Mollusca: a large group of soft friends!

Mollusca is a major phylum with a large number of species including snails, clams, oysters, squid and octopus. Molluscs have successfully adapted to fresh and salt water as well as the terrestrial environment. Of the 110000 existing species of molluscs, about the half are marine species.

The phylum is represented by a very high diversity in body forms. The general characteristics are the presence of an external calcium carbonate shell, a prominent foot, a feeding structure called radula and a mantle cavity. Many species don’t present some of the mentioned structures.

The calcium carbonate shell is secreted by the mantle on the dorsal surface of the body. It is secreted in a series of layers: the external or periostracum, is made by a horny protein called conchiolin, the medium one, called prismatic layer, contains calcium carbonate deposited in prism shape and the inner one, called nacreous or mother of pearl is smooth and lustrous.

The inner layer produces pearls when irritant bodies entered in the shell with the aim to include and to neutralize them. The shell grows with the mollusc body and the animal does not need to change it.

The foot in molluscs is normally well developed and assures the locomotion by muscle contraction and movement of the hydrostatic skeleton. In some species, like in limpets, the foot acts like an adhesive gland, working like a suction cup.

The radula is a sort of tongue, armed with a series of rows of small, chitinous teeth, which can scrape food from the substrate.

The mantle cavity has a number of important functions as to hold the mollusc gills and the pores from the digestive, uric and reproductive systems. The mentioned apparatus release their products in the mantle cavity before the same are released in the environment.

Mollusca are divided in 7 Classes, 5 of them exclusively marine. Only the 2 classes of Gastropoda and Bivalvia count together the 95% of the total number of molluscs.

Let’s see now all the classes, one by one, giving more focus on that ones which are possible to observe during diving or snorkelling.

Monoplacophora is a small group of primitive molluscs, living only at great depth on sea rocky bottoms.

Aplacophora is a small group as well of worm-shaped molluscs without a shell, living only at very high depth, too.

Polyplacophora (chitons) is an ancient class of mollusca of about 600 species found commonly in shallow rocky bottoms. They are very simple with a massive flat food, and a flat body covered by a shell divided in 8 overlapping plates, which permit to the animal to enrol in a ball in case of danger. Most of the chitons are herbivorous with a well developed radula that they use to scrape algae and diatoms.

Gastropoda (snails, slugs and limpets) is a very large group of mollusca (75000 species) which have successfully colonized marine, fresh water and terrestrial environment. The great majority are benthic animals of shallow waters with a shell of a conical spire. Exceptions are represented by the limpets, which present a cone-shape shell and by the nudibranchs, which have lost all traces of a shell and they instead show a beautiful variety of shapes and colours. There are both herbivorous species and carnivorous ones, which use the radula to drill holes through the prey body to eat it.

Pelycepoda or Bivalvia (clams, oyster and mussels) is a large group of molluscs (30000 species) that live buried in the sandy or muddy sediments. Some species are attached permanently to the solid substrate. Their shell is formed by two valves joined together by a hinge ligament that open the valves, while an adductor muscle close them. Most bivalves are filter feeders and use the mucus of the gill to trap small particles of food (often phytoplankton and zooplankton).

Scaphopoda is a small group of 350 marine species with a white shell presenting the shape of an elephant tooth. They live inside the sandy and muddy bottoms, feeding on microscopic organisms of the sediments. They are difficult to see during diving.

Cephalopoda (squid, cuttlefish, octopus and Nautilus) is the most developed class of molluscs, counting about 600 of marine species. Their brain and sense organs are extremely developed and give them the possibility of learning and memory. Except Nautilus, all cephalopods have lost the external calcium carbonate shell, which in squid and cuttlefish is substituted by an internal skeleton. They are large, active carnivorous animals where the foot has been modified in a set of tentacles and a siphon for feeding, reproduction and locomotion. They use to feed actively during the night, and squid are used to live a high depth.

... it continues!...

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