Taxonomy
Phylum - Mollusca
Class - Gastropoda
Subclass - Prosobranchia
Order - Archaeogastropoda
Family - Turbinidae
Genus - Turbo
Species - marmoratus
2.1.1 Distribution
Green snail distribution ranges from Western Indian Ocean localities of Kenya, Tanzania, and the Seychelles to Western Pacific and Southeast Asian countries like Indonesia, Malaysia, Philippines, and upward to the Ryukyus and the South Pacific Islands of Vanuatu, Solomon Islands, and Papua New Guinea (Figure 1). Vanuatu is the eastern-most limit of the species distribution. Introduction of green snail to other Pacific Island countries such as Tonga and Tahiti has been successful, and this has increased the distribution range of the species eastward. However the new localities cannot be included as natural distribution range for green snail.
Figure 1. Distribution of green snail
(Arrow shows transplantation of green snail Turbo marmoratus to islands in the West-tropical pacific. The sriped area is the natural habitat of green snail (Kikutani, 1998))
2.1.2 Habitat
Green snail Turbo marmoratus is a tropical coral reef-dwelling species. Juvenile green snails from 2–6 cm shell height (“SH”) inhabit the coral reef tidal zone, although sub-adults and adults are found in deeper waters of reef margin areas 5 to 25 meters deep (Yamaguchi, 1993). Movement from shallower to deeper water varies with the size, which is a survival strategy adopted by the species. Juvenile green snail inhabit small holes and crevices made by burrowing sea urchin, Echinometra mathaei, while adults stay in large holes and on shelves during the daytime (Yamaguchi and Kikutani, 1989) and emerge to feed at night.
2.1.3 Reproduction
The sexes of the green snail are separate, and gametes of both sexes are released into the water column. Green snails were estimated to be sexually mature at their natural reef habitat at about 110–120 mm SH (Devambez, 1961, Kikutani, personal communication). At the Sopu hatchery in Tonga, sexually mature sizes of hatchery-produced individuals are smaller, from 70–90 mm SH. In higher latitudes, green snail appears to breed only in the summer months when the water temperature is higher, but in lower latitudes, mature animals breed repeatedly throughout the year (Yamaguchi, 1993). Fecundity increases with size, but has been estimated at up to 7 million eggs in a 2-kg female snail. Unlike trochus, the egg of green snail does not possess a jelly layer.
The spawned eggs are slightly heavier than sea water but are kept suspended by current movement and are likely to be easily dispersed. The eggs hatch into trochophore larvae and the larvae go through veliger stages before settling on the substrate. In the hatchery this process is completed after three to four days depending on water temperature (Yamaguchi, 1993). Settling juveniles go through a series of morphological stages before reaching adult-like morphology six months after fertilisation. Growth of green snail varies at different localities, but they tend to reach around 2–3 cm at 12 months of age and become sexually mature at 3–4 years of age (Yamaguchi, 1989). The life cycle is illustrated in Figure 2.
2.1.4 Feeding
Green snail adults are nocturnal, hiding by day and emerging at night to feed. The larvae and early juveniles feed on microalgae such as diatom. Large juveniles and adults feed on red algae Gracilaria, Hypnea and Echeuma, but laboratory experiments showed that their most favoured food is red algae Gelidium, which grows on substrate as short turfs (Yamaguchi, 1995).
2.1.5 Predators
Predation experiments of green snail juveniles in the laboratory revealed that many carnivorous animals such as flat worms, crabs, mantis shrimps, carnivorous gastropods, octopuses and fishes preyed on juvenile green snails (Yamaguchi, 1988). There is not much information on the natural predators of juveniles and adults at this stage; however, Kikutani's personal observation at Tokunoshima Island reveals that juvenile green snail were attacked by Horse conches Pleuroploca spp. and Ceramic vase shell Vasum ceramicum in the daytime, land adult green snails were preyed upon by a hermit crab Dardanus sp. at night.
Figure 2. Life cycle of the green snail Turbo marmoratus
2.1.6 Sex determination
Sex determination of live green snails is now possible without any special apparatus (Kikutani et. al. 1994). The genital papillae, (or right kidney opening, “RKO”) of both male and female snails can be observed externally, when their soft bodies are extended outward. If the observer faces the shell aperture and holds it above eye level, the tip of the organ appears clearly against the inner shell wall on the left side of the animal. This time consuming method can be speeded up by stimulating the snail applying running seawater into the shell cavity. The female organ is broad, bean shaped and long, while the male's is tubular and long, as seen in Figure 3. Both male and female organs are pale white/yellow.
