What are gastropods?

Marine gastropods are a diverse group of mollusks that inhabit the waters of seas and oceans across the globe. These animals are remarkable for their extraordinary adaptability, as they can live in a wide variety of marine environments, from rocky coastal zones to the deepest ocean trenches. Their main characteristic is the presence of an external spiral shell, though some species have lost this feature through evolution. Marine gastropods – from the colorful nudibranchs, characterized by striking shapes and vibrant hues, to sea snails and whelks – play a vital role in aquatic ecosystems, acting both as herbivores and predators. In this article we’ll show you fascinating facts about one of the most studied and captivating groups of marine invertebrates in nature.

What are gastropods: their key features

Gastropods are a class of mollusks that includes more than 60.000 species, such as snails, slugs, and limpets. They are one of the most numerous and diverse groups of animals in the world, inhabiting terrestrial, freshwater, and marine environments.

The distinctive feature of gastropods is their body torsion: an evolutionary process in which their digestive organs and shell rotate 180 degrees relative to the body axis. This unique twist results in a body structure where the mouth is located near the foot (hence the name “gastropod,” meaning “stomach-foot”) and a spiral-shaped shell, present in most species.

Gastropods possess a muscular foot used for movement, often secreting mucus to help in gliding over surfaces or along the seabed. They have a well-developed head, complete with eyes and sensory tentacles to help explore their surroundings. The shell , if present, acts as a protective barrier into which the gastropod can retreat in case of danger. Another distinguishing feature is the radula, a ribbon-like tongue covered with rows of tiny chitinous teeth, used to scrape and grind food. About that: gastropods are heterotrophs, and their diet varies depending on habitat and species. Some are herbivores, feeding on algae or aquatic plants, while others (such as marine snails) are carnivorous predators, consuming small invertebrates or other marine creatures.

Gastropod reproduction is highly varied. Many terrestrial gastropods are hermaphrodites (they have both male and female reproductive organs) and able of self-fertilization or mating with others. Marine gastropods, on the other hand, usually have separate sexes. Some lay eggs that hatch into free-swimming larvae before developing into adults.

Gastropods are part of the food chains and contribute to the recycling of nutrients. In marine environments many species of gastropods serve as prey for fish and seabirds, while terrestrial species are hunted by mammals and reptiles. Additionally, their feeding behavior helps control algae and plant growth, and some gastropod species are considered bioindicators, meaning they can reflect the health of the ecosystems they live in, thanks to their ability to respond quickly to environmental changes.

The difference between gastropods and bivalve mollusks

Gastropods and bivalve mollusks are two distinct classes within the large phylum Mollusca and have significant differences in morphology, behavior and ecology, which we’ll analyze in detail below.

Morphology

Gastropods have a long, asymmetrical, body, usually protected by a single coiled shell. A key feature that distinguishes them is torsion, an evolutionary process in which the posterior part of the body twists 180 degrees, bringing the anus and gill or lung cavity near the head.

Bivalves, on the other hand, are characterized by a shell made of two symmetrical valves connected by a hinge and held together by strong adductor muscles. Their body is more compact and entirely enclosed within these two shells. Unlike gastropods, bivalves do not have a well-developed head and lack obvious sensory organs such as eyes and tentacles (which are present in many gastropod species).

Feeding

Many terrestrial gastropods are herbivores or detritivores, feeding on leaves, fungi, and decomposing organic matter. Marine gastropods may scrape algae off rocks using their radula, while some species use it to drill through shells and feed on other mollusks.

Bivalves, in contrast, are mostly filter feeders. They obtain food by filtering suspended particles from the water, such as plankton, bacteria, and organic debris. They use modified gills (called ctenidia) to filter the water and separate food from non-edible particles.

Movement and habitat

Many gastropods move using a muscular foot that slowly glides over land or marine surfaces. Some marine gastropods, like nudibranchs, are very good swimmers.

