Opisthobranchia

(Sea slugs)

Phylum Mollusca

Class Gastropoda

Subclass Opisthobranchia

Number of families 110

Thumbnail description
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

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 characteristics

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 parapodial lobes and are used for swimming. The visceral mass contains 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.

Distribution

Opisthobranchs occur worldwide. The greatest diversity is in the tropical seas, but several species have been reported from the polar waters of Antarctic.

Habitat

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.

Behavior

Most species of sea slugs are benthic and live on some type of substrate as adults; a portion of these species can swim for

short periods of time as well. Benthic sea slugs are slow-moving organisms. They live relatively sedentary lives, and some species spend their entire life on one prey organism such as a sponge or coral reef. Almost all dispersal to new areas occurs during the veliger larval stage; veligers will settle out of the water column only when suitable substrate is present. Some larvae have crossed entire ocean basins because of lack of suitable settlement substrate.

A smaller portion of species remain planktonic and float in the water column throughout their lives. Most pelagic species migrate up and down the water column on a daily cycle. They move closer to the surface at night and to deeper waters during the day; however, in some species, this pattern is reversed.

Sea slugs exhibit several defensive behaviors. Some species secrete toxic chemicals or retain stinging cells from animals they eat; these defenses can be used to ward off or harm potential predators. Other species, typically benthic, swim to escape predators, while pelagic species sink in the water column to avoid predation.

Feeding ecology and diet

Sea slugs feed on a wide variety of organisms. Some are herbivores that eat algae, while others are filter feeders that take in particles from the water. Most are carnivores that eat many types of animals, including hydra, sponges, corals, barnacles, worms, other mollusks, and even the eggs of other sea slugs, cephalopods, or fishes. Despite the diversity of organisms consumed by sea slugs as a group, many species are highly specialized feeders that consume only a single type of food item; some will only prey upon a single genus or species.

Most sea slugs have a pair of jaws and a radula, a rasping, tonguelike organ with rows of small teeth, that are used for feeding. The radula is capable of scraping, piercing, tearing, or cutting food particles. Species of sea slugs that lack a radula may suck in whole prey like a vacuum, pry tissues off of their prey, or have special adaptations for trapping food items. To overcome defensive mechanisms of their prey, such as spines, exoskeletons, or stinging cells, many sea slugs cover their food with mucus.

Few organisms prey on sea slugs; some sea slugs use chemical defenses to keep predation to a minimum. The sea slugs often take up distasteful or toxic chemicals from their own prey and, in turn, use them to deter predators; some nudibranchs even eat the stinging cells off of jellyfish and Portuguese man-of-war and use them to ward off potential predators. One of the most frequently reported acts of predation occurs between two different types of opisthobranch mollusks. Navanax inermis, a species of the order Cephalaspidea, feeds on nudibranchs as it crawls along the substrate; it follows the trail of slime left by a nudibranch, sneaks up onto the prey, and sucks

the organisms in with a sort of suction tube from inside its body.

Reproductive biology

Sea slugs are hermaphrodites; each individual has both male and female sexual organs and produces sperm and eggs. Some simultaneous hermaphrodites possess male and female organs at the same time, while others begin life as one sex and then transform to the other gender. Individuals do not exchange their own sperm and eggs; the reproductive system of sea slugs keeps these products separate within an individual.

Mating behaviors are highly variable in sea slugs. Some species mate as pairs; others form long chains of individuals. In some species, particularly those that are simultaneous hermaphrodites, both sperm and eggs are reciprocally exchanged between partners at the same time. Individuals in other species behave as either male or female during a mating session. Mating may last from minutes to days, depending on the species. Fertilization may not take place immediately; an organ called the seminal receptacle can store sperm for several months until the eggs they will fertilize have reached maturity.

Eggs are laid in masses ranging from hundreds to millions of individual eggs; the masses are sometimes formed into chains or globules on the substrate or in the water column, and some species protect their egg masses with a mucous covering.

When eggs hatch, planktonic larvae emerge; these larvae, called veligers, soon develop a shell around them. A few species retain their eggs inside their bodies, and young emerge as juveniles, thereby eliminating the planktonic larval stage.

Conservation status

Although little is known about their populations, sea slugs are not considered threatened or endangered. No species are listed by the IUCN.

Significance to humans

Many sea slugs are admired by humans who are fortunate enough to view them during snorkeling or diving activities. Beyond their aesthetic appeal, they are of little significance to humans.

Species accounts

List of Species

No common name

Pupa solidula

order

Cephalaspidea

family

Acteonidae

taxonomy

Pupa solidula (Linnaeus, 1758).

other common names

None known.

physical characteristics

Among the most primitive of the opisthobranchs, it has a hard calcified shell that is adorned with a spiral pattern of black dots. A relatively small, colorless animal lives inside the shell. Flaps on the head overlap the front of the shell and protects the mantle cavity from sand as it crawls along or burrows into the substrate. Most adults are approximately 0.39–0.59 in (10–15 mm) in shell length.

distribution

Tropical IndoPacific.

habitat

Sandy substrates of marine waters.

behavior

None known.

feeding ecology and diet

Feeds on polychaete worms; other prey items and its predators have not been reported.

reproductive biology

Hermaphroditic and reproduces on an annual cycle. Eggs are laid in the spring, and veliger larvae settle to the substrate a few months later. Young produced in one year reach maturity and reproduce the following year. Most individuals die after spawning, but a few survive into their second year of life.

conservation status

Not listed by the IUCN.

significance to humans

None known.