Figure 3. Male and female organs are seen when the
fleshy part of the green snail is extended outward. (Kikutani et al., 1994)
2.2.1 Classification and description of topshell Trochus niloticus
The species Trochus niloticus commonly known as Topshell, belongs to Super-family trochacea and Family Trochidae of Order Archeogastropoda, the most primitive order of the prosobranch gastropoda. The species name mistakenly referred to the Nile River by Aldrovandus, who described it first in 1606, and confused it with another gastropod found in the Nile River, Egypt. Linnaeus repeated the name niloticus in 1767 (Nash, 1993).
T. niloticus has two different growth forms. One form is conical with straight sides and a flat base. In the second form, the final whorl of the shell expands greatly to form a wide basal flange. These two different forms are considered as phenotypic variants of a single species (Asano, 1963).
The trochus shell is smooth and heavy with a thickened, spreading peripheral rim. The body whorl has concave sides. The columella is long, curved, smooth and ends at a basal notch. The outer lip and the aperture are smooth, and the colour is off-white with oblique reddish stripes (Figure 4). Trochus niloticus is classified as follows:
Taxonomy
Phylum - Mollusca
Class - Gastropoda
Subclass - Prosobranchia
Order - Archaeogastropoda
Super-family - Trochacea
Family - Trochidae
Genus - Trochus
Species - niloticus
2.2.2 Distribution and habitat of the topshell, Trochus niloticus
T. niloticus is found in the tropical & subtropical waters between the eastern Indian Ocean and the western Pacific Ocean. Its natural distribution extends from Sri Lanka in the west to Wallis Island in the east. The northern limit is in the Ryukyus Island of Southern Japan and New Caledonia and Swain Reef at the southern end of the Great Barrier Reef in Australia in the south. Because of successful transplantation, trochus distribution now has extended far to the east (Figure 4).
The natural habitat of Trochus niloticus is the coral reef, and particularly the reef-flats. It generally inhabits in the windward margin of the reef, but is sometimes found on the leeward side particularly in the intertidal and shallow sub-tidal zones. Juvenile trochus are found among the boulders and rubble on the reef top. Adult trochus are usually found from reef margin to 25 meters deep. Maximum density occurs on the dead coral slabs covered with small algae, diatoms and foraminiferans. Size segregation is also noted in trochus: the juveniles are mainly found on the intertidal reef-flats, while adults prefer the sub-tidal and fore-reef slope areas. A transplantation of trochus was attempted in Tongatapu, when 1,019 trochus were introduced from Fiji. Of that number, 500 individuals were released at Fukave Island and 400 were released at 'Euaiki Island.
2.2.3 Nutrition and feeding
T. niloticus is a herbivore feeding on either turf or fleshy algae. It has a rasp-like radula made up of about 150 teeth, which is used to graze on the substrate. It generally feeds on green algae (Chlorophyceae) and brown algae (Phaeophyceae). Analysis of the digestive tract contents showed a small amount of red algae and large quantities of bottom deposits like sand and sediments. Remains of foraminifera, sponge, hydroids, crustaceans and molluscs were also found. Rao (in Nash, 1993) concluded that T. niloticus “extracts its nourishment mainly from the bottom deposit consisting of organic and inorganic materials, but supplements it with nutrients derived from an inconsiderably portion of vegetable matter”.
Figure 4. Gross external anatomy of the Trochus niloticus
Figure 5. Distribution of Trochus niloticus and its transplantation
(Arrows show transplantation of Trochus niloticus to islands in the West-tropical Pacific. The striped area is the natural habitat of the trochus. (Bour, 1990))
2.2.4 Gross external anatomy, reproduction and life cycle
T. niloticus usually reaches a maximum diameter of 12–15 cm at the base of the shell or the shell width (“SW”). The gross external parts of the shell are shown in Figure 4.
Top shells are dioecious, having separate sexes, but they do not exhibit external dimorphism, meaning that the sexes cannot be differentiated by external morphology. Sexes are readily distinguished by histological examination of the gonads: male gonads are pale brown to creamy white in colour, and mature female gonads are dark green.