Bivalves are mostly sessile or semi-sessile. Many species, such as oysters and mussels, live attached to hard surfaces, including rocks and artificial structures. Others, like clams, burrow into sand or mud. Some bivalves, like scallops, can move by rapidly clapping their shells, but in general, their mobility is far more limited than that of gastropods.

Respiration and adaptation

Regarding their breathing system, both classes use gills for respiration, but their use varies depending on the habitat. Terrestrial gastropods breathe through a pulmonary cavity, while aquatic species use gills to absorb oxygen. For bivalves, gills serve a dual function: they are essential for both respiration and feeding, filtering water to extract oxygen and capture food particles.

Reproduction

Many gastropods are hermaphrodites, having both male and female reproductive organs, though they often still engage in mating to fertilize their eggs. Some species lay eggs that hatch into planktonic larvae, while others deposit them directly on substrates or inside protective capsules.

Most bivalves are dioecious, meaning they have separate sexes. In many species, sperm and eggs are released straight into the water, where external fertilization takes place. After fertilization, bivalve larvae often undergo a planktonic phase, floating in the water column before settling on the seafloor to develop into adults.

Ecological Role

Some gastropods, just like bivalves, function as filter feeders. In this case, both species have a major impact on aquatic ecosystems, as by removing suspended particles from the water, they help maintain the quality and clarity of the marine environment.

Which animals belong to the gastropod class?

Gastropods represent the largest class of mollusks and among the most known species that inhabit the Mediterranean Sea we find: limpets, sea snails, spiny murex, pelican’s foot snails and land snails. Each species has unique characteristics, which we’ll explore below.

Limpets (Patella caerulea)

Limpets are marine gastropods recognizable by their flattened conical shell. They typically live on rocky intertidal zones, where they cling tightly to surfaces using a strong muscular foot that helps them withstand wave action. Limpets feed on algae, which they scrape off the rocks with a radula, a robust, tooth-covered tongue. These mollusks are important bioindicators of coastal health, as their presence signals good water quality.

Sea Snails (Nassarius mutabilis)

Belonging to the Nassariidae family, these small sea snails are usually found on sandy or muddy seafloors of the Mediterranean. They feed on organic detritus and small invertebrates, playing a crucial role in the breakdown and recycling of organic matter within marine ecosystems.

Spiny Murex (Bolinus brandaris)

The spiny murex is one of the most fascinating Mediterranean gastropods, known for its ornate, spiny shell and its historical role in producing purple, a valuable violet pigment in antiquity. This carnivorous mollusk preys on invertebrates like bivalves and crustaceans, which it drills into using its radula to reach the soft tissue inside. The spiny murex typically inhabits rocky and sandy seabeds and is common along the Mediterranean coastline.

Pelican’s Foot (Aporrhais pespelicani)

This species is found on sandy and muddy seabeds and feeds on small organisms and organic debris. Along the Marche coast of Italy, pelican’s foot snails are considered a culinary delicacy. In the city of Fano, they are known as garagòj, while in Ancona, they are called crucéte (Italian: crocette). A traditional local recipe involves cooking them “in porchetta”, both at home and specialized restaurants. Before cooking, the shells are trimmed and chipped at the tips to allow seasoning to penetrate. In Ancona, these mollusks have even inspired a beloved dialect poem titled “Cume se magna le crucete in porcheta” (“How to Eat Crucete in Porchetta”).

Sea Snails

The spiral shell of sea snails provides them with protection and is one of the most distinctive features of these mollusks. In terms of diet, they are primarily herbivores, feeding on algae, organic detritus, and other small organisms found on rocks or the sea floor.

Regarding their use in cuisine, sea snails are highly prized in countries like Italy and France. They are often boiled or sautéed with garlic and parsley, and are considered a true delicacy with a delicate flavor.

The gastropods of the Mediterranean are a remarkable example of biological diversity and adaptation, enriching not only marine ecosystems but also our gastronomic culture. We must respect and protect these fascinating creatures by consuming only responsibly farmed species, for the sake of our health and the wellness of marine ecosystems.