MOLLUSK

Mollusc-info.

- Check the sub-folders for the specimen information files -

Some might find it strange to learn that clamshells, octopus and snails are related. All of them are members of the phylum Mollusca and are divided into three groups, the bivalve (two-shelled) such as file shells or oysters, the univalve (single shelled) like snails and the cephalopod (head-footed) like octopus and squid. All mollusks have true organs, including a heart, stomach, intestines, kidneys and gonads. The bivalves are named for their two shells which are connected by a hinge. They feed by filtering small particles from the water. Most of the bivalves live on or underneath the substrate, which is why we only really see them on night dives. They also don't have to fear predators because of their nocturnal lifestyle and their double shell. Some bivalves, like the clam shell, are active during the day, even though their only protection is their ability to close themselves. This is how they protect themselves from intruders. However giant clamshells have been and still are very popular in many countries for their meat and for their large shell. The univalve (single-shelled) is the largest subdivision of the mollusk group. Most single-shelled mollusks have a foot upon which they crawl or glide. Their eyes are not particularly developed, but can detect light intensity and they have a coiled shell that protects their internal organs. Unlike filter-feeding bivalves, gastropods are algae-grazers or carnivores. Apart from a few gastropods that are used as food, it is their beautiful shells which have always interested by people. In fact, there are places in the Indo-Pacific region where they used to use their shells as money. Another type of mollusk is the worm snail or tube mollusk. The tube-like ends of their shells are visible, but apart from this, they look very much like any other gastropod mollusk. However they can't move because their shell is fixed firmly in place between or in corals. The gastropod feeds by spreading out a mucus layer into the surrounding water to trap tiny organisms. They then deliver these to their mouths by retracting the mucus. A shell-less mollusks that is not a cephalopod is known as a nudibranch, although technically there are several defined groups. More information on nudibranchs can be found in the nudibranch section. Cephalopods are almost certainly the most fascinating and intelligent of all mollusks. Like most other mollusk, cephalopods have a radula (a cover for the internal organs); a foot that has developed into arms which are also based around the mouth. Their eyes are well developed and are very much like the human eye. Cephalopods are the only mollusk that can change color, and the only animal that can control its color changes by muscular action. Squid suck in water around their mantles and eject it forcefully through their funnels to aid movement. In fact this jet propulsion system makes squid the best swimmers of the cephalopods. They can easily move by changing the direction of the funnel and fin-tune with the fin they have on either side. Squids have eight arms and a pair of longer, extendable tentacles, which they use for catching prey. Reef squid spend much of their time hovering in mid-water or close to the surface. Others like the giant squid, which can grow up to eighteen meters long, live at depths greater than three hundred meters. The octopus also has eight arms like squid, but no fins. Most of the time they walk along the sea bed but are also able (if necessary) to swim quickly by using jet-propulsion. They tend to live around their den where they rest, clean and feed. These dens are recognizable because of the amount of mollusk- and crab shells which cover their ‘front yards’. Many species live on and around rocky or coral reefs where there are many places to hide. Others live in sand or mud, but need to bury themselves to hide from predators. Most octopus are active at night, and only some will be seen going out during the day to feed. They are masters in camouflage and are able to match their surrounding in a second. This ability to change colors is also used to warn off predators or to mimic poisonous marine creatures. All octopus have some special (salivary) glands which contain a strong neurotoxin. This toxin is used to immobilize fish. The blue ringed octopus or Hapalochlaena lunulata also have a bacteria inside which produces tetrodotoxin; a toxin which even immobilizes humans. There are octopus that weigh less than one gram, although there are some like the cold water living giant pacific octopus that can weigh more than one hundred kg. These can have an arm-span of more than four meters. During mating, the males bring their modified third arm into the female and insert sperm. The female will store the sperm till she is ready to lay her eggs. All female benthic octopus takes care of the eggs, and then sadly dies around the time the young will hatch.

- Check the sub-folders for the specimen information files -