No common name

Tylodina corticalis

order

Notaspidea

family

Tylodinidae

taxonomy

Tylodina corticalis (Tate, 1889).

other common names

None known.

physical characteristics

One of the more primitive members of its order. Retains a large heavily calcified shell of circular shape on the exterior of its body. When crawling, the body becomes elongate and extends beyond the shell. Has a large head with prominent feeding tentacles and rhinophores. The body is bright yellow, and individuals may reach size of 2 in (5 cm).

distribution

In Australia from southern Queensland to southwestern Australia around the southern coast.

habitat

From the intertidal zone to depths of around 328 ft (100 m), typically on its food sponge.

behavior

None known.

feeding ecology and diet

Feeds on sponges in the family Aplysinellidae. Predators have not been reported.

reproductive biology

Mating behavior has not been described, but it is known to lay ribbons of eggs on its food sponge.

conservation status

Not listed by the IUCN.

significance to humans

None known.

No common name

Aeolidiella sanguinea

order

Nudibranchia

family

Aeolidiidae

taxonomy

Aeolidiella sanguinea (Norman, 1877).

other common names

None known.

physical characteristics

An elongate body that is covered in cerata, thin, fleshy projections that arise in regular rows from the digestive tract. Long oral tentacles and prominent rhinophores are present on the head. The body, cerata, and tentacles are pale yellow, orange, or red in color, depending on the individual's diet. The sole of the foot and tips of the cerata are white.

distribution

Atlantic coasts of Ireland, France, and Scotland.

habitat

Found in the intertidal zone and sublittoral areas; often in muddy inlets or on rocky coasts.

behavior

Has been described as slow and aggressive. Is more active at night than during the day.

feeding ecology and diet

Feeds on sea anemones. Predators have not been reported.

reproductive biology

Mating behavior has not been described. Eggs are laid in a spiral thread with scalloped sections.

conservation status

Rare, but not considered threatened. Not listed by the IUCN.

significance to humans

None known.

No common name

Glossodoris atromarginata

order

Nudibranchia

family

Chromodorididae

taxonomy

Glossodoris atromarginata (Cuvier, 1804).

other common names

None known.

physical characteristics

Characterized by an elogate body with a highly sinuous mantle edge. Has two rhinophores on the head that are similar in appearance to small horns; gills that undulate like coral polyps are found on the dorsal portion of the body. Although the body may range in color from cream to pale brown, the mantle edge, rhinophores, and gill edges are always black. Adults reach lengths up to 3.15 in (8 cm).

distribution

Throughout the tropical and subtropical Pacific and Indian Oceans.

habitat

Inhabits reefs, intertidal, and subtidal areas of coastal waters; ranges from low intertidal to depths of 92 ft (28 m).

behavior

Nothing is known.

feeding ecology and diet

Feeds on several species of siliceous sponges. Predators have not been reported.

reproductive biology

Simultaneous hermaphrodite; it typically mates as pairs. The individuals position themselves alongside one another and exchange both sperm and eggs. Eggs are laid in ribbon-like masses with a mucous sheath surrounding them.

conservation status

Not listed by the IUCN.

significance to humans

None known.

No common name

Elysia viridis

order

Sacoglossa

family

Elysiidae

taxonomy

Elysia viridis (Montagu, 1804).

other common names

None known.

physical characteristics

A flat, elongated, leaflike body that may grow to 1.7 in (45 mm) in length; the parapodia may stretch alongside the body or be held over the dorsal mid-line; body color varies from green to red depending on the types of algae that have been the recent diet of the organism; red, blue, and green specks of color are typical on the body; white patches may occur near the edge of the parapodia, and black markings sometimes occur on the head. Feeding tentacles and rhinophores are prominent on the front of the head.

distribution

North Atlantic from Norway to the Mediterranean. Also observed in North Africa and China.

habitat

Found on a variety of shallow-water algae.

behavior

Nothing is known.

feeding ecology and diet

Eats filimentous (Cladophora and Chaetomorpha) and coenocytic (Codium and Bryopsis) algae. Retains plastids, often chloroplasts, from the algae it eats and uses these cells to continue photosynthesis within its own body. Predators have not been reported.

reproductive biology

Hermaphroditic; spawning occurs from May to October. Egg masses contain as many as 10,000 eggs. Embryos develop for 5–12 days before veliger larvae emerge. Probably has an annual life cycle.

conservation status

Not listed by the IUCN.

significance to humans

None known.

No common name

Corolla spectabilis

order

Thecosomata

family

Cymbulidae

taxonomy

Corolla spectabilis (Dall, 1981).

other common names

None known.

physical characteristics

A small gelatinous species; its body is nearly transparent, with the dark gut being the most visible part. Lacks an external shell, but a gelatinous internal structure provides skeletal support. Has large oval wing plates that extend laterally from the body; the lobes of the foot have become fused to form a proboscis. Large individuals can reach lengths of 3 in (8 cm), with wing plates spanning 6 in (16 cm).

distribution

Atlantic and eastern Pacific Oceans between approximately 40°N and 5°S latitude.

habitat

Near the surface of marine waters.

behavior

Flaps its large lateral wing plