clams, and that the presence of eggs in the water is a cue for clams still further downstream, to release sperm (Lucas, 1988). Under hatchery conditions sperm release can be artificially induced by introducing macerated clam gonads (fresh, frozen or freeze-dried), or neurotransmitters such as serotonin (Hesiinga and Fitt, 1987). In the home aquarium, tridacnid clams have been known to spawn both spontaneously and after some sort of disturbance in the aquarium i.e. after adding large amounts of freshwater, increasing
lighting, adding large quantities of activated carbon, excessive IJV exposure, etc. (J.C. Delbeek, pers. obs.; pers. comms. M. Paletta, J. Burleson, L. Jackson).
In closed system aquariums, sometimes the clams die a few days after spawning. This is most likely a direct result of the severity of the disturbance, and not an end result of spawning. It is also possible that sperm release is toxic, and the concentration of the toxin in a small closed system could kill the clams (B. Carlson, G.
Hesiinga pers. comms.). After the clams spawn we suggest performing a partial water change.
Strong contractions of the adductor muscles, during spawning close the valves vigorously, sending the sperm or eggs out of the exhalent siphon and into the water column. These contractions and expulsions can continue for over 30 minutes during which time millions, and in the case of the larger species, hundreds of millions of 100 micron diameter eggs are expelled into the water. When the eggs hatch (roughly 12 hours after fertilization), the larvae are called trochophores. This stage only exists for 12-24 hours, during which no solid food is ingested. Within two days metamorphosis occurs and they become 160 micron long, bivalvecl veligers. At this point, the veligers begin to take up dissolved nutrients from the surrounding water, and start to ingest zooxanthellae and other phytoplankton. Symbiosis, however, doesn't occur until after the final metamorphosis. Generally about a week after fertilization, the veligers will transform into pediveligers (pedi = foot), developing a larval foot, and begin to settle. During this period they alternate between swimming and resting on the substrate. Within 9 days, they settle permanently onto the substrate, using byssal threads to attach the 200 micron o J
juveniles. Nevertheless, they can still travel short distances using their foot until a suitable place is found. The factors responsible for triggering metamorphosis and substrate selection are not yet known. The time from fertilization to settlement and establishment of a symbiosis with zooxanthellae usually takes about 1 to 2
weeks, with the larger species having the shorter larval periods (Heslinga and Fitt, 1987).
As mentioned above, zooxanthellae are introduced into the stomachs of the developing clam during the veliger stage. The zooxanthellae may remain in the stomach for as long as a week. A
* o few days after metamorphosis, zooxanthellae are seen in tissues adjacent to the stomach and are subsequently found in rows in the tubules extending into the developing mantle (Heslinga and Fitt. 1987). The zooxanthellae are moved along the tubular system by the beating of cilia that line the tubule (Norton, et al., 1992). The final step in the development of the symbiosis is the growth of the zooxanthellae population within the mantle.
In the last 10 years a great deal of information has been acquired on the artificial propagation of tridacnid clams (see Heslinga et al., 1990). Various commercial breeding programs have arisen in Palau, Australia, Micronesia, the Philippines and Tonga, to name just a few. Currently T. gigas, T. derasa, T. squamosa and H. bippopus are the main species being propagated for food, restocking programs and the aquarium industiy. However, more colourful, commercially raised T. crocea, T. maxima and T. squamosa are now being produced for the aquarium market, and especially colorful varieties of T. derasa way appear in the near future.
The white margin along the edge of the shell of this Tridacna maxima is new growth. J.C. Delbeek.
With the mantle retracted, the new growth is evident on this T. derasa, one of the fastest growing species. J.C. Delbeek.
The propagation of triclacnid clams in the home aquarium is a very real possibility. In fact, they may prove much easier to breed and raise than clownfish. The main hurdle is to acquire specimens that can produce eggs; spenn production is easily induced. Once the larvae reach the veliger stage, they can be fed unicellular algae such as Isocbtysis galbana but success can be had without feeding. After metamorphosis, zooxanthellae need to be introduced into the clam. This is another hurdle. Although growing cultures of zooxanthellae is not difficult, acquiring a suitable strain may be. Once symbiosis is established, all that is required is light and the proper nutrients to promote shell and tissue growth i.e. calcium, strontium, iodide, ammonium, sulfate and nitrate. Some initial mortality results from bacteria, but these losses can be curbed with antibiotics (Fitt et al., 1992). For those of you interested in pursuing this topic further, a
One more source of tridacnid clam mariculture information is Biology and Mariculture of Giant Ciams (W.K. Filt Ed.) ACIAR Proceedings No. 47, Canberra: Australian Centre for Internationa! Agricultural Research.
breeding manual Giant Clam Farming was published by the Micronesian Mariculture Demonstration Centre, P.O. Box 359, Koror State, Republic of Palau, 96940. and it covers the subject thoroughly. It was possible to purchase the manual at one time through the Pacific Fisheries Development Foundation, P.O. Box 4526, Honolulu, Hawaii, USA 96812, but the supply has run out. We hope that it will be published again, but for now one may have to find this manual in a library. Another book, Giant Clams in Asia and The Pacific, also contains a wealth of information about giant clam farming. It is available from the Australian Center for International Agricultural Research G.P.O. Box 1571, Canberra, A.C.T. 2601 Australia.
When one looks at the large size attained by some species of Tridacnidae, it is easy to imagine that these individuals became so large because they were very7 old. Although some species such as T. gigas can be over a hundred years old, it is now believed that their large size is more a function of their rapid growth rate than their extreme age. The two largest species, T. derasa and T. gigas, can grow over 10 cm (4 in.) per year, while the smaller species such as T. crocea and T. maxima grow much slower, only 2-4 cm (0.8-1.6 in. ) per year. On average, T. gigas can reach a length of over 60 cm (2 ft.) within 10 years (Crawford and Nash, 1986). Although growth is relatively slow in the first year, it increases rapidly after that for the larger species but slows for the smaller species (Lucas, 1988). As the clams become sexually mature their growth and calcification rates can also slow noticeably ( Jones et al., 1986; Lucas, 1988). For example, at an age of approximately ten years, growth of T. maxima slows appreciably, which was found to coincide with sexual maturity (Jones et al., 1986).
The life span of these animals can range from 8-200 years depending on the species (Achterkamp, 1987a), however, veiy little work has been done on age measurements of tridacnid clams. Since clams form seasonal growth bands in their shells it is possible to age sections of dead shells; perhaps in the future, more accurate measurements will be made (Lucas, 1988).
"My object in writing this article is to introduce to you this new system, (no, it is not new, it is as old as when God first created the oceans and their contents) to help marine hobbyists all over the world to enjoy their hobby with the greatest ease and simplicity."
Lee Chin Eng. 1961 Nature's System of Keeping Marine Fishes
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