In the wild, small tridacnid clams are heavily preyed upon. Many species of fish (triggerfish, large wrasses, puffers, etc.), crabs, lobsters, shrimp, polychaetes (Bristleworms, Fireworms, etc.), octopi, and snails prey on clams. Even burrowing sponges! Since most of the clams available to aquarists are juveniles, hobbyists should be extra cautious about both predators and environmental factors that can wreak havoc with a clam.
Fish Predators:
Certain wrasse species (Family Labridae) are bad tank mates. Species like the Twin Spot Wrasse Coris aygula and Bird Wrasse Gomphosus varius have been known to attack and devour juvenile clams in the aquarium. Sometimes the clam is eaten from above or knocked over and eaten through the soft and unprotected byssal opening. Any large wrasse species should be watched closely when introduced into a tank with giant clams.
Other fish can also irritate clams. Fish that are constantly grazing like as Centropyge sp. (Pygmy Angelfish), Ctenochaetus sp. (Chevron Tang, etc.) and Acanthurus sp. (Achilles Tang, etc.) Tangs, and Ecsenius sp. (Blennies) will occasionally nip at a clam in passing.
Sometimes small pieces of tissue are removed but the problem is that the clam is now irritated and it will expand less and less. It may eventually expand so little that it won't receive enough light and will slowly die.
Crab Predators:
Large crabs will eat clams and usually shortly after they are placed in the tank, before they can attach to the substrate. They attack the clam through the byssal opening, but larger crabs just crack the shells open.
Shrimp Predators:
Certain species of shrimp can also prey on clams. Large shrimp such as Marble Shrimp Saron marmoratus and Buffalo Shrimp Saron sp. will attack clams at night.
The common Cleaner Shrimp Lysmata amboinensis has been known to attack clams. It's rare, but it does happen, especially when the clam is injured and the shrimp hungry.
Snail Predators:
Parasitic snails are sometimes imported with tridacnid clams as mentioned earlier. Examine new clams closely and remove any snails or eggs right away.
Environmental factors:
A number of environmental factors can also irritate giant clams, creating potential problems:
Algae:
Algae is another problem for clams. If algae begins to grow over the lip of the shell, the mantle may become irritated and it will not expand as much.
Macroalgae:
Macroalgae like Caulerpa can irritate the clam from underneath if allowed to grow under the byssal opening. If this happens, the clam will produce large amounts of mucus from below that surrounds the base.
Clam Mucus:
Producing large amounts of mucus is a normal means of protection for the clam against algae, stinging corals, or predators. The mucus is thick, clear, and often contains brown patches (brown jelly). The effect of noxious by-products of soft corals (example: Xenia spp.) can also cause clams to produce large amounts of clear mucus. The mucus can quickly clog prefilters
Aiptasia anemones:
If Aiptasia are allowed to grow on tridacnid clams, they can reach underneath the mantle and sting the clam. This will result in the mantle pulling away and the clam will eventually die.
Clam worms:
Polychaete worms such as the larger Nereis sp. and Eunice sp. can prey upon tridacnids. They are usually active at night and feed on the clam from below, through the byssal opening or by boring a hole through the shell.
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