity and quality, which will enable the larva to hatch in good conditions, and find its food. In this way, the biological cycle of each species is settled.
Both for crustaceans and fish an important part of the food ingested is directed towards the ovaries or testicles. The mechanisms of transport, the detailed structure of the carrier macromolecules, the vitellogenesis, the place and the mechanism of their synthesis require yet still numerous research especially for the part of the reproduction cycle which is situated on one hand between the digestive tract and the metabolic reserves of the animal and on the other hand, between the tissues which synthetize the vitellogenesis and the ovary. It is important however to underline that depending on the quality of food absorbed and especially in accordance with the lipidic part of lipoproteins, the reserves accumulated in the ovocytes will permit the acquisition of eggs of more or less good quality.
NUTRITION AND RESERVES
Reserves are consumed according to requirements. It is known that fish employ rarely their hepatic glycogen, only in the case of necessity, even if they are fasting. They prefer to catabolize the proteins and preserve their hepatic glycogen.
Crustaceans do not abide by the same rules for the consumption of reserves according to the different groups or the different environmental conditions which were studied: lipids are sometimes consumed first, glucids in other cases. Triacylglycerol, diacylglycerol, muscular glycogen which are abundant in crustaceans are the first to be consumed. The principal reserves are located in the hepatopancreas and are liberated during the moulting season.
NUTRITION AND AQUACULTURE
The aquaculture operations simulates, either by accelerating or delaying, the biological phases which occur in nature. Thus it is not surprising that, at least at larval stage, the animals reared are placed into environmental conditions similar to those found in natural conditions, or the most suitable conditions for the species in question which may not be identical. The food furnished must be more or less the same as that found in natural conditions or at least, suited to the species in rearing.
Beside these natural biological requirements, economic rules must be follo wed; it is necessary to make proper use of the facilities existing, which represent more often very important investments.
In a hatchery, it is absolutely necessary to have under control algae cultures, which will be used at the first larval stages, both for fish and crustaceans. Besides this, algae in a rearing environment will permit the natural elimination of excretion matter coming from the animals in rearing. Artemia and rotifer culture such as briachionus, will then be employed for the rearing of both fish and crustaceans. Finally, particles of animal origin, such as the minced washed flesh of molluscs or compound food particles will be used to feed both fish and crustaceans, the former however will adopt to this food more quickly.
In any case, the reconstitution of the different stages of the natural biological cycle in controlled conditions always nearly requires very similar hatchery and production structures, even for the way in which they are operated. sequence lighting, regular spaced out feeding, environmental control.
CONCLUSION
To conclude, the cyclic variations of the environmental conditions, the existence of the same natural food and prey in the biocenoses bring about a similarity, concerning the feeding methods employed, the feeding behaviour and the coverage of the physiological requirements, between fish and crustaceans, especially at larval and post-larval stages. This great original similarity develops during their ulterior differenciation, as they grow, both in natural environments as in aquaculture.
As for compound food, there exist many common points: crustaceans and fish require the same ten essential amino-acid and a rather high protein content rate in their food. Very often, the same basic meal employed in the manufacture of compound food will be used. So as to cover the fatty acid requirglements, the same appropriated oils are employed. Research on the eventual origins of proteins or of lipids used in the manufacture of commercial type food, whether for fish or crustaceans have turned out to be much more general and of major interest for the rearing of these aquatic animals.
Thus we see, that even though of very different phylogenic origine, fish and crustaceans have sufficient points in common so that the aquaculture structures where they are reared can be similar trophic system, in the natural environment, is a basis for the ressemblances of the rearing structures in aquaculture.
GENERALITIES
Digestion can be defined in many ways depending on the specialist studying it. Indeed, nutrition typically includes, the intake of food, from the exterior environment its assimilation and its role in the maintenance of the growth of the organisms which ingest it. This is the most typical way in which to consider nuritional mechanisms.
But digestion can be studied in a more precise manner, in taking into consideration the processes which take place at cellular level or the metabolic cycles, the enzymatic activities and carrier mechanisms in the interior medium.
Finally, we can enlarge yet more the examination of phenomena linked with nutrition by examining the endocrinological regulations linked with digestion, along with the excretion and respiration processes which are in control of what happens] to the food after it has been assimilated.
This report will not take into account the excretion and respiration functions.
