Snails, Chitons and Clams

Chitons are sluggish, slow-moving creatures that crawl about on a large foot, much like a snail. Across its upper surface are eight calcareous plates, that articulate and overlap, forming a perfect protective shell if needed. The normal appearance of the chiton, uncurled, looks like the tail of a lobster, but when it curls up it has a completely different appearance, like a cradle, from which its nickname was derived.

Chitons have a rasping tongue which is used for scraping algae off rocks. The chiton will scrape an area clear and then move off to greener pastures. They browse at high tide and "hang on" when the tide drops, conserving water until it comes back.

One must strike fast collecting these animals. If you don't get them on the first try they can clamp down on the rock tightly. Once that happens, they usually cannot be budged without damaging them.

Anyone who has walked along the beach has stopped to pick up and admire marine snails. A live snail can be recognized by the flat, tight-fitting "door" or operculum with which it blocks entry into a shell. This is its principal means of protection, not only from predators but from the danger of drying out. When tightly closed a small amount of water is kept inside to keep the gills moist in order to be able to get oxygen from the water.

ChitonChiton
source: bethel.edu

Most snails feed with the aid of a rasping organ called the radula. It is used to scrape the fine algae off the surface of rocks. The "track" of a snail can easily be seen and its path for the past several hours indicated by the bare rock it has left behind. As the radula wears out new rasping teeth replace the worn ones.

Among the more prized snails are cowries. These have smooth, colorful, oval shells which do not outwardly show the spiral structure common to all snails. A fleshy membrane, the mantle, covers the shell keeping it smooth and repairing any damage that occurs.

The hunters of the snail group are the cone shells. They hunt and capture other marine creatures including fish. Poison darts are jabbed into a victim by muscular action and the prey is engulfed by the muscular foot and feeding takes place at will. The poison from this snail can be dangerous to man - handle with care.

Tiger Cowry Cyprea tigriTiger Cowry Cyprea tigri
source: uhhmop.hawaii.edu/ Giant land snailGiant Land Snail

Reproduction in snails is well known. Some, like the top snail, release their eggs freely into water. After a short pelagic life they metamorphose into recognizable snails, some drop to the bottom, and begin crawling. Most snails lay eggs enclosed in protective capsules or jelly-like masses. These are often found lying on the bottom around the reef or grass flats. If far enough along the development the miniature snails can easily be seen when the eggs capsule is held up against light.

The snails will probably make short work of the algae on the aquarium glass. They are, however, wanderers and if the tank is not covered they often disappear. It is also difficult to determine if they are dead and they can quickly pollute an aquarium.



Among the mollusks probably the most sought after animals are the nudibranchs, snails without shells and as the name implies with bare gills. They are sometimes called marine slugs and are often quite beautiful. The exposed gills add to their charm, being numerous and frilly giving the appearance of delicate feathers. Fortunately for them they have a rather bad taste, an effective defense against a hungry fish. The nidubranchs are also provided with a radula and are therefore browsers.

Clams, oysters, mussels, etc. are related to snails but instead of having a single coiled shell, they have a pair of hinged shells. Their dreaded enemy is the starfish. These animals are usually not kept in the home aquarium because, as with snails, it is difficult to determine when they die and fouling of the tank can quickly occur. A closed clam shell gives no indication of whether it is alive or dead. However, there are attractive clams and some are well worth the risk. Scallops are not attached and have the ability to "fly" through the water by clapping the shells together in combination with a sort of propulsion. A group of scallops quietly resting on the bottom of the tank will immediately "take off" the instant the starfish is placed near them.

Another favorite is the dwarf relative of the giant clam. Its fluted or scalloped shell is very attractive and the mantles come in a variety of colors. Clams in general feed by pumping water into the shell, through a sieve-like structure, and out the other end. The edible particles caught in the sieve are moved by cilia to the mouth and ingested while the inedible materisl are moved also by cilia to the outside and discarded.

A living filter, Tridacna gigas rows very large, weighing up to a ton or more. The mantle at the edge of the clam is brightly colored.


 

 

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