nematocysts radula veliger

Nudibranchs and sea slugs are mollusks that belong to the Class Gastropoda, which includes about 80 percent of all mollusk species, including snails, abalones, sea hares and garden slugs. Many species of sea slug and nudibranch have vestigial shells during their veliger larval phase but lose them as they mature. They resemble their terrestrial cousins, garden slugs, but because of their aquatic medium, appear more graceful and delicately colored. Sea slugs have a pair of oral tentacles on their head and another pair of sensory tentacles, called rhinophores, on their dorsal surface, toward the back of their body.

Eight species of sea slug inhabit the Chesapeake Bay, including herbivorous species, known as sacoglossans, and carnivorous species, the nudibranchs. Most Bay species are smaller than those found in deep ocean waters.

Cratena sp. nudibranch. Photo by K. Hill, courtesy of The Marine Biological Laboratory.

• Striped nudibranch, Cratena pilata: A small (to 1 3/16 inch) greenish-gray carnivorous sea slug with reddish stripes along its body and elongated tentacles. Several cerata, or fingerlike respiratory structures, extend along its back. Striped nudibranchs feed on tiny organisms such as jellyfish polyps, among hydroids—plantlike organisms that are actually animals, related to jellyfish and anemones—in the seagrass meadows of the Bay.
• Limpet nudibranch, Doridella obscura: One of the smallest yet most common nudibranchs in the Bay, this species is a third of an inch long and lives on bryozoans. Its surface is spotted yellow, black and brown, and some individuals appear darker or colorless. They have two rhinophores on their dorsal surface at one end. They can be found in seagrass meadows and along pilings and reef structures.
• Rough-back nudibranch, Doris verrucosa: This is the largest sea slug in the Chesapeake Bay—yet it is still only about 1 ½ inches long. Its flat oval-shaped body is orange and has a round structure of feather-shaped gills at one end, and a pair of rhinophores at the other. This species prefers high-salinity waters and lives and lays its egg cases among eel grass meadows.
• Ridged-head nudibranch, Polycerella conyma: This miniscule (1/8th of an inch) species disappears among its prey—Bowerbankia gracilis, a creeping bryozoan—by adopting its translucent greenish coloration. Its three-part gills emerge from its dorsal surface, which appears to be covered in warts.
• Dusky sea slug, Stiliger fuscatus: Another tiny (to ¼ inch), common sea slug in the Bay has a dark gray surface, a yellow foot, long, pale rhinophores and a black-and-white cerata gathered at the rear end of its dorsal surface. It consumes the algae that grow on other plants.
• Cross-bearer sea slug, Hermaea cruciata: An uncommonly glimpsed species in the Bay, this sea slug has many clear cerata through which can be seen brown, cruciform extensions of the organism’s liver. The rest of the body is covered with tiny, iridescent-white spots. It lives among eelgrass and other sea weeds.
• Emerald sea slug, Elysia chlorotica: An inch-long, bright green-and-white mottled sea slug found so far only in the Maryland portion of the Bay, this species also has tiny red spots along its back and bilateral ‘wings’.
• Kitty-cat sea slug, Elysia catula: Just a quarter of an inch long, this species earns its common name from the shape of its rhinophores, which are shaped like cat’s ears. It has graceful, bilateral ‘wings’ and lives among the Bay’s sea grass meadows, but has not been seen in Maryland waters.

Nudibranch means ‘naked gill’—a reference to the exposed position of the organisms’ gills, usually along the dorsal surface. It’s thought that many species of nudibranch are selective feeders, preferring one or two species of prey. This carnivorous group has a strip of teeth, called radula, that generally are adapted to the preferred food; for example, some species have broad radula for feeding on sponges, while others have narrower radula that are better adapted for consuming hydroids.

Nudibranch Life Cycle and Defense

• Nudibranchs are simultaneous hermaphrodites, and thus contain male and female sex organs at the same time (and can potentially mate with any member of the same species).
• Fertilized nudibranchs lay egg cases near their food source; sometimes it’s easier to spot the egg cases than the organism itself—but it’s likely to find one near the other. Some lay spiraled ribbons of egg cases, others a single coil of eggs, which, depending on the species, may be colored white, red, orange or pink.
• Eggs may take between 5 and 50 days to develop into their first larval form, or veliger phase, in which they become pelagic plankton. Warmer water temperatures tend to speed up the development process.
• Since they are not armed with shells like many other gastropods, nudibranchs require other defense mechanisms, including bright coloration and strong scent. Some nudibranchs that feed on Cnidarians, including hydroids and sea anemones, consume the stinging cells (nematocysts) of those organisms without being stung, and store them in their cerata. These ‘adopted’ cells then form part of the nudibranchs’ defense against predators.

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Last modified: 02/01/2006