Snails, Clams, Squid, and Relatives (Phylum Mollusca)
With over 100,000 species having been described, and with untold numbers likely as yet unseen by scientists, the mollusks are among the earth's most successful creatures. Inhabiting freshwater, marine, and terrestrial environments, many mollusk species are commercially important as food animals and are also much in demand as aquarium subjects.
The mollusks form an unlikely group of animals, whose members seem, in many cases, to possess no similarities to each other whatsoever. They are, however, united in the possession of a shell comprised mainly of conchiolin and calcium carbonate, although this has been lost in the octopus and the squid. This phylum numbers among its members creatures that are vastly dissimilar, from the basically sedentary clam to the highly intelligent and active octopus.
The irregularly spaced lines that occur on the shells of mollusks are actually growth rings. Much like the more familiar growth rings on trees, these markings can, in some cases, be used to age the animal. This is especially true for temperate species, most of which appear to create one ring per year.
Individual mollusk species may reproduce utilizing separate sexes or may be hermaphroditic. Most simply release eggs and sperm into the water, but squid and octopus practice internal fertilization. Many species begin life as planktonic larvae and are important components of the food chains in which they occur. Marine larvae may drift for hundreds of miles before settling down and maturing into the adult form.
Although people may tend to think of snails and slugs as somewhat simple, inactive creatures, the over 70,000 members of this group actually follow quite a diverse number of lifestyles. Individual species may be scavengers, carnivores, or herbivores. Some are surprisingly active predators. Marine snails known as whelks feed upon bivalves (two-shelled creatures such as clams) by prying open their shells or by boring holes through them. Other sea-dwelling snails known as cone shells impale their prey on retractable tongues, or radula. The venom thus injected paralyzes their victims and can be fatal to humans.
Sea slugs, or nudibranchs, are brilliantly colored, free-swimming animals that appear, unfortunately, to be difficult to maintain for any length of time in captivity. All species of this group dwell in saltwater and seem to be fairly specialized feeders upon sea anemones, sponges, and corals (a fact that does little to simplify their care in captivity). Those species that feed upon sea anemones incorporate the anemones' stinging cells, or nematocysts, into their own gill tufts as a form of protection.
Numbering over 6,000 species, the bivalves form the second-largest group of mollusks after the gastropods. Many are extremely important commercially as human food items, and a number are adaptable to life in the aquarium.
Bivalves are unique in that they have gills that function not only as organs of respiration but also as a means of acquiring food. All species studied thus far have been found to be filter feeders, most of which trap plankton in the sticky mucus that covers the gills.
At weights in excess of 220 pounds the giant clam (
Shipworms are, despite their appearance, not worms at all. These commercially important pests in the dock-building and shipping industry are actually bivalves that have retained only a vestige of shell. The animal pushes this shell through water-logged wood, boring a hole as it moves along and feeding on the resulting wood shavings.
Although basically sedentary creatures, bivalves in the aquarium often surprise their caretakers with their degree of movement. Particularly unusual are several species of scallop, which can rapidly open and close their shells, expelling water in the process and propelling themselves quickly.
The squids and their relatives comprise a group of nearly 700 species of extremely active and intelligent invertebrates. Their lifestyles and appearances render them distinctly unlike the rest of their mollusk relatives. The shell is much reduced or completely absent, with only the chambered nautilus retaining what appears to be a full shell. Even in this species, however, the animal lives in a small portion of the shell, with the rest being devoted to gas-filled chambers that control the animal's buoyancy. In the cuttlefish and squid, the shell has been internalized. The familiar cuttlebone that pet keepers feed to birds is actually the remnants of the cuttlefish's shell.
The eight tentacles of the various cephalopods are extremely effective organs of touch and taste. Octopus and squid are equipped with sharp, parrotlike beaks, which they use to help overcome their prey. All cephalopods are predators, feeding upon crabs, fish, shrimp, and other marine creatures. They move by crawling on the substrate or by actively swimming using the force of water expelled from the mantle. Several species of squid are actually able to soar above the surface of the ocean using this form of jet propulsion.
The giant squid is the largest known mollusk, and the largest invertebrate. Growing to a length of at least 66 feet, this deep-sea predator has been the subject of legend for centuries. Formerly known only from beached corpses, it was not until the year 2005 that Japanese scientists finally captured a living giant squid on film.
Those cephalopods that have been studied have been found to be extremely intelligent and capable of learning simple tasks. Octopus seem especially quick to make various associations and to remember those that result in a food reward. And, as anyone who has kept these fascinating creatures can attest, their ability to escape seemingly secure enclosures seems to evince a degree of mental acuity. Octopuses are capable of rapid and quite startling color changes, which they use as a form of communication, camouflage, and defense. At least one species of octopus changes the actual form of its body to mimic creatures as diverse as sea snakes and flounders, in an effort to escape or threaten predators.
Many cephalopods are equipped with an inklike substance known as sepia, which can be released as a “smokescreen,” apparently to divert the attention of would-be predators so that the animal can escape unnoticed.
Cephalopods reproduce by internal fertilization. The males of most octopus species insert a sperm packet into the mantle of the female. In what appears to be a quite unique and unusual fertilization technique, male squid of certain species seem somehow to thrust a sharply pointed sperm packet deep into the arms of the females. Female octopuses guard their eggs and often die shortly after they hatch.
Until recently, it was believed that female squid of all species released their eggs into the ocean without further parental involvement. Photographs taken in the year 2005, however, revealed a species of squid that carries its eggs wrapped in the arms and actively aerates them as well. Amazingly, this shallow-water species descends to great depths during the eggbrooding process.