ON THE HALF SHELL		BY DANIEL KNOP

Do you have questions, want to discuss the issues raised in this column, or read the comments of other aquarists and the answers from columnist Daniel Knop? You can do so by going to: On the Half Shell Interactive.

What’s in a Shape — Variations in tridacnid shell formation

One of the most common of these “unusual” shell shapes occurs in T. crocea. Normal T. crocea shells are much longer than they are tall, but in this case, the shells look as though they are taller. To be more exact, the height of the shell is normal, but it was unable to increase in length because it was prevented from doing so by the surrounding hard lime substrate into which the clam had lodged itself. This resulted in it being much taller than it was long.

T. crocea is also known as the “boring” clam. It is able to bore a hole into the substrate using acidic secretions to weaken the substrate that are produced in the tissues near the byssal opening. At the same time that secretions are boring down into the substrate, the clam mechanically “shifts” itself from side to side to increase the diameter of the indentation until the hole is big enough to house the entire clam. In a relatively soft substrate, such as live rock or a porous coral skeleton, the clams usually won’t have difficulty increasing the diameter of the indentation so they are able to grow into a natural shape. But, if the hard lime material is able to resist the sideways shifting of the clam, the hole may just get deeper, but not wider. In this situation, the clam adapts to these conditions by altering the shape of its shell to conform to the shape of the space available to it. This results in a “tower-shaped” shell that makes it hard to identify the species by simply looking at empty shells.

DANIEL KNOP

A T. crocea shell with a flat bottom beside a normally developed T. crocea shell. Flattened undersides are a common deformity in farm-raised individuals.

Most clams that exhibit this “deformity” are mature adults and can even be considered to be relatively old. You can verify this by looking at the thickness of the shells — the thickness of the shells increases as the clams get older. The unusual shape does not affect the clam’s health in any way. In fact, these unusual shell dimensions will not even be noticeable as long as the clam is still in its natural habitat imbedded in the rock. It is only when the rock around it is split open revealing the clam’s “cavity” in the substrate that this will be apparent.

Similar variations in shape can be seen in T. maxima. This species normally develops a pronounced asymmetrical shell, leading to the popular name of “elongated clam.” The shell appears to be longer on one side, and this asymmetrical characteristic is one of the main means of distinguishing this species from the relatively similar T. squamosa. Under natural conditions, T. maxima tends to hide its underside between pieces of coral rubble, so just the upper half is visible. (Sometimes in wild-collected T. maxima the lower portion of the shell is clean and white, while the upper half is overgrown with algae and a wide variety of invertebrate organisms, indicating that the “clean” portion of the shell had been buried in the substrate where it did not receive any illumination.) Because the coral rubble does not affect shell growth, the clam develops a natural shell shape.

However, depending on what substrate is available, sometimes T. maxima also buries itself into solid limestone or hard stony coral skeleton. Occasionally, these individuals will develop shells with a pronounced “symmetrical” shape because the shells are prevented from increasing in length by the hard rock material (Lucas 1988). These clams can be easily confused with the similar T. squamosa if you are only looking at the empty shells. You can only distinguish the “unnatural” T. maxima from T. squamosa by examining the dense rows of scutes on the upper part of the shells. In T. squamosa, these rows of scutes are farther apart and not as narrow as in T. maxima. In my book Giant Clams (1996) I have included a picture that shows a normal T. maxima shell underneath an unnaturally formed T. maxima shell that is almost symmetrical.

Variations in shell shape on clam farms

Another type of shell alteration that develops on clam farms is much less obvious. In an ocean nursery located in shallow coastal waters, living conditions are much different than those in the shallow tanks of a land-based hatchery, where the clams normally spend the first six to eight months of their lives. This means that when they are transferred from the hatchery to the nursery, environmental conditions such as water current, temperature and light intensity, among others, change drastically. This has a definite effect on shell growth, leading to althered growth.

The change in location causes a step-like mark to form on the shell, which, in my opinion, allows one to differentiat that portion of the shell that developed in the land tank. In some specimens, this makes it quite easy to distinguish the part of the shell that had once been the outer shell margin before the clam was transferred to the nursery. When viewing it from the bottom, it sometimes looks as though there is a pair of little shells lying on the big shells. The theory that this is caused by relocating the clam from one envirnoment to another is further supported by the fact that the size of the mark is identical to the size of the shells at the time they are usually brought to the ocean nursery.

