by
D. I. WILLIAMSON
Marine Biological Station
Port Erin, Isle of Man, United Kingdom
Abstract
Larvae of decapod Crustacea usually require different food and environmental conditions from juveniles and adults. It is suggested that in culture the problems of rearing larvae could be eliminated or greatly reduced by selecting species with direct or abbreviated development. Examples of shrimps and prawns with rapid post-embryonic development are given, and studies on their biology in captivity to assess their suitability for culture are proposed.
LE TYPE DE DEVELOPPEMENT DES CREVETTES EN TANT QUE FACTEUR DETERMINANT LEUR APTITUDE A L'ELEVAGE
Résumé
Les larves des crustacés décapodes ont en général besoin de nourriture et de conditions de milieu différentes de celles qui conviennent aux jeunes et aux adultes. On pourrait peut-être éliminer ou réduire considérablement les problèmes que pose l'élevage des larves en choisissant des espèces ayant un développement direct ou abrégé. La communication donne des exemples de crevettes ayant un développement post-embryonnaire rapide et propose des études sur leur biologie en captivité afin de déterminer si elles conviennent à l'élevage.
EL TIPO DE DESARROLLO DE LOS CAMARONES COMO FACTOR QUE DETERMINA SU ADAPTABILIDAD AL CULTIVO EN ESTANQUES
Extracto
Las larvas de los crustáceos decápodos requieren por lo general alimentos y condiciones ambientales distintas de los indivíduos juveniles y adultos. Se sugiere que en el cultivo se eliminen o reduzcan grandemente los problemas de la cría de las larvas mediante una selección de especies que tengan un desarrollo rápido o abreviado. Se indican ejemplos de camarones y langostinos de rápido crecimiento postembrónico y se proponen estudios sobre su biología en cautividad para determinar la conveniencia de su cultivo.
Not all decapod crustaceans are equally suitable for human food, and not all are equally easy to rear in captivity. The range of edible species is, however, very broad, and few species which appear suitable for farming or culture are inedible. The first criterion of suitability for culture must be the ease and cost of rearing on a large scale. Stock selection at sub-specific level, based on such factors as growth rate, flavour and resistance to disease can be envisaged in the future, but the problem at the moment is that of selection at the specific level.
Cultivation by impoundment or any form of culture in which the young of wild stocks are trapped must be dependent on local species. A considerable amount of species selection can, however, be exercised by the method of trapping and its timing. The present paper is chiefly concerned with methods of cultivation in which the young are reared in tanks, and consideration need not be limited to local species.
Most decapods pass through a number of larval stages, and the environmental requirements of larvae are almost invariably different from those of juveniles and adults. The larvae are normally planktonic while the later stages may be either nektonic or benthic. In some cases the larvae have different temperature and salinity requirements from the later stages, and in all cases they have different food requirements. In order to farm a typical prawn it is, therefore, necessary to have two fairly distinct units, one catering for the physical, chemical and food requirements of the larval stages and one supplying the needs of the post-larval stages.
Prawns which are cultivated on a commercial scale are drawn almost entirely from the families Penaeidae and Palaemonidae (species listed by Holthuis and Rosa, 1965) and all are species which pass through five to ten feeding zoeal stages. The Penaeidae also pass through a number of naupliar stages, but these are of brief duration and do not feed (Gurney, 1942). Today, prawn cultivation is mostly by impoundment, and since the larval stages are not reared in captivity their number, duration and feeding habits are of little direct concern to the cultivator. These factors are, however, of very great concern to the farmer who raises all developmental stages of his stock in captivity, and interest in this form of prawn cultivation is now growing rapidly in several countries. The few pre-farming studies conducted have concentrated on determining and supplying the larval requirements of species which are already commercially important in the area, and such investigations have met with considerable success in the case of the penaeid Penaeus japonicus Bate (Hudinaga, 1942) and the palaemonid Macrobrachium rosenbergii (de Man) (Ling, 1962). While these examples show that the problems of large-scale rearing of larvae may in some cases be successfully surmounted, it is the purpose of this paper to point out that, by the selection of suitable species, these may largely be eliminated. A number of Caridea and Stenopodidea hatch in near-adult form and in others the larvae feed only on internal yolk. It is suggested that some of these species may well prove suitable for large-scale farming and that the difficulty and cost of rearing them could be much less than for species with typical larvae. Some suggestions for further investigations of such species are made below.
