Derasa Clam

Care

The Derasa Clam - also known as the Smooth Giant Clam because of the relative lack of ribbing on the exterior of the shell - is one of the larger growing Tridacna species and is treasured by aquarists who appreciate an intriguing centerpiece in their reef aquarium. These clams are highly variable in colour and markings, dependent on the original collection location and where subsequent specimens were aquacultured. They may show striking green, gold, and blue hues, and although brown-gold specimens tend to predominate in the hobby, they still tend to show beautiful blue or purple coloration along the mantle edges and some degree of marbling or stripes on the mantle itself. The thick shell usually has 6 or 7 vertical folds with very low ridges, lacking the rough, concentric sculpture seen in other Tridacna species. As the mantle edges are very symmetrical, the clam can shut its shell incredibly tightly if required. In the wild, Derasa Clams are known from clear, shallow reef habitats, most individuals living on sand, and a few on hard substrates or amongst colonies of Acroporacorals. In the home aquarium, they are best placed on the sand wherever possible (providing sufficient light can reach them there), but acclimatization must be carried out very carefully as clams are extremely sensitive to minute environmental changes. Unlike some other members of the Tridacna genus, they do not firmly anchor themselves to the substrate, as their byssal opening is very small, and therefore only a limited amount of byssal threads are extended. These threads are more common in smaller, younger specimens, and these tend to disappear as the clam grows and can hold itself in position by its own weight. Although Derasa Clams must be considered one of the hardier clams to keep in the home aquarium, they do have some specialist care requirements. First of all, young specimens of 10cm/4" or smaller do not tend to fare well in captivity due to their regular and intense requirement for phytoplankton. Larger specimens tend to acclimatize better and although they do require feeding to supplement the nutrition they gain from their symbiotic zooxanthellae, they are not quite so tricky to nurture. A large, mature aquarium is required, with strong lighting provided by VHOs or power compacts. If the tank is particularly deep, metal halides will be required. Bright lighting will enable the zooxanthellae within the clam's tissues to photosynthesize and provide nutrients to its host. Be aware that smaller specimens will be much more sensitive to very intense illumination (as the skin is much thinner compared to that of more mature specimens) and they will need to be carefully and gradually acclimatized to the aquarium lighting.Even if you eventually hope to site the clam in a large rocky crevice higher up the aquarium, it is best to start by slowly acclimatizing the clam to your water conditions (drip acclimatization is best, over around an hour) and then place the clam as low down in the tank as possible, gradually moving it higher over time. It mustn't be moved repeatedly over a short period unless you have to - allow plenty of time in between moves, ideally at least 2 weeks each time. Clams have a lot of trouble adapting to changes in lighting and current as it is, and moving them over and over again in a short period may prove too stressful. If at all possible, find a concave piece of rock (shaped a bit like a shallow bowl, and which is large enough to allow for growth), and place the clam onto this at the bottom of the tank; this then makes it easier to move rock plus clam gradually higher in the tank without disturbing the underside, should it have put out some byssal threads. The clam should be stable, but never placed in a restrictive crevice that prevents it from opening fully, and it should not be placed in an area of high flow. Too much current will cause the clam to close, and the zooxanthellae will then not receive enough light and the health of the animal will deteriorate. Having said this, gentle-moderate water movement will be necessary around the clam to ensure that particulate food reaches it and also to make sure that no heavy detritus builds up around the base; Care must be taken to ensure that flow is not too vigorous, the correct balance being crucial to its continued wellbeing. It is not unheard of for clams to move around if they are unhappy where they have been placed, and they should be observed in case they propel themselves into an even more favorable position, such as on their sides. Watch out for tankmates accidentally knocking them askew too. In the wild, clams grow with the mantle perpendicular to the sunlight, so they should be placed perpendicular to the aquarium lighting, even though that may result in more of a side-on view for the aquarium. This ensures that none of the mantle loses zooxanthellae, as it is exposed to the light in its entirety. When maintained under ideal conditions, it is not unusual for a Derasa Clam to more than double its size in a year, particularly during the earlier stages of growth. Tankmates should be peaceful and any species that would be tempted to nip at the mantle should be avoided e.g. angelfish, large wrasses, some butterflyfish, and crabs. Avoid placing the clam too near stinging corals, particularly those with long sweeper tentacles, and be sure that none of your corals shade out the clam as they grow. Small water changes performed on a frequent, regular basis tend to be more beneficial to clams than larger water changes on a less frequent basis since vital trace elements will be added more steadily. Pyramidellid snails do predate on clams and can become a problem, so before purchase, do inspect their mantle and foot carefully for snails and their eggs. Aquacultured clams are readily available in the trade, and these are always preferable to wild specimens as not only are they hardier, but they are much less likely to have suffered any sort of byssal gland damage due to improper harvesting techniques.

Feeding

Zooxanthellate, but should befed regularly with phytoplankton, zooplankton, rotifers etc.

Reproduction

Tridacna derasa reproduce sexually and are hermaphrodites (possessing both male and female reproductive organs). However, the release of sperm and eggs are separate to prevent self-fertilization. This not only reduces the burden of finding a compatible mate but simultaneously doubles the number of offspring produced by the process. Since these clams do not move about, they adopt broadcast spawning. This means that when it is time to reproduce, sexually mature clams will eject firstly sperm, then eggs, into the water where they can mix in the currents with those of nearby clams. This is relatively energetic, as usually the shell and body contract vigorously, expelling the sex cell with some force out of the smaller of the two openings in the clam's fleshy mantle (known as the exhalant siphon). Releasing the sperm first, over the space of several minutes and several ejaculations allows for the currents to disperse them to other clams before it begins releasing its eggs in the same manner. At the same time the gametes are released, the clams also eject a communicative substance known as SIS (Spawning Induced Substance), which elicits spawning in other sexually mature clams that are nearby, sending colonies of clams into a spawning frenzy. There is then a much higher chance of genetic mixing with neighboring clams. It is thought that spawning coincides with incoming tides near the 2^nd, 3^rd, and 4^th quarters of the moon's phases during the spring and summer. Derasa Clams do sometimes spawn in aquaria, either as part of their normal life cycle or if they are particularly stressed. Unfortunately, in the confines of the average aquarium, the release of such a vast amount of gametes can lead to huge problems if they aren't removed promptly from the tank, as they will all start to decompose at the same time, causing a mass die-off that could wreak havoc with water quality and oxygen levels. Due to the fragility of the fertilized eggs, only a minuscule percentage tend to make it through metamorphosis, and those that do often succumb to microbial attacks, polyspermy, predation, damage from filtration equipment, etc. so it is highly unlikely you'll end up with a tank full of baby clams.