GIANT CLAM ANATOMY: THE SHELL Most animals have an internal skeleton. Tridacnid clams, as well as all other clams and mussels, have an external skeleton. It consists of two convex, calcareous plates that are connected to each other along the bottom. Four to seven vertical folds that originate on the bottom of the shell run across the entire shell to the upper rim, giving it a more or less undulated shape. Viewed from the top, the pair of shells reminds one of a pair of jaws with teeth. Several species develop sharp scutes on the outer surface of the shell that act as a defense against predators. Interestingly, the scutes are larger in clam species with thinner shells (T. squamosa, "T. rosewateri") and they are reduced in species with relatively thick shells (T. crocea). Also, some species that do not have scutes as adult will have these scutes as juveniles with very thin shells. Those scutes frequently allow the settling of sessile invertebrates of a huge variety of species. Colorful sponges, corals, sea squirts and many other animals can be found here in wild clams. When looking at the inner surface of the shell, the central portion seems to be rather dull, while the distal portion is shiny. The border between those two areas, called the pallial line, is where the syphonal mantle is connected to the shell. The dull area in the center of the shell is covered with the soft tissue of the clam (the lateral mantle), while the shiny, distal portion represents the area where the overhanging syphonal mantle protracts and retracts.

I have seen this phenomenon mostly in T. gigas and T. derasa, but also in other species. Again, this is not a sign of disease. However, in cases of heavy infestation by boring algae or boring predators, such as sponges that develop inside the clam tissue, it sometimes allows us to determine whether the clam was infected during the land-based hatchery phase or the nursery phase. I have seen many tridacnid shells that were heavily infested by boring organisms in those portions of the shells that developed in the ocean, while the portion that developed in the land-based phase were absolutely free of these predators.

Shell deformities of “unknown” origin

An explanation for this that has been offered by some marine biologists is simple: These deformities develop at a very early stage in the development of the clam when the clam is not yet visible due to its small size and the shells are still soft. Pressure of some sort is put on the soft shell, causing damage. As they grow, the clams are unable to compensate for this damage, so the shells continue to grow in this deformed shape.

At one time I accepted this explanation. This changed, however, when I looked at the undersides of the deformed shells. Near the hinge of the shells I was able to detect what had once been the outer shell margin when the clam was still in a land-based hatchery tank, and this area exhibited no deformities. Had the deformity developed at a very early stage, this — the oldest part of the shell — would also be deformed in some way. Instead, the shell developed normally until, I believe, there was a drastic change in environmental conditions — the transfer to the ocean nursery. The portion of the shell situated next to this “step-like” mark (which in my opinion developed directly after relocation to the ocean nursery) is the area where the deformity is first noticeable. Except for this flaw, the clams seem to be healthy, they grow to normal sizes and all organ systems seem to function normally.

Foreign bodies

This has several advantages. First, the shells become stronger, which helps to withstand predation. Second, they get heavier, which protects them against strong water turbulence. As this coating is produced it also seals any holes drilled by predators, and even encases anything (such as a predator) that is still inside the shell. In many cases there is a thickening of the inner shell surface at the location where a predator has entered.

Chambers inside the shells

My guess is that this chambering was caused by an accumulation of gas inside the shell of the clam. Let me hypothesize a bit in order to find an explanation for this very unusual chambering in giant clams. Because there was a hole in the area of this chambering proves that a predator at least tried to enter. It may have been a creature without calcareous parts (such as a snail shell) — for example, a worm. This worm could have entered the shell by the hole with the help of acidic secretions, as several worms do, and then may have been unable to leave after its meal and died inside the clam. As stated above, clams usually seal these holes by producing new material to thicken their shell, and, in fact, this was the case here. However, the decomposition of the dead worm might have caused gas to be given off, which then accumulated between the inner shell surface and the lateral mantle, dislodging the lateral mantle from the shell. The lateral mantle would now be in the position it was here, producing a calcareous chamber membrane. Of course, this is just a hypothesis.

DANIEL KNOP

Bottom view of a T. derasa shell showing the step-like mark that, in my opinion, allows us to identify the portion of the shell that developed in the land-based tank phase of the clam’s development.