A REMINDER CONCERNING THE PHENOMENA LINKED WITH GROWTH IN CRUSTACEANS
Growth occurs with successive moults in crustaceans. The moment when the animal looses his old tegument or exuvia, is known as exuviation. DRACH, 1938, to his merit developed a system permitting a cleat comprehension of the evolution during the different stages which takes place from one exuviation to the next. Immediately on loosing his exoskeleton, the animal increases in volume by absorbing great quantities of water. Being very limp, it cannot eat for several hours or even days, and will rely solely on its reserves, which it has accumulated during the previous stages, to survive and overcome this natural physiological crises. The moults become more and more spaced out as the animal grows older. it must be remembered that with crustaceans, growth is discontinuous and is characterized by a particular cycle: The constitution of reserves, the consumption of reserves, from one exuviation to the next, in other words, during each intermoulting cycle.
The reserves of crustaceans are found essentially for adults, in the cells of the digestive glands. In decapoda, this gland, located at mid-gut level, is known as the hepato-pancreas. This denomination is not exactly valid, and certain authors have rightly tried to define it otherwise in proposing, for example, the as term the mid-gut gland. The reserve substance are principally made up of glycogen, lipids, Vitamins, pigments and mineral salts, principally calcium. Glycogen reserves can also found in the muscle.
The reserves found in the eggs during their development are constituted by the vitellus which develops during the ovogenesis. As for many species, the embryo, void of a digestive tract, but consuming however reserves, sustains on the vitellus. When there is a feeble quantity of vitellia reserves, the larvae hatch quite quickly after the spawning has taken place, when the last vitellin of the egg has been consumed. Young Larvae feed on phytoplankton more or less at first.
It is of prime importance to know the quantitative and especially qualitative variations of the food which is consumed by the crustacean planktonic larvae during growth. In natural environments, the first larvae choose cells coming from diverse species of phytoplankton, according to the size of their mouths or the shape or hardness of these cells. From then onwards they choose either organic particle or more often small living animals, larvae or small invertebrates which live in the plankton. They thus acquire progressively a carnivorous behaviour and physiology as they get older.
The feeding behaviour of adults is practically acquired as soon as the benthic way of life has been established by the species in question.
DIGESTIVE SYSTEM IN CRUSTACEANS
Food diets
The food diets of crustaceans very greatly from one group to another. Indeed certain species consume algae, while others are strictly carnivorous, detritivoros necrophagous or filterers of particles.
Appendixes employed to obtain food
For Decapoda, the most anterior appendixes are generally employed to obtain food, in very developed and specific way as they are located near the mouth. The appendixes located on the cephalic metameres; the mandibles, maxillas, maxillulas surround the mouth and dilacerate the food before the latter is introduced into the oesophagus. The two pair of antennas: antennules and antennas, play a role in the chemical reception, in other words in the reconnaissance of food thanks to dissolved molecules that are released into the environment.
For Decapoda, the three anterior pairs of thoracic appendixes are adapted to the function of nutrition: they transform into claw jaws or maxillipeda. These appendixes also, manipulate and dilacerate food, while the posterior appendixes, five pairs, are employed for the locomotion.
The food thus dilacerated reaches the stomach where it is reduced to a very finely minced gruel.
Digestive tract
The digestive tract in Copepoda is rectilineal. the anterior part is generally enlarged while at the same time, it does not really form a stomach. Copepoda do not possess a specialized digestive gland. The absorbing cells and the reserve cells constitute the walls of this digestive tract.
Isopoda have generally three pairs of digestive caecums located on either side of the digestive tract and which are constituted by a basement membrane which supports an epithelium. Numerous cells with brushy borders constitute the epithelium. The absorption and the storing of reserves take place in the caecums.
The digestive tract of Decapoda is divided in three parts, the head-gut or stomodaeum, mid-gut or mesenteron, the hind-gut or proctodaeum. The stomodaeum and proctodaeum are coated in chitin and this coating is rejected at each exuviation.
The stomach
The stomach comprises two pouches, the anterior cardiac pouch (or stomachic bag) where the food ingested accumulates and the posterior pyloric pouch which possesses calcareous parts, bristles, needles, filters along with recesses and proeminences where the food will be pulverized successively as it passes through each one.
The posterior part of the cardiac stomach and the pyloric stomach is reinforced and supported by a group of articulated calcareous parts, plates and ossicles. These parts and area are thickened by the chitinous coating of this organ.
The mucus of the stomach is similar to that of the oesophagus. The walls of this organ are not flat and include a great number of recesses of diverse form and size. The height of the cells constituting the stomach walls vary from one place to another in this organ. There are also a certain number of bristles and needles located inside the stomach, of different size depending on where they are located.
