The phylum Mollusca is one of the largest and most diverse groups of animals, encompassing familiar creatures like snails, clams, and octopuses. While most people associate mollusks with aquatic life, where gills are the standard method for extracting oxygen from water, the respiratory structures show remarkable variation across the phylum. A large majority of mollusks do have gills, but this diversity is a direct result of adaptation to different aquatic and terrestrial habitats.
The Gills of Mollusks: Structure and Function
The ancestral and most common respiratory organ in aquatic mollusks is the ctenidium, a specialized gill structure. It is located within the mantle cavity, a space formed by the fold of tissue known as the mantle. Structurally, the ctenidium often resembles a comb or feather, consisting of a central axis from which numerous flattened filaments or lamellae project.
These filaments greatly increase the surface area available for gas exchange, allowing oxygen to diffuse efficiently into the blood. Water is drawn over the ctenidia, typically by the beating of tiny hair-like structures called cilia on the gill surface. Deoxygenated blood enters the filaments through afferent blood vessels and returns to the heart via efferent vessels after oxygenation. The primary function is to transfer dissolved oxygen from the water into the circulatory system while simultaneously releasing carbon dioxide.
Respiratory Adaptations in Aquatic Mollusks
While the ctenidium serves the function of gas exchange, its form and role are highly specialized across different aquatic mollusk classes. In bivalves, such as clams and oysters, the gills are significantly enlarged and have evolved a dual function. Their heavily ciliated ctenidia create a strong water current used not only for respiration but also for filter feeding.
The filaments in bivalve gills trap suspended food particles in mucus, which are then transported to the mouth, making the gills a primary feeding apparatus. In contrast, cephalopods like squid and octopus have a high metabolic rate demanding a more active and efficient respiratory system. Their two gills are non-ciliated and rely on muscular contraction of the mantle to rapidly push water through the mantle cavity and over the gills. This muscular pumping ensures a continuous, high-volume flow of water, supporting their fast-moving, predatory lifestyle.
Respiratory Evolution in Land-Dwelling Mollusks
The transition from water to land required a complete overhaul of the molluscan respiratory system, as gills are ineffective in air and prone to desiccation. Terrestrial snails and slugs, collectively known as pulmonate gastropods, have entirely lost their ancestral ctenidia. In place of gills, the mantle cavity has evolved into a highly vascularized structure that functions as a lung, sometimes called a pulmonary sac.
Gas exchange occurs across the moist inner surface of this cavity, which is rich with blood vessels. Air enters this modified “lung” through a small, muscular opening called the pneumostome, or breathing pore. The pneumostome can be opened and closed to regulate airflow and minimize water loss.
Some aquatic pulmonate snails, though living in water, must still regularly surface to draw air into their lung cavity, demonstrating their terrestrial evolutionary heritage. Other mollusks, particularly certain sea slugs (nudibranchs), rely instead on cutaneous respiration. Gas exchange occurs directly across the general body surface or through specialized secondary projections, having foregone specialized gills or lungs altogether.