Unequal curving of the shells

In my experience, most of the tridacnid clams that exhibit this unnatural growth are T. maxima species. While other species might be able to develop this same pattern, they live in different habitats that seldom requires this type of adaptation. T. gigas, for example, prefers sandy bottoms in sea grass beds, and T. crocea bores into limestone. When situating a clam with this type of deformity in the aquarium, it is advisable to place it in a similar sideways position, because it has already adapted to this position and might not be able re-adapt to a completely upright position.

DANIEL KNOP

A T. gigas with a severe deformity of its shells.

Reefer’s Questions

Q. I just noticed that there is a very small shrimp living inside my T. maxima. The shrimp is only ¼ inch long, almost transparent, but with some black dots on it. I can see it (through the incurrent syphon) “walking” inside the giant clam! The clam is fine, but I’m really worried about the other three clams in the same tank. If the shrimp is a pest, I will remove the clam immediately so the others won’t be infested. Can you give me some advice?
Thanks. N. N.

A Don’t worry about the shrimp in your clam. It’s probably just a commensal that does not disturb the clam at all. Many people would love to have those commensal shrimps or crabs in their clams, including me.

Tridacnids have spawned!

Q. I have heard from one of the CompuServe moderators, in a personal conversation we had one day, that one of the members of his marine society has successfully spawned tridacnids in his home tank under VHOs. Is that possible? We were talking lighting, so didn’t go much into it, but next time I talk with him I will ask about it.
Brian

A. He has spawned the clams, but I am pretty sure he was not able to raise the clams from this event. Spawning frequently happens, but the problem is raising the larvae. Once the egg cells of the clams meet the sperm in excessivea concentrations (which you cannot avoid in a closed tank system), development of the larvae is blocked. Breeding them requires several special containers and a controlled spawning.

DANIEL KNOP

A T. gigas with a distinct chamber in its shell. This chamber had previously been sealed shut, but it broke open during rough handling.

Spawning after a water change

Q. One of our customers reported that after a partial water exchange in his reef tank, one tridacnid clam started to secrete a whitish substance and another tridacnid clam followed. Now, the clam that started the secretion looks bad, although it is still alive. Can you give any advice? What happened? What has triggered the event and what can the customer do? He is in a hurry to get the answer.
K. B., Reef Aquarium Supply Wholesaler, Germany

A. Your customer has created an environmental change by doing the partial water exchange. Many animals are sensitive to this type of a sudden change in their environment, possibly because they are not going to be able to cope with what could follow. They might react by releasing gametes. In tridacnid clams, this usually leads to the release of sperm cells, but if the clam has fertile egg cells, they may also be released into the water in an effort to let them be fertilized and carried away by the water currents to different locations in the reef where they will have a chance to survive and settle.

Your customer’s clam has very probably released sperm cells. Tridacnid clams are simultaneous hermaphrodites, which means they contain both sexes and can produce sperm as well as egg cells when they are adults. Sperm release occurs much more frequently because even semi-adult tridacnid clams can do this — egg release is only possible in mature adult specimens. Also, the sperm release always occurs first before egg cells are released, which further supports the assumption that your customers’ clam has released sperm sells. During sperm release, a hormone-like substance is released to “inform” other tridacnid clams about the gametes in the water, in an effort to encourage them to join in and do the same so egg cells can be fertilized and turn into larvae. This explains why the second clam joined the spawning event.

If the clam was just irritated by the environmental changes caused by the partial water exchange, it should be feeling well again soon. In some cases, however, a tridacnid clam in an aquarium may be ailing without this being noticed by the aquarist, and before dying, it will expel sperm or egg cells, trying to hand its genetic material to the next generation and let the species survive. This can also be triggerd by the partial exchange of water. In this case, the clam did not die because of the sperm release, but just after it.

In other cases, tridacnid clams have killed themselves by releasing gametes in the aquarium, especially sperm cells, which die relatively rapidly and consume a huge amount of oxygen. Something similar was observed after the gamete release of stony corals in a reef tank of an aquarist in Norway, which was reported by Alf Nilsen a while ago. However if this were the case in your customer’s tank, both the clams should show more or less the same degree of damage.

I would advise your customer to examine the clam and remove it from the tank if it does not react to touch or or strong irritation of the syphonal mantle. The tank seems to be stressed due to the gamete release, and it is questionable if the other tank inhabitants would tolerate the further water pollution from dissolving tissues of a dying clam. Also, the water should be cleaned by a strong foam fractionator, and the use of granular activated carbon is recommended.