What Do Mollusks Eat? It's Not What You Expected

The term Mollusca refers to a large phylum of invertebrate animals, known as mollusks. Being the largest marine phylum, it is represented by more than 85,000 species.

These soft-bodied invertebrates vary in size, but most have a protective covering known as a shell. Although most species live underwater, some also have a terrestrial habitat. Given the huge number of species, the phylum Mollusca is divided into 3 main groups: cephalopods, bivalves, and gastropods. Here are the dietary habits of each group.

Cephalopods

Of all mollusk species, cephalopods are the largest in size, and essentially carnivorous. They are known for their unique hunting style. This group is primarily represented by cuttlefish, squid, octopus, and nautilus. They are equipped with long tentacles used for catching prey.

Because cephalopods must hunt to get their food, they are highly mobile and known for their voracious appetite. Their diet includes crabs, crayfish, and small mollusk species such as gastropods. Cephalopods also prey on shellfish, crustaceans, myctophids, shrimp, and smaller fish such as sardines and pilchard. Some species, such as small squid, feed on plankton, but the larger ones are carnivores.

Bivalves

Also called pelycopoda, bivalves are sessile, meaning they cannot move because they are fixed to their substratum. Oysters, clams, scallops, and mussels belong to the bivalve class. What exactly do bivalves eat? Although different species exist, most are filter feeders that live primarily on phytoplankton and algae, organisms that float freely through the water.

Bivalves have a unique filter-feeding system that separates food particles from water. Their gills are lined with cilia (tiny hairs) that capture plankton particles as small as 1 micrometer. The collected plankton is then directed toward the mouth for digestion.

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Gastropods

The gastropods class contains a wide range of species and is considered the largest class of mollusks. The group is diverse, including both land-dwelling and aquatic species. Land species such as snails and slugs generally have a uniquely coiled shell. Important aquatic members include tusk shells, sea butterflies, conchs, periwinkles, limpets, whelks, nudibranchs, oyster borers, and abalones.

Frequently Asked Questions

What do mollusks eat?

It depends on the group. Cephalopods (octopus, squid) are mostly carnivores; bivalves (oysters, clams) filter-feed on plankton and algae; gastropods (snails, slugs) eat plants, algae, or other animals depending on species.

Are bivalves herbivores?

Most bivalves are filter feeders that primarily consume phytoplankton and algae floating in the water.

How many species of mollusks are there?

The phylum Mollusca is represented by more than 85,000 species, making it the largest marine phylum.

The aquatic habitat of these species can be freshwater, marine, or deep sea environments. Here are the dietary habits of some gastropod species:

• Snails and slugs feed on various types of leafy plants and rotting vegetation.
• Oyster borers (aquatic snails) prey on shelled creatures. They attack by drilling a hole through the shell and eating the soft body parts inside.
• Conchs, which are large sea snails, feed on living seaweed, algae, and organic debris.
• Limpets, soft-bodied invertebrates, graze on plants, usually algae that grows on the surfaces of rocks.
• Whelks (a type of sea snail) are meat-eaters that prey on worms and bivalves such as clams, oysters, and mussels. These marine gastropod mollusks are experts at opening the protective shells of bivalves, then use their toothed tongue to grab the prey.
• Periwinkles (small marine snails) are plant-eaters found on rocky coastlines. They eat algae scraped from rocks.
• Nudibranchs, soft-bodied sea-slugs, are carnivorous and prey on corals, sponges, crustaceans, jellyfish, hydroids, and even members of their own species.

A Note on Flexibility

An important point to note here is that, although cephalopods are carnivorous by nature, this doesn't mean they will never eat plants. For instance, if an octopus cannot catch sufficient prey, it may eat aquatic plants. The availability of food plays a crucial role in deciding dietary habits at any given moment.

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