The masticatory parts of the stomach
The anterior and posterior portions are lined inside with a proteic chitin membrane which joins up with the exoskeleton. It is also subject to periodical moults like the exoskeleton. The anterior part of the digestive tract thus possesses a reinforced coating, on the inner surface, prominences, which can submit the food to extreme trituration. These prominences can become calcareous and create numerous skeletic parts of different forms. The whole lot form a real inner skeleton with a very special articulation. Each part is activated by the muscles located on the outside of the stomach wall, the movements of which are controlled by a group of characteristic nervous elements. The promineces and recesses differ in form and disposition from one group to another. These systems have been more or less largely described over the past century. The studies carried out show that the stomach framework can be summarily compared to a “three forked claw that the food must pass through to reach the pylorus” (MILNE - EDWARDS, 1834). HUXLEY (1880) compared the stomach system to a gas tric mill and the pyloric portion to a filter.
The stomach thus comprises, on its inner surface hard elements or ossicles, with the same function as teeth, forming a triturant or masticatory device and a group of recesses and valvules. There also exists near the posterior part of the stomach and the pylorus, bristles, needles and tubules which play the part of a complex filter.
Structure and terminology of the ossicles
All the ossicles with the exception of those situated in the symmetry axis of the stomach, are symmetrical, bilateral and in twos. A general terminology for ossicles was proposed by MOCQUARD (1883) with some modification which were proposed by COCHRAU(1935).
Thirty three ossicles at least have been described during the studies carried out on compared anatomy. They can be divided into seven categories, depending on what role it is beleived that they must play. Indeed their exact role requires to be defined in detail.
Seven fundamental categories have been described and show great variations joinings and expansions. In certain species, the ossicles form calcified plates: their form and size can vary greatly, which makes their homology difficult from one species to the other.
The masticatory parts have thus roles, differing in number and in quality, depending on where they are located respectfully. The first fifteen parts located in the cardiac bag constitute a first anterior sub-system often known as the gastric mill. The anterior parts, stronger and more calcified are special ossicles which take the name of teeth. The second sub-system, made up by at least eighteen parts, which are smaller and less calcified, participate in the functioning of the filter sub-system in the pylorus region.
Several specialists have admitted that the efficiency of a stomach depended on its complexity. Finally, it must be remarked that the complexity of the mandibles varies in an inverse manner to that of the stomach.
Trituration in the stomach
The food passes down the digestive tract through complex passages. Movement depends greatly on the size of the food.
The cardiac bag of the stomach is separated from the pyloric bag by a cardio-pyloric valvula, thus giving two adjacent bags, Big particles remain in the cardiac bag; they are digested, by the movement of the stomach muscles, in the cardiac bag; they are digested, by the movement of the stomach muscles, in the dorsal part of the bag where the gastric mill parts intervene.
The particles can pass beyond the cardio-pyloric valvula while remaining in the plane of symmetry of the stomach and enter into the dorsal part of the pyloric bag. The finer particles pass into the mid-gut gland. The bigger ones are retained back by a filter located at the entrance of the hepatopancreas and are directed later on towards the intestine.
The rhythmic movements of the different regions are ensured by a striated musculation, the contractions are entirely controlled by neurons.
The stomato-gastric nervous system of Decapode crustaceans is doted with an important ganglion which innervates the anterior part of the digestive tract. This ganglion can control two neuronic systems:
- The system which ensures the rhythmic motricity of the gastric teeth (12 neurons in the Parlinurus vulgaris spiny lobster.
- The system which ensures the rhythmic motricity of the pyloric region and which comprises 12 neurons (14 neurons in the Palinurus vulgaris spiny lobster.
These systems organize alone all the rhythmic activity of the head-gut.
The rhythmic activities of the digestive tract
The cardiac bag of the stomach is the centre of rhythmic contractions, with a frequence of one contraction every 8 to 10 seconds. These movements are ensured by the activity of the 12 neurons: 10 motor neurons and 2 interneurons. The side teeth, fixed laterally, in the most posterior part of the cardiac bag, have transversal rhythmic movements which are controlled by four motor neurons.
The medium tooth fixed on the roof of this bag at the level of the lateral teeth, has coordinated rhythmic movements. It has longitudinal movements controlled by six motor neurons.
The pyloric bag is characterized by construction movements, the first in the anterior part. These successive and coordinated movements ensure the filtration mechanisms and permit the progression of the food towards the mid and end-gut. The contractions take place every 1 to 2 seconds.
When food is given to a Jasus lalandei lobster, it swallows the food although the rhythmic movements of the cardiac bag do not commerce immediately.
The pyloric bag, which was the centre of contractions of regular intensity and with frequencies of around 1 every 3 seconds, becomes the centre of rhythmic contractions which are more regular and shorter, one every second, which commence within one minute after the ingestion of food. The rhythmic contractions of the cardiac bag don't begin until 3 hours later, although the food lies there.
The apparition of the rhythmic contractions of the cardiac bag could be linked with the secretion of the digestive enzyms from the tubules of the hepatopancreas to the stomach.
