Phylum Mollusca Class Gastropoda Subclass Opisthobranchia Number of families 110
Marine snails found in oceans throughout the world; the diversity encompasses species that range in habit from pelagic to burrowing, in color from transparent to vividly colored, and in structure from those with protective shells to soft-bodied forms with flowing movements
Photo: This small nudibranch (Glossodoris stel-latus) is laying eggs. The "swirl" pattern of egg laying is a common one; the color of the eggs is dependent upon the species. (Photo ©Tony Wu/www.silent-symphony.com. Reproduced by permission.)
Evolution and systematics
Opisthobranchia is a subclass of gastropod mollusks whose evolutionary ancestors date back to the Paleozoic period (543-248 million years ago). The earliest ancestors were probably all benthic, burrowing animals with rigid shells covering their soft body tissues. As members of this group evolved, a major trend was towards reduction, internalization, or loss of the shell. Numerous lineages have arisen independently that have exhibited this same evolutionary trend. In place of the shell, opisthobranchs have evolved other defensive mechanisms, including cryptic or warning coloration, secretion of toxic chemicals, and the ability to fire stinging cells obtained from animals on which they feed. The subclass, Opisthobranchia, includes a great diversity of species. More than 3,000 species are recognized, and these are classified into more than 110 families and eight orders.
Physical appearance varies greatly among members of the Opisthobranchia. Generally, their bodies are divided into three sections: head, foot, and visceral mass. In some species, these components are easily visible; in others, the sections may have merged in various ways. The head is flattened in some species, but in others, it may bear up to four types of sensory tentacles: oral tentacles, which project from the sides of the mouth; rhinophores, which are located on top of the head; propodial tentacles, which occur on the front of the foot; or posterior cephalic tentacles, which project from the posterior portion of the head.
The foot has a flattened sole that is used for creeping over substrates. Some species have wide lateral extensions of the foot that appear winglike; these structures are called parapo-dial lobes and are used for swimming. The visceral mass con tains the digestive and reproductive organs. It is covered by the mantle, which generates a shell in some species and hangs like a skirt around the body in others. The shell has become reduced, internalized, or lost in some of the more evolved species. The more primitive species have a gill located between the mantle and visceral mass near the head. In place of the gill, some of the more advanced species have cerata, small projections from their bodies, or a ruffled mantle edge to facilitate gas exchange.
Species range in size from some that are so minute that they can move between grains of sand to others that reach lengths of more than 17.7 in (45 cm). They vary in shape from rounded, shell-covered forms to elongate, often ornate, bodies. Some species are cryptically colored to blend in with their natural surroundings, while others are brightly colored, perhaps as a warning signal to predators; some of the pelagic species are nearly transparent.
Opisthobranchs occur worldwide. The greatest diversity is in the tropical seas, but several species have been reported from the polar waters of Antarctic.
Sea slugs are found in marine habitats such as reefs, intertidal areas, and the deep ocean. Some species live on the substrate, while others remain in the water column.
Most species of sea slugs are benthic and live on some type of substrate as adults; a portion of these species can swim for
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