Mollusks are a huge phylum of invertebrates that comprises more than 85,000 species (according to some classifications, there are around 120,000 mollusk species distributed all around the world) and seven classes, including large and very common groups such as gastropoda (snails and slugs) or cephalopoda (octopuses, squids, cuttlefish and nautiluses). Though some mollusk species live on land and there are some that dwell in freshwater environments, most of them prefer marine habitats, making up almost one-quarter of all marine life forms.
Mollusk species can be found anywhere in the world. Most marine species prefer shallow waters such as sandy areas close to shores or coral reefs, but there are some species that live only in the deep waters, while terrestrial and freshwater species can be found everywhere from habitats located at sea level and up to the rocks and the waters in the highest mountains.
Mollusk species are so diversified that it is very difficult to find physical features shared by all of them. The task of making generalizations is actually so problematic that many books on the topic provide an image of a “generalized mollusk”, describing the most common features. This generalized mollusk would have:
a bilaterally symmetrical body that carries a shell on the top;
a mantle that accommodates the vital organs such as the gills, the excretory pores and the anus; it is also responsible for excreting the animal’s protective exterior shell, creating it from the materials available in the habitat;
a strong and muscular foot and soft, non-muscular visceral mass inside the shell.
open circulatory system and a pair of gills (some species have only one gill).
Mollusk species are varied in terms of their propulsion method as well. Some of them, such as snails, crawl very slowly, while others, such as squids, are able to swim faster than most fish, often reaching the speed of 25 miles an hour while moving from one place to the other.
Being extremely adaptable creatures, mollusk species adjust their feeding habits to the environment they live in. Some of them, especially terrestrial species, but some marine species as well, are herbivores preferring larger plants and kelp when in a marine environment, while others are omnivores or exclusively carnivores. Some species feed by means of grazing, eating the parasites they find on the body of other animals. They all make use of the nutrients ingested by means of intracellular digestion, a method that involves the breakdown of food at cellular level, inside the cytoplasm of the cell. They also have a radula, a rough-surfaced, tongue-like organ they use for fragmenting food.
How Do Mollusks Proliferate?
All mollusk species have complex reproductive organs. Most of them reproduce by means of external fertilization, but there are some species that rely on internal fertilization for proliferation and are known to be hermaphrodites. The females of all mollusk species release eggs that transform into larvae, but there are exceptions even from this rule: when the eggs of cephalopods hatch, small adult individuals emerge rather than larvae. Most species of mollusks are relatively long-lived, with life spans of around 15 years or more.