It is in the stomach that the food is transformed into a liquid gruel, and here again that the greater part of the chemical digestion of the food takes place.
In the other groups of free crustaceans, the mechanical degradation depends on the food diet and on the morphology of the appendixes. For example, filterer Copepods and Cirripeda have certain appendixes doted with byssus with which they collect the food particles in suspension in the water. Parasites, on the other hand, possess special organs for each group.
FOOD REQUIREMENTS
Global requirements
Crustaceans must cover important requirements: locomotion, exuviation, the constitution of an new exoskeleton at each moulting period, growth, excretion, maintenance of the osmotic pressure and especially for females, the production of gametes. The latter is important as the ovaries represent a very remarkable volume of the body weight.
These requirements are rather badly known, even from a quantitative view-point. The Copepoda Calanus finmarchicus filters 70ml of water per day so as to ensure its food, in water which has and average planktonic resource.
Studies have proven that planktonic crustaceans barely succeed in feeding themselves correctly with the quantities of plankton available during the year. They sometimes acquire a cannibal behaviour. The same applies for crustacean larvae during growth, in both natural and hatchery environments.
The requirements vary greatly depending on growth. The oxygen consumption per weight unit, which is a convenient way to define the metabolism, is all the greater when the animals are small in size.
It seems that larvae of Decapoda crustaceans feed at all hours of the day and night. The post-larvae of Peneidae take four meals per day while adults are satisfied with two or even one meal per day. The progressive acquisition of cyclic feeding activities develops at the same time with the acquisition of the possibilities to ensure food reserves and the establishment of a digestive enzymatic secretion rhythm.
The quantities of food that must be furnished to the same species of animals of different size, varies according to the size. The following formula is employed:
Q = n Pi
where Q is the quantity of food absorbed per day,
P is the weight of the animal,
n is the quantity of food per time unit in given conditions,
and i is the rate increase of the food absorbed, when the weight of the animal increases.
Food quantities
Few species have been studied from this point of view. As has been already stated here above, the oxygen consumption is one of the best ways to define the metabolism of animals, but the food absorbed by the food canal(and not swallowed) must be taken into account, as the food rejected by the organisms studied and not digested must be taken into account.
Quantitative requirements in basic nutriments
They vary greatly depending on the food diets of the species taken into consideration. Thus, the more carnivorous species can not be reared if the food furnished has not a high protein tenor. We can give as very approximative values for the global composition of food, the following figures: proteins 45%, glucides 30%, lipids 10%, mineral mixture 5%, vitamins 2%, binders 3%, indigestible ballest 5%,
It must be remarked here that the protein requirements vary according to the size, thus the age of the animals. On general, the optimum protein tenor decreases during growth. REGNAULT showed with Crangon that the best growth rates were obtained with food containing 60% of proteins for juveniles, while 30% was sufficient for older animals.
As for the amount of food, the requirements will also decrease during growth. During the rearing of Peneidae shrimps, juveniles weighing between 0,1 and 0,5 g consume daily 50 % of their live weight in food. The values decrease and reach 25 % between 1 g and 2 g, 15 % at 10 g, and 5 % at 20 g.
Qualitative requirements
The essential amino - acids have been identified: arginin, threonin, methio nin, valin, isolation, leucin, lysin, histidin, phenolalanin, tryptophan. The protein consumed in natural environments or employed in rearing must contain these diverse amino - acids in sufficient amounts. In certain cases, basic or sulphur amino - acids are added to the mixture so as to try and compensate deficiencies. The results obtained are not always better and sometimes they are even very bad.
Lipids play an important part in the metabolism of crustaceans: a certain number of fatty acids prove to be necessary. Long chain unsaturated fatty acids, especially the eicosapentanoic (c20: 5) and docosahexanoic (c22: 6) fatty acids are especially important. The linolenic series of fatty acids are more effective than the linoleic series of fatty acids. Those whose first double link is found in position n 3(ou w 3) are more effective then those with their first double link in position n 6 or n 9.
The fatty acid composition of reared crustaceans varies depending on the lipid composition of the food ingested.
The vitamin requirements are not known precisely; However, certain works carried out have proven that crustaceans must receive in their food vitamin C, pentothenic acid, sterols. The latter plays a particular important part as the moult hormones, the ecdysons, are synthetized by crustaceans from sterols.
Natural food diets
In a natural environment, the crustacean populations depend on biotops in which they find both in quality and quantity, food which is apt to cover their requirements.
As these requirements vary during the growth of each animal, the biotop where they live must not only be to supply their instantaneous requirements but also those for later on.
Research work has been carried out on crustaceans in rearing concerning their stomach content, in spite of the fact effect of the masticatory parts of the stomach make the organisms during ingestion and ingested unidentifiable.
