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Interesting Facts about Mollusks

[Kingdom: Animalia] [Subkingdom: Eumetazoa] [Superphylum: Lophotrochozoa] [Phylum: Mollusca]

Biological resources suggest 85,000 types of marine molluscs exist in the world today. Nonetheless, the recognised number of living mollusk species excludes another 100,000 fossil species.

This section contains a comprehensive list of mollusk species (the largest marine phylum), with fun and interesting facts about the behavioural patterns of mollusks in the ocean.

What is a Marine Mollusk (Phylum Mollusca)

Oceanic invertebrates with a soft body and a hard carbonate shell are most likely marine mollusks (British spelling 'molluscs').

This diverse clade of invertebrate species accounts for around 23% of all known organisms living in the seas and oceans worldwide.

In fact, if we exclude the phylum Arthropoda, mollusca (Latin word for 'soft') make up the largest phylum of extant (surviving) animals.

As a consequence of that, the species differs in ecology, habitats, diet, and especially in their body structure.

Simply put, marine mollusks make up an enormous group of gastropod invertebrates that includes various species of sea snail, sea slug (nudibranchia), mussels, nautiloids, oysters, clam, squid, and octopodes.

List of Mollusks and Taxonomy

In reality, the phylum mollusca contains two entirely extinct members (Ammonites and Belemnites). However, the taxonomic classes that make up the major clades of living soft-bodied organisms, include:

The majority of mollusk specimens live in the oceans and have a protective hard shell usually as a single unit and not jointed exoskeletons like crustaceans have. They broadly exhibit an open circulatory system.

Examples of Molluscs Invertebrates

Bivalve Molluscs

Most of the 30,000 bivalve marine mollusc species thrive in saltwater and freshwater environments. The headless bivalvia is a mollusc with a hinged shell.

Nonetheless, some of the typical molluscan organs are absent, including the odontophore and several other radula structures.

Instead, they rely on a muscular, wedge-shaped, ventral foot to carry out most of their bodily functions. The twin shells can also lock closed as a defence mechanism, as in giant clams (Tridacna gigas) for example.

Cephalopod Mollusks

In fact, squids and octopuses don't have an outer shell and they only exist in saltwater biomes. In general, these marine predators are bigger and faster than other species in this mollusks list.

The planispiral shell of the nautilus means they are the only cephalopods that have a robust external casing. The biggest mollusc is the giant squid (up to 13 metres long), Whereas, most ocean snails are good examples of small mollusks (micromollusk).

They feed on other mollusks, crustaceans, and small fishes. More than 800 different Cephalopods display bilateral mollusk symmetry.

Gastropod Mollusks

Studies suggest there are around 60,000 mollusk snail and slug species living in various seawater and freshwater habitats around the world.

Snails also exist in terrestrial environments. Snail mollusks are characterised by their single spiralling shell (e.g. pelagic sea butterflies). Whereas, the vast majority of snails without shells are slugs.

Fun Fact: Living gastropods (marine snails, cowries, and sea slugs) account for around 80% of all ocean mollusks, believed to be close to 120,000 different specimens.

List of Marine Molluscs




The common octopus (Octopus vulgaris) is a marine mollusk belonging to the biological order Octopoda. In reality, there are at least three hundred (300) different types still in existence today.

This section contains a collection of fun and interesting facts about octopus, including where they live, what they eat, and how these eight-limbed mollusks reproduce.

Sea Snails

You can find this group of slow-moving marine gastropod mollusks living in every ocean. Even so, there are some key differences between the different species of these shelled organisms.

This section contains interesting sea snail facts with extra details explaining how their role in saltwater seas and oceans differs to that of their land-based counterparts.

5 Fun Facts about Mollusks

Here are five cool facts about mollusks to help beginners distinguish these unsegmented, soft-bodied, animals from other species in a comprehensive list of invertebrates.

  1. Most molluscs have eyes, but it's also common for some to have more than one heart. For example, the octopus (Octopus vulgaris) actually has three (3) hearts.
  2. Even though most bivalves use mollusca appendages to attach themselves to rocky surfaces, some use a muscular foot as a means of locomotion.
  3. Cuttlefish, squid, and octopus can all force water out from a funnel-like structure to generate propulsion through the water column.
  4. Some examples of the colossal squid (Mesonychoteuthis hamiltoni) can measure up to fourteen metres (46 feet) long and weigh more than 500 kilograms (1,100 pounds). Yet, a tiny micro mollusk existing in Borneo is less than one (1) millimetre long (0.027 inches).
  5. For the most part, marine mollusks are nocturnal creatures, meaning they are most active after the sun has gone down.

Mollusk Facts: Anatomy and Behaviour

The anatomical diversity, in both appearance and behaviour, differentiates sea mollusks from other ocean animals and plants.

Most of the phylum mollusca are soft-bodied and nonsegmental, usually with bilateral symmetry. A single, limpet-shaped calcareous shell, a tough exoskeleton made from chitin, covers the head and foot (the main body) in many specimens.

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