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Mollusks, or Mollusca, are a large phylum of invertebrate animals that includes clams and oysters. Scientists recognize more than 90,000 species of mollusks, and this is the largest phylum of marine animals, with almost 1/4 of all named marine organisms.

Although they originated in the seas more than 500 million years ago, some have evolved adaptations to live in brackish and freshwater. Today, a large number of mollusk species, including freshwater mussels and snails, live in freshwater and terrestrial habitats.

Diversity in size, shape, color, habitat, and even behavior is typical of the Mollusca group, which is normally divided into 8 extant taxonomic classes. These classes include gastropods, the largest group, with more than 65,000 species of snails, slugs, conchs, and relatives, and the bivalves, with about 20,000 species of clams, oysters, scallops, and mussels.
The most complex class within the mollusk group is the cephalopod mollusks, which includes octopus, squid, and cuttlefish, some of the most cognitively advanced of all invertebrate species. The giant squid is the most massive of all known invertebrate species, at 18 meters long and weighing almost 900 kilograms.
These marine mollusks have diversity in movement as well; octopi and squid move quickly via jet propulsion. Even scallops can “clap” themselves away from danger, or they can remain glued to a rock in the tidal zone, like oysters or mussels do for most of their lives.
The bivalves have so much morphological diversity that it is most useful to define them by the features that unite them. All bivalves have a hinged outer shell (also called a valve) and a mantle, and most have a foot or threads used for burrowing or anchoring the creature into the substrate.
Clams, oysters, and other bivalves breathe solely through gills that are part of the animal’s mantle. The gills are so different in different types of bivalves that gill morphology is a major indicator for bivalve systematics.

Reproduction in bivalve creatures is bisexual. For fertilization to occur, it is crucial that as many oysters as possible spawn at the same time. Spawning is cued by water temperature and salinity in midsummer. During an oyster’s first spawning period, the creature will be reproductively male, and then it will transform to female for subsequent spawning cycles.
Some types of bivalves, notably mussels, have a byssal gland that produces a byssal thread to attach them to a rocky surface. It is sometimes easy to observe groupings of these proteinaceous threads extending from one mussel to another in a mussel bed. clams, including surf clams, clams, are burrowers that lack a byssal gland and develop a specialized foot as they grow. This wedge-shaped muscle can be expanded and contracted so that the clam can burrow into soft sand.
The biomass, or the weight of individuals in an area, of these creatures is as important from an ecological point of view as is the harvest from an economic point of view.
Much of the American oysters’ historic beds disappeared over the last 200 years due to construction of cities and suburbs along seacoasts. However, some good habitat remains, and here the oysters provide substrate for marine creatures such as barnacles and worms.
A high density of oysters over a high percentage of their historic range within an estuary and with a growing population is a good sign for the ecosystem as well as the economy.