BEN MUSTAPHA (1962) obtained the following composition for species consumed by shrimp:
- Crustaceans 14.2 %
- Polychaeta 16.6 %
- Molluscs
Pelecypoda 39.5 %
Gasteropoda 9.3 %
Scaphopoda 17.5 %
Cephalopoda 0.4 %
- Echinodermata 2.2 %
The stomach also contained a non negligeable fraction of sandy-muddy sediment.
On the other hand, the composition of the benthic populations and the number of crustaceans in nature depends especially on the granulometry and hydrodynamics of the beds of the zones in question.
FOOD EMPLOYED IN AQUACULTURE
Natural food
The development of natural populations is always limited by the production of food available in the biotop where they are found. In Intensive rearing, the to the rearing tank where it will be consumed.
The animals the most frequently employed as food are lamellibranch molluscs for example Venerupis ( = Tapes) philippinarium, or Mytilus, in japan. Cephalopoda ceans fished in cold seas (krill) are employed in japan as food in tank rearings. Waste fish from trawler fishing is also often added.
Research carried out by HUDINAGA and KITTAKA (1967) on Penaeus japonicus post larvae has permitted to obtain relation between the live weight of Venerupis the weight of P 21 post-larvae of Panaeus equal to 2.4 for 21 day-old post-larvae after metamorphosis.
Natural compound food
For economic reasons as well as to cover adequately the requirement of crustaceans in rearing, food manufactured while employing natural ingredients which have been reduced to power form and processed by the use of physico-chemical methods seem to be becoming more and more renown. The animal or vegetable meal which contains protein and glucid tenors capable of covering the requirements of the animals, is the basic point of this compound food (NEW, 1976). Lipids, vitamin and mineral salts are then added. The mechanical cohesion and maintenance in the water is ensured by diverse binders, whose properties have sometimes been studied in detail (FOSTER, 1972).
Proteins represent the prime fraction of the composition of compound food. Traditional proteins may be employed in feeding. The meal of animal origin which is employed, is fish meal of crustaceans, obtained from the cephalo-thorax of shrimp, cephalopoda meal obtained from squid, and soluble concentrated meal from fish. The proteins of vegetable origin usually employed is soya or copra meal. Proteins obtained by the employment of more recent methods, for example from Spirulin algae, yeast growing on alkans, or brewers yeast are also employed.
The glucids employed most frequently are corn meal, wheat, lucerne, rice. different starches are employed, but their digestibility by the amylases of the digestive tract of crustaceans varies greatly from one starch to another.
The Lipids Employed are vegetable oils or oils obtained from marine organisms, such as cod liver oil especially, or other fish liver oils or squid liver oil. It is necessary, that the lipid furnished, contain sterols and carotenoid pigments. The food is completed on one hand by vitamins presented in the form of mixtures which have been previously prepared, and on the other hand in the form of mineral mixtures which contain principally elements such as calcium, phosphorous and certain metallic oligo-elements such as copper, zinc, iron, manganese for example, which are also added to the compound ingredients during manufacture.
When the compound food possesses the basic ingredients obtained by means of processing or new technological methods, it is sometimes incorrectly called synthetic food.
Presentation
Pellets must have a consistance so that the crustaceans may easily manipulate them by means of their appendixes, while not crumbling them into bits; also] they must be able to remain in the water a rather long time; several hours.It is thus necessary to employ binders which permit the fine particle ingredients to stick together. These binders can consist in polysaccharides from algae, such as agar, alginates, carragheen or terrestrial vegetable by-products such as pectine gum and guaranates. Other binders of animal origin such as gelatine or chitine by-product and even synthetic binders manufactured by chemical industries such as carboxymethy-cellulose or polyvinyl are also employed.
In order that the food be not only accepted but also searched for by the animals, the pellets are coated with appetizers. These compounds are generally marine organism extracts such as polycheta, cephalopoda, or lamellibranches; organic molecule mixtures, the appetizing properties of which are known, are also employed for example certain amino-acids or certain nitrogenous molecules such as trimethylamin oxide.
Vacuum processing is employed for the manufacture of compound food. This method permits the acquisition of pellets can be conserved and handled more easily than fresh food.
However, soft food presented in the form of pastes or jellies, ensures a better growth than that obtained with dry pellets.
The chemical degradation of food
The chemical degradation of food is caused by the action of the digestive enzymes from the hepato-pancreas in particular. This complex organ ensures several functions. As a rule, we generally admit that along with its functions in the secretion of digestive enzymes and in the temporary and cyclic retention of reserves, the hepato-pancreas is the principal organ which ensures the absorption of digestive products (GIBSON and BARKER, 1979).
