HomeSea LifeMarine SpeciesInvertebratesMollusksSea Snail › Anatomy

Interesting Facts about Sea Snail Body Parts

Despite being known as slow-moving grazers, or because of it, the intricate anatomy of marine gastropod mollusks reflects their fascinating lifestyles.

The information in this page explains how the internal and external anatomical features of sea snails function, with extra details about some special anatomical adaptations.

What is the Basic Anatomy of a Sea Snail?

It's true to say that the sea snail species belongs to a diverse group of marine animals.

Almost all marine snails use one or more gills (respiratory organs) for respiration (breathing).

In spite of that, the intertidal species have a lung instead of a gill. So, they wait for low tide before they get active, and they move around in air - not submerged in water.

The false limpets (Siphonariidae), and those belonging with the family Trimusculidae, are typical examples of air-breathing snail species.

Another crucial feature of bivalve marine mollusks is their short, triangular tentacular organs. Sea snails use their tentacles and eye spots for sensory roles (chemoreception) and for protection.

Some species only have one retractable appendage. Whereas, others have two or four located close to the snail's head.

The purpose?

Through a process known as olfactory orientation, scent cells on the surface of each tentacle create a picture of its surroundings. In turn, it helps snails search for sediment and pick up food as they glide along the ocean floor.

As a matter of fact, the abalone and top snail species (family Trochidae) have many small tentacles at the edge of the pallial curtain (the mantle).

Sea Snail Anatomy: External

The outer shell of most marine snails is spiral shaped (like a coil or helix) and made of calcium carbonate (CaCO₃).

However, the skeletal case of some aquatic snails (especially limpets) is more patelliform (conical). The shape and thickness of the body wall varies greatly, depending on the species and their particular habitat.

Most marine snail shells are small. Yet, the shell of tiny bivalve gastropod mollusks in the family Juliidae (e.g. Julia zebra) consists of two individual and hinged valves, like plates.

Either way, this rigid structure provides some protection from their fiercest predators (crabs and groupers), strong wave action, and from desiccation (water loss). Hence, a snail cannot live without its shell because it is an integral part of the body.

Key points:

The opening section of the shell, called the aperture, is the part where the body extends. The operculum is a hard plate on the foot and acts in a similar way to a "trapdoor". Simply put, it seals the shell's opening when the snail retracts inside.

The snail's head contains the primary sensory organs - usually two pairs of tentacles and the eyes. The tips of the longer pair of tentacles contain the eyes. Snails use the shorter pair for smell and touch.

So, even though sea snails have eyes that appear as small black dots in most species, the eyes of conch snails are a lot bigger and more mobile - a feature that helps to provide better vision.

They use a large muscular organ located underneath their body (called a ventral foot) for movement around the ocean floor and for burrowing.

Interesting Fact: One of the biggest living shelled gastropods is the Australian trumpet snail (Syrinx aruanus), with an exoskeleton measuring up to ninety one (91) centimetres (36 inches).

Sea Snail Anatomy: Internal

Another interesting fact about sea snails is that they do not have normal teeth. Instead, they have a radula located inside the mouth and used to scrape, grind, and tear apart their favourite food (e.g. algae in herbivorous snails).

So, even though the major parts of a sea snail include teeth, they are in the form of a ribbon-like organ covered in thousands of microscopic, chitinous teeth-like structures.

In fact, radula teeth in snails are polysaccharides and one of the strongest biological materials. This kind of rasping tongue is actually capable of grinding rock to extract food (present in many limpet species).

Mantle and Mantle Cavity

Along with other types of shelled molluscs, such as clams, the mantle is a thin layer of tissue located underneath the gastropod shell. Its main function is to secrete calcium carbonate, which helps to enlarge the shell as the snail grows.

Labelled Diagram Showing Sea Snail Anatomy and Body PartsThe mantle cavity refers to the space below the mantle.

It's an area that contains either the snail's ctenidia (gills) or other forms of modified lung-like structures.

Sea snails are marine invertebrates so they do not have a spine or any kind of backbone.

Digestive and Circulatory Systems

Having an open circulatory system means the tissues receive fluid in the form of a blood equivalent, called hemolymph.

Even though snails have a heart, it's usually a small two-chambered pump with an atrium (to receive hemolymph) and a ventricle (to pump the circulatory fluid around the body).

Nervous System and Brain

The complexities of the nervous system vary according to the species. Excluding some of the predatory snails (e.g. the venomous cone snail), these sea snail parts are simple nerve cords.

Nonetheless, they include sensory ganglia used for smell and taste, and statocysts for orientation and balance. The nephridia are kidney-like organs that filter their waste products.

Here's the thing:

So you may be wondering if sea snails can feel pain? The simple answer is very likely yes... even though sea snails do not have a brain in the natural sense.

Snails exhibit protective nocifensive traits, which means they react to harmful stimuli. As a result, it indicates that they can experience discomfort or some form of pain.

Snail Reproductive Organs

There can be separate sexes (known as dioecious) where males and females have their own reproductive organs, and some species are hermaphroditic. In other words, most snails can produce sperm to fertilise eggs.

Special Anatomical Adaptations

In cone snails (Conidae), a modification in the venom glands that connect to the radula teeth act like harpoons and they use them to immobilise their prey.

Abalones (of the genus Haliotis) and a small group of aquatic snails called limpets, have flattened shells that have developed into structures that are capable of clinging to rocks in adverse and wave-swept habitats.

Another special adaptation occurs in cowries (Cypraeidae). Being able to extend their mantle over the shell helps them stay camouflaged and highly polished - which is why they have ornamental significance in some cultures.

Pro Tip: What's the lifespan of a sea snail? Even though it varies within the species, living in harsh environments with an abundance of predators means it's rare for sea snails to live much longer than a few years.

Divers also enjoyed reading about...