They release their gametes (eggs and sperm) directly into the water and fertilisation is external. The eggs are lecithotrophic (containing a yolk), usually 220–240 μ m in diameter with larger jelly. Fecundity ranges from 100,000 to 1,000,000 eggs for female shell (Heslinga & Hillman 1981 in Nash, 1993), and could possibly reach 2,000,000 (Nash, 1985). A general cross-section of gastropods showing the position of the genital gland and other internal organs is shown in Figure 6.
The ratio between males and females as reported by Asano (1963) is generally equal. Spawning occurs throughout the year at low latitudes, but only during summer months at the southern or northern limit of its range.
Spawning behaviour is preceded by movement of the trochus to a high point in the tank, either to the water line, or to the top of other trochus in the tank. Males always initiate spawning. Females begin to spawn from 10 minutes to an hour after commencement of spawning by males. Spawning takes place at night, and usually a few days before and after the new moon and full moon.
The life cycle of T. niloticus is shown in Figure 7 with permission from Kikutani (1992). Spawning of sexually mature trochus usually occurs in the reef slope. Fertilised eggs undergo a series of cleavages, from 2–4–8–16–32 morula and to the gastrula stage. The enclosed trochopore stage follows and hatching occurs within 10–12 hours, depending on the water temperature after fertilisation. After hatching, the trocophore develops a larval shell, the protoconch, and swims towards the surface of the water using ciliated velum. It feeds on its own yolk reserves, thus becoming a lecithotrophic veliger. Roughly 3–4 days later the veliger larva settles down to favourable substrates and metamorphoses by shedding its velum. It begins to creep, feeding on microscopic algae in the reef-flat until it becomes a juvenile, and later moves to the reef slope during its adult stage.
Figure 6. The general internal anatomy of gastropods, showing the gonads
Figure 7. Life cycle of Trochus niloticus (Kikutani, 1992)
2.3.1 Distribution
Donkeys ear abalone H. asanina is widely distributed along West Pacific waters from Southern Japan to Southeast Asian countries. H. ovina is distributed throughout the tropical pacific region but rarely in abundance nor is of any value to the pacific people. Therefore, the distribution is unknown or there is less interest of research into distribution and biology of the species.
Taxonomy
Phylum - Mollusca
Class - Gastropoda
Subclass - Prosobranchia
Order - Archaeogastropoda
Family - Haliotidae
Genus - Haliotis
Species - ovina
2.3.2 Habitat and Ecology
Abalone species are tropical coral reef dwelling species. They live in habitat similar to other gastropod species such as green snail and trochus.
2.3.3 Feeding
Like green snail and trochus the larvae feeds; on microalgae diatom while the larger juveniles and adults of tropic species feeds on Gracilaria and Gelidium. In the hatchery, formulated feed in the form of pellets mixed with red algae such as Gracilaria spp., Eucheuma sp. and Hypnea spp. is mostly used.
2.3.4 Predators
Natural predators for newly settled juveniles are flatworms, fish, crustacean and certain species of poly chaete worms. Sea urchin Centrostephanus rodgersii could remove juvenile abalone during grazing. Larger juveniles and adults are preyed upon by sea stars, crabs, octopus and fish such wrasses and parcupinefish.
2.3.5 Reproduction
Similar to other marine gastropods, the sexes of abalone are separate. The reproductive behaviour is the same as both trochus and green snail. Development of egg to larvae stage is also the same except at the late larval stage when the shell shape begins to change. Refer to reproductive biology of green snail and trochus.
Sex determination in abalone is possible without any special apparatus. The gonad is developed around the hepatopancreas at the edge of the shell muscle (shown in Plate 1). It is externally visible by gently opening a space between the shell and the soft body with a finger or spatula. The matured testis is milky white (male) and easily defined, while the ovary is dark green in color (female) and a little difficult to distinguish from hepatopancreas. It was noted that the most active specimens are usually the best spawners.
Plate 1. Gonads of abalone; Male gonad is shown on the left and female gonad on the right
The number of eggs spawned varies largely among individuals. Based on the individual spawning test, it was estimated to be 200,000 to 600,000 eggs for the shells of 5–8 cm in shell length. Biological minimum size of females is about 4.4 cm in shell length for smaller sizes such as H. asinina. Among larger species, sexual maturity is reached at more than 6.7 cm shell length (Singhagraiwan and Doi, 1992).