The hepato-pancreas consists of a set of blind end tubules; The open ends permit the products by secretion to be discharged into the stomach.
The walls of the tubules combine cells of several types: absorbtion and accumulation cells, secretion cells, embryonic cells and fibrillar cells.
The secretion cells or B cells (taken from the german word Blasenzellan) have a basal nucleus and large cytoplasmic vacuoles containing an acid matter which flocculates easily. They have a striated border. These cells have a restricted quan lity of reserves in the form of lipids, glycogen, and calcium of phosphate. The mechanism of secretion differ depending on the groups: apocrin in Panulirus, holocrin in Astacus or mecrocrine in other species. The mechanism of enzym evacuation is not well known in detail, due to the difficulty to observe the phenomena which take place inside a massive organ. It is at present believed (LOIZZI, 1971) that a cells (from the german word restzellen) or absorbing cells, accumulate the nutriment which are found in the light areas of the hepato-pancreas tubules, and that they synthetize glycogen and lipids. F cells described as being “fibrillar” synthetize the digestive enzymes and keep them in reserve in a supernuclear vacuole. The latter will fatten by pinocytosis, in taking the nutriments from the light areas at the tubules until they form a typical B cell.
When the B cell are full, they are the most voluminous cells of the hepatopancreas. They contain an unique central vacuole, representing at least 4/5 of the cell volume. The cytoplasm is dense and thin, containing myelinic forms, groups of parallel filaments and other varied inclusions. There is a complex apical which separates the vacuole of the tubule light, having a brushy border, small vacuoles of pinocytosis, dense cytoplasm, small mitochondries, microtubules. When at full maturation stage the vacuole will compress the basic cell nucleas. No glycogen granulas nor lipidic drops are found present in cell B.
It is believed that the secretion is of merocrine or apocrin type in normal physiological conditions but that in intense stimulation it can be holocrin type.
The expulsion of digestive secretions from cell B could be caused by the contrasting and constricting muscle network which surrounds the external wall of the tubules (LAEVITT and BAYER, 1982).
The secretion of cells has been remarked as simultaneous for several cells located at the level in each tubule of the hepato-pancreas (BOCHEN, comm. pers.)
The digestive enzyms
The digestive enzyms secreted, differ greatly from one type to another. Although some studies have been carried out on enzyms, during the past century, the identification and detailed description of each one has only been commenced. We know however that several proteases exist in certain peneid shrimp, such as the Astacus. Carboxypeptidase A and B activities (GATES and TRAVIS, 1969, 1973: GALGANI, 1985) similar to trypsin activity have been remarked and show a molar mass of 34 200. Aminopeptidases and dipeptidases were discovered after a chromatographical or electrophoretical separation (DE VILLEZ, 1965; DEVILLEZ and BUSCHEN, 1967; LEE, 1980; MURAMATSU and MORITA, 1981). There exists especially a very active protease of a feeble II 000 Daltons molar mass (PFLEIDERER et al., 1981): this enzym seems only to exist in Decapoda. it has the same role as that of pepsin, the latter being absent in the hepatopancreas.
In certain crustaceans such as Penaeus japonicus, there exists a feeble collagenolytic activity.
The trypsine studied in the same species in made up of 6 isoenzyms. Its molar mass is 25 000 Daltons. It alone represents more than half of the proteasic activities of the hepatopancreas in Peneid crustaceans. An immune serum obtained with a trypsin of Penaeus japonicus reacts positively against the trypsins of eight other species of peneids. The sequence of the trypsin from the Astacus fluvialitia Crayfish has been recently defined (TITANI et al., 1980) and presets around a 50% homology with bovine trypsin.
The echymotrypsin activity, revealed in several species of crustaceans is generally feeble (BRUN and WOJTOWICZ, 1976; TRELLU and CECALDI, 1977; GALGANI, 1965). In Palaemon serratus, chymotrypsin has an average activity and appears as soon as embryogenesis begins.
Some enzyms capable of hydrolizing native collagen has been revealed and characterized in several species (GRANT et al., 1981; GALGANI, 1985).
Enzyms digesting glucids and polysaccharides also exist; amylases, maltases, saccharases, and sometimes cellulases. In Palaemon serratus, beta-glucosaminidase, beta-glucosidase, alpha-mannosidase, B-fructofuranosidase and alpha-fucosidase have been defined.
The α-amylase of Palaemon serratus has a molar mass of around 50 000 (VAN WORMHOUDT, 1980). It is made up of 2 and sometimes 3 isoenzyms depending on their geographical origin.
Three glucuronidases of 235 000, 275 000 and 370 000 molar masses have been defined in Palaemon serratus (TRELLU and CECCALDI, 1976).