Among species which feed in the zoeal stages, those which hatch as large larvae and complete their larval development within a few moults seem to be better suited for cultivation in captivity than species which hatch as small larvae and pass through many moults before metamorphosis. Their larval life will be shorter, and they should, in general, accept a greater variety of foods. Suggested foods include adult copepods from natural plankton and young amphipods from either wild or farmed populations. Some large larvae may accept non-living food, whereas all small zoeas seem to feed only on living planktonic species. Large larvae would not be dependent on the newly-hatched nauplii of Artemia, which form the standard laboratory food for most zoeas. Artemia nauplii are invaluable for the laboratory rearing of decapod and fish larvae, but the number of large-scale rearing projects relying on this food is steadily increasing, and there is a danger that the supply of Artemia eggs will eventually limit such operations, apart from being a major expense in the budget of any farm dependent on it.
Decapod Crustacea which undergo direct or rapid development tend to occur more commonly in fresh and brackish waters and in the deeper and colder parts of the marine environment, although other species which pass through a long series of larval stages may also occur in the same environments (Gurney, 1942). Species with abbreviated development almost invariably have fewer and larger eggs than comparable species with a more typical development. An extreme case is provided by Richardina spinicincta A. Milne Edwards (Stenopodidae) in which a female may carry as few as six eggs (Kemp, 1910), although females of most Decapoda with direct or abbreviated development can carry over 50 eggs and many carry several hundreds. The prospect of rearing eggs to juveniles with lower mortality and at lower cost will generally compensate for the smaller number of eggs per female. Hardly any prawn with a suitable type of development seems likely to be rejected as a potential species for farming on the grounds that the female carries too few eggs.
Species from water of reduced salinity are frequently tolerant of a wide range of environmental conditions and can often withstand comparatively rapid changes in these conditions. This hardiness makes many of these species suitable for cultivation by impoundment, and a number of them grown in this way are listed by Holthuis and Rosa (1965). In the case of impoundment the length of larval life is of little importance, but those species which are suitable for this form of cultivation and which also have abbreviated larval development should be particularly suitable for farming.
Few attempts have been made to keep deep-sea prawns alive in captivity and difficulties can be foreseen in many cases. Some species may require very constant conditions, others may need to carry out extensive vertical migrations, and some may require the presence of undefined chemical factors which are not present in coastal waters (Wilson, 1951). Further knowledge of the habits and requirements of deep-sea species is, therefore, very desirable, and a number of species may eventually be found which are suitable for farming.
Arctic and antarctic seas and sub-polar waters offer what appears to be the most promising source of shrimps and prawns for farming. Many of the species from these areas are quite large and a high proportion have abbreviated larval development. It seems likely that some will breed only in relatively cold water, and a prawn farm in temperate or tropical regions might have to supply cooled water for the breeding stock. There is, however, the exciting possibility that juvenile prawns may be acclimatised to the local sea temperature, where they might be expected to show an increased growth rate. Such a system, involving the cooling of relatively small volumes of water, seems preferable to systems which have been suggested for temperate regions which involve the heating of considerable volumes of water for the growing stock. Species which occur in cold coastal waters may well prove the most suitable for farming in warmer coastal waters.
