Whether a mollusk is a crustacean is no, despite the common practice of grouping them together as “shellfish.” This confusion arises because both groups contain many aquatic species that are harvested for food, like clams and crabs. However, the biological differences between them are significant. Understanding these distinctions requires looking past their culinary uses and examining their fundamental organization.
Classification
The separation between mollusks and crustaceans occurs at the level of Phylum, the broadest category of animal life after Kingdom. Mollusks belong to the Phylum Mollusca, which is the second-largest phylum in the world. This phylum includes organisms like snails, octopuses, and clams.
Crustaceans are members of the Phylum Arthropoda, the largest animal phylum, which also contains insects, spiders, and centipedes. They are further categorized into the Subphylum Crustacea, signifying their close evolutionary relationship with all other joint-legged, segmented animals.
The Mollusk Body Plan
The body plan of a mollusk is characterized by four features. The first of these is the muscular foot, which is a large, ventral structure used for locomotion, burrowing, or modified into the arms and tentacles seen in squid and octopuses.
A second defining feature is the visceral mass, a centralized region containing the digestive, excretory, and reproductive organs. This soft body is typically covered by the mantle, a specialized layer of tissue. The mantle secretes the shell, if one is present, and creates the mantle cavity that houses the gills for respiration.
The final characteristic is the radula, a ribbon of microscopic teeth used for rasping or scraping food. This is found in most mollusks but is absent in bivalves like clams. This combination of features allows for a wide diversity of forms, such as the shelled terrestrial snail, the two-shelled scallop, and the shell-less octopus.
The Crustacean Body Plan
Crustaceans exhibit the segmented body structure typical of all arthropods, which contrasts with the soft, unsegmented body of a mollusk. The crustacean body is divided into three main regions—the head, thorax, and abdomen—though in many common species, the head and thorax are fused into a single cephalothorax. This body plan is encased in a rigid, non-living exoskeleton, or cuticle.
To grow, a crustacean must periodically shed this hard, external shell in a process known as molting. Their key feature is the presence of jointed appendages, which are highly specialized for various functions. These appendages include two pairs of antennae on the head used for sensory perception, along with specialized mouthparts for feeding.
The limbs are often biramous, meaning they branch into two parts, and are used for walking, swimming, or grasping. This segmented, jointed architecture is evident in familiar examples like the crab, the lobster, and the shrimp, as well as less obvious forms like the sessile barnacle.