The digestion of lipids is ensured by lipases and esterases. The lipases have an effect on the lipids present in the form of emulsions and the esterases continue the enzymatic digestions on the hydrosoluble products presented. This has been established for more than a century by HOPPE-SEYLER (1877) who demonstrated the digestion of olive oil by the digestive juice of the Astacus lobster. Several esterasic activities have been discovered in the digestive juice of different species. Although the energetic metabolism of crustaceans is largely under the influence of Lipids, there also exists a right number of esterases. Each one plays a role, which is sometime very specific and much greater, on the molecules to be digested having ester functions. TRELLU and, CECCALDI (197§) have proven the existance of 20 sets in using as substratum α-naphtyl-acetate in Palaemon serratus.
Chitinases also exist, permitting the digestion of the chitin which forms the exoskeletons; numerous crustaceans are indeed predators of other crustaceans and on the other hand, some of them consume their own exuvia after moulting. Taking this biochemical definition of chitin as an example, JEUNIAUX defined real chitinases as enzyms which liberate, by their action, n-acetyl-glucosamine.
Other enzymatic activities, such as desoxyribonuclease of 33 000 molar mass, ribonclease of 25 000 molar mass and alkaline phosphatases have been defined.
Crustaceans also have emulsifying compounds which play the same role as bile in mammals, in other words it disperses fats before their digestion. These compounds have been defined and studied by VONK. They are formed of taurin by-products, taurocholic and taurodesoxycholic acids.
The optimum pH activity of the different enzyms is very variable, ranging from 5.5 to 9. In most case, optimum pH is much higher than in vertebrates and especially in mammals.
The global proteolytic activity of the hepato-pancreas of Penaeus kerathurus has an optimal pH of 8.5 for gelatine and 9.5 for casein.
The optimal temperature of this activity is 50° C (GALGANI, 1985).
The trypsic activity of 5 peneid species has an optimum pH around 8.3, a carboxypeptidase A activity of 7.5, a carboxypeptidase B activity of 9.2.
α-amylase shows a optimal pH of around 6.3 to 6.8.
Variation of digestive enzymatic activities
Variations during the intermoult cycle
During the intermoult cycle defined by DRACH (1939), the cycle of reserve storage - reserve consumption which characterizes the physiological variations caused by the exuviation and the formation of a new exoskeleton, is closely connected with the digestive activities. The digestive enzyms indeed show variations in their activities during the intermoult season (BEUCHAU and MENGEOT, 1965; WORMHOUDT et al., 1972 b; TRELLU and CECCALDI, 1977).
Feeble, at stages A and B, after exuviation, they increases during the storing periods of reserves, during the different stages of stages C. During the active growth of crustaceans, in other words during the Summer months and beginning of Autumn, two digestive enzymatic activities are at maximum during the intermoult season, the first during stage C, the second during stage D1 - D2. On the contrary, during the cold months, only one maximum during the intermoult cycle at stage D0 is remarked.
Variations during the circadian cycle
During the circadian cycle, the digestive enzymatic activities are submitted to important variations. Two maximums have been defined in some species such as Penaeus japonicus and Palaemon serratus: The first maximum takes place in the morning, the second one in the evening, twelve hours after the first. It has been established through recent research that the first maximum was started by the beginning of the light phase and was produced around five hours after the passage obscurity-light (VAN WORMHOUDT, 1977).
The daily duration of the photophase plays an important role in the circadian cycle of the digestive enzymatic activities. Preliminary studies were carried out, but this date must be specified by systemized by systemized studies (VAN WORMHOUDT and CECCALDI 1974; TRELLU and CECCALDI, 1980 b). The length of the light wave employed also plays an important role on the digestive enzymatic activities, the blue green light being beneficial (VAN WORMHOUDT and MALCOSTE, 1976).
The circadian rhythm of digestive enzymatic activities is not acquired at first level stages during which irregular cyclic variations are remarked. The enzymatic activities pass through a tetracircadian phase at Zoea stage 4, and then a bicircadian cycle by a progressive decrease of both the digestive enzymatic activity peaks. Some observers have indicated that enzymatic activity increases from 1 to 4 hours after feeding.
Variation during larval development
It is of prime importance to feed the larvae during their growth, food compositions which have been adapted to the physiological digestive capacities of the different larval stages. in particular, the composition of the food must correspond to the enzymatic activities of the digestive tract of the larvae and of their evolution during growth. Specific activities are generally expressed in milligrammes of soluble proteins of the heapto-pancreas.