Examples of shrimps and prawns showing different types of rapid post-embryonic development are given below, and remarks are made on some aspects which are likely to affect their suitability for farming. In addition to the species mentioned here, many more are known to have large eggs and it is probable that their larval development is abbreviated, but there is no published indication as to whether or not the larvae feed or in what form the animal hatches (Gurney, 1939). It should be noted that different species of the same genus may show considerable variation in the state of development at hatching. The wide variation within the genus Pandalus Leach is discussed by Pike and Williamson (1964).
3.1 Species with no free larval stages
The most satisfactory zoological definition of “shrimps and prawns” is to equate them with the Macrura Natantia, and the only practicable distinction between shrimps and prawns is one of size. The fresh-water crayfishes (Astacidae and Parastacidae) cannot be included as prawns in the zoological sense, but they lend themselves to similar culinary treatment and to similar farming methods. Development is direct in all known members of both families; the only larval characters of the newly-hatched young are the absence of first pleopods and uropods and the virtual absence of setae on any of the appendages. The young are never pelagic and they remain attached to the pleopods of the parent for at least one instar. They hatch with considerable supplies of yolk and probably take no other food until after one or two moults. The culture of fresh-water crayfishes is discussed by Smolian (1925).
Several Macrura Natantia have no pelagic larval phase in their development. In Sclerocrangon boreas (Phipps) and S. ferox (G.O. Sars) the newly-hatched young show rather similar development to those of the Astacidae and Parastacidae, and they apparently remain attached to the pleopods of the female, without feeding, until they reach Stage III (Wollebaek, 1906, 1908, and personal observations). On the other hand, S. intermedia (Stimpson) and S. salebrosa (Owen) have recently been shown by Makarov (1966) to pass through several pelagic zoeal stages. The genus contains at least eight other species whose development is not described. All species are bottom-living as adults.
S. boreas appears to have considerable potential as a species for farming. It is a large prawn, reaching a length of over 12 cm, and is widely distributed in the colder waters of both North Atlantic and North Pacific Oceans. It is most common in waters of less than 100 m and occasionally occurs intertidally (Wollebaek, 1908; Heegaard, 1941). S. ferox occurs in the arctic Atlantic at greater depths than S. boreas and usually in colder water. Its suitability for rearing in captivity also merits investigation, and the type of development of other species of Sclerocrangon G.O. Sars should be studied.
There is also no pelagic zoeal stage in the development of some species of Synalpheus Bate (Alpheidae) and in all known species of Cryptocheles G.O. Sars and Bythocaris G.O. Sars (Hippolytidae) (Dobkin, 1965a; Sars, 1912). The young of these forms do not attach to the pleopods of the parent; they may emerge either as juvenile shrimps, with no larval characters, or as ‘pseudo-larvae’, with vestigial exopods on some of the legs. The ‘pseudo-larva’ is incapable of locomotion and quickly moults to the juvenile form. Although there is a tendency towards abbreviated development throughout the genus Synalpheus, the pelagic larval phase is completely suppressed in only a minority of species. The small size of adults of Synalpheus and the fact that they normally live inside sponges and other animals probably makes them unsuitable for farming. Most of the known species of Cryptocheles and Bythocaris are also rather small, but B. leucopis G.O. Sars can exceed 9 cm in length. Records of this species are from 320 m to over 2000 m depth, chiefly from the Norwegian Sea (Heegaard, 1941).
3.2 Species in which the larvae do not feed
A number of Decapoda are known to pass through one or two zoeal stages which are nourished entirely on internal yolk and which have no functional mouth parts. Most of the known prawns in this category occur in the families Oplophoridae and Pasiphaeidae; they are pelagic species often inhabiting deep water and they may be difficult to keep in captivity. A benthic prawn in the same category is Glyphocrangon spinicauda A. Milne Edwards (Glyphocrangonidae), whose development is described by Dobkin (1965 b). Other members of the Glyphocrangonidae probably develop in the same way; all appear to be restricted to deep water.
3.3 Species with few larval stages
Prawns which hatch as large larvae and pass through only one or two zoeal stages are known from the families Pandalidae, Hippolytidae and Crangonidae. The following examples are all coastal species:
Pandalus kessleri Czerniavski is fished in parts of northern Japan but is apparently not farmed. It hatches as a very advanced zoea, about 8 mm long, which moults within a few days to give a megalopa (postlarva) with functional pleopods (Kurata, 1955). Pandalopsis coccinata Urita, also from Japanese waters, is about 15 mm long on hatching. The stage which emerges from the egg is probably bottom-living, its only truly zoeal characters being the form of the telson and the antennules (Kurata, 1964). This species might equally well be included in section 3.1 (“Species with no free larval stages”).
There is no complete description of post-embryonic development in the hippolytid genus Lebbeus White, but L. polaris (Sabine) and L. groenlandicus (Fabricius) probably hatch as very advanced zoeas, measuring about 8 mm, and pass through not more than two zoeal stages (Pike and Williamson, 1961). L. polaris has a pan-arctic distribution, and L. groenlandicus extends from north-eastern Asia through North American waters to Greenland (Heegaard, 1941, both as Spirontocaris).
Argis lar (Owen) (Crangonidae) measures 8 – 10 mm in stage I and probably passes through only two zoeal stages, although Squires (1965) thought that there may be other zoeal stages which have not been observed. The distribution of this species (Heegaard, 1941, as Nectocrangon) is very similar to that of L. groenlandicus. Several other species of Argis Krøyer occur in the colder parts of the North Pacific Ocean but their development has not been described.
There is an obvious need for extensive studies on the biology of prawns in captivity to determine their suitability for farming, and it is urged that cold water species with abbreviated development should receive particular attention. The species which have been suggested in this connection in this paper are mostly from boreal and arctic seas, but it is likely that equally suitable species are to be found in the colder waters of the southern hemisphere. There are also a great many northern prawns whose development has not been studied, and it is probable that a number of these have types of development similar to those which have been mentioned.
The chief object in choosing prawns with abbreviated larval development for farming is to reduce or eliminate the problem of feeding the larvae. It does not, of course, reduce the problem of feeding the juveniles and adults, but this should be no more of a problem than for prawns with typical larval development. Suitable food for the juveniles and adults will in most cases have to be determined by trial, although knowledge of natural foods may be very helpful. It is suggested that gammarid amphipods, which often thrive on a diet of partly decayed vegetable matter, may prove a relatively economical food for some prawns. It is also suggested that, provided recently hatched nauplii of Artemia salina are not required, parthenogenetically reproducing cultures of this species are likely to provide a cheaper source of live prawn food than populations raised directly from resting eggs. It is probable that many prawns will take a variety of non-living foods, including ‘prepared’ foods based on fishmeal and other inexpensive sources of protein.
One of the most difficult factors to control is the chemical composition of the water in which prawns are to live, and in many cases it is not known which chemical factors are important for the growth and reproduction of the species (Wilson, 1951). In most cases, therefore, selection of species which will thrive in the local water is another matter which will have to be determined by trial, although, in general, shallow water species are likely to be tolerant of a wider range of both chemical conditions and temperature than are deep water and oceanic species.
Pre-farming studies on prawns should include observations on the effects of temperature, salinity, food and crowding on growth-rate and survival at different stages in the life history and the effects of these conditions on breeding. If the breeding stock is kept under different conditions from the growing stock, then the time of transfer of the young prawns and the method of acclimatisation must be matters for further experiment. It is to be hoped that many different techniques will be tried in many different regions with a view to finding a range of species suitable for farming under a range of conditions. Not only will these conditions vary geographically but within any region the potential growing habitats will include man-made tanks and ponds and natural bodies of water of a great variety of shapes and sizes.
Finally, it is recommended that consideration of species for farming should not be limited to those which are abundant in nature. The experience of many biologists seems to show a complete absence of correlation between the commonness of plants and animals in natural aquatic environments and the ease with which they may be reared in captivity.
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