The Palaemon serratus has a high growth rate in the first stages, which decreases during the last larval stages. On parallel, the activity of the digestive enzyms is reduced, along with that of the amylase which greatly decreases. The relation amylases/prolease takes place. Later on, it again increases(VAN WORMHOUDT, 1981). At first larval stage the larvae don't feed. First Zoea stages are fed mostly phytoplankton rich in polysaccharides. The increase of proteases takes place at Zoea 4 stage when the larvae begin to feed on live animal prey.
The Macrobrachium rosenbergii, at first larvel stages; Zoea 1 and 2 do not feed and live solely on their reserves. When they reach the following stage the amylasic and proteasic activities increase greatly at first and from then afterwards in a more regular and moderate way. At Zoea stage 6, the specific activity of the amylase increases and is multiplied by 5 while that of proteases is only multiplied by 2.
The feeble digestive enzymatic activity of Penaeus japonicus during nauplius stages is greatly increased at metanauplius stages. The increase in the specific amylasic activities is multiplied by 15 while it will only increase 5 times between Zoea and Mysis stages. The digestive enzymatic activities are at maximum at Mysis stage 1; They decrease until the metamorphosis into post-larvae takes place, and then increase once again progressively (LAUBIER - BONNICHON et al., 1977).
On general, these variations take place at the same time as the change in food diets is carried out. Simultaneously, special neuro-secretions appear in the neuro-secreting tissues of the ocular peduncle in certain species (BELLON - HUMBERT et al., 1978). In addition, these digestive enzymatic activities are correlative with nucleic acid Denors (REGNAULT, 1977).
These biochemical variations are connected with the ecology and feeding behaviour of the species studied in their natural environment. When low protein content food is given to the animals in rearing, their proteasic activity is feeble. Progressively as the protein tenor is increased, their proteasic activities will also increase until they reach their maximum value. Then when protein tenors go above 45 %, the proteasic enzymatic activities will keep decreasing as the protein tenor increases. A similar mechanism was defined for amylases, but the optimal percentage permitting a maximum amylasic activity is around 6 to 8 % of glucids in the food.
The variations in salinity have hardly no effect on the digestive enzymatic activities.
The acclimatation of crustaceans to temperatures which are different from those of the biotop from where they come, lead also to adaptations at molecule level (VAN WORMHOUDT, 1980; TRELLU and CECCALDI, 1980 a). These regulation mechanisms of the digestive enzymatic activities can be carried out in many ways, the principal ones being the modification of the cinetic parameters of the enzyms and the quantitative variations of isoenzyms.
Endocrine regulation of the synthesis of digestive enzyms
Recent research has permitted the establishment of new facts in this sphere, but a lot of work remains to be developed.
Gastrin, localized through immunocytochemistry in the walls of the stomach, in the neurosecreting cells and in the sinus gland, increases the synthesis of digestive enzyms and especially -amylase. It provokes an increase in the proteic synthesis of the hepato-pancreas.
Ecdysteroids secreted by the Y organ stimulate the synthesis of digestive enzyms.
Cholecystokinine or CCK, peptidic hormone, is also present in the neurosecreting cells and the sinus gland, of the ocular peduncle. It increases the synthesis of the digestive enzyms. The same applies for secretin, localized through immunocytochemical techniques in the neurosecreting cells of the ocular peduncle and which increases the synthesis of the digestive enzyms.
On the contrary, the Molt inhibiting Hormone (M.I.H) present in the neurosecretions, provokes an inhibition of the proteic synthesis by blocking the ecdysteroids in the Y organ; It will also inhibits therefore the digestive enzyms synthesis.
The use of immunocytochemical techniques in employing vertebrate antiserums has permitted to localize several hormonal activities in the neurosecreting cells of crustacean Decapoda. This is how leucine - enkephalin activities substance P, glucagon have been defined.
Endocrine regulation of the Physiological functions linked with nutrition
Little precise data exists in this particular sphere. The Crustacean Hyperglycemic Hormone or C.H.H. (known, some years past, as Hyperglycemic Hormone or H.C.R) is synthetized by the neurosecreting cells and is located in the gland of the sinus of the ocular peduncle. Its role is to increase the glucid tenor of the hemolymph.
Inversely, insulin of mammals has no glycostatic effect but a stimulating effect on the synthesis of glycogen.
CONCLUSIONS
Although research should is carried out so as to understand in detail the physical, chemical and biochemical processes which take place when crustaceans start feeding. The metabolic cycles, the regulations caused by the modification of the environmental factors, the hormones implicated in these mechanisms, the roles they play, the implications on the ecology of the animals in question, the biochemical studies concerning the structure and action method of the enzyms, the comparisons of the phylogeny and the biochemical evolution should certainly give new very interesting and promising results (CECCALDI, 1982). Their application would lead to the perfection of efficient cheap compound food for both extensive and intensive rearing along with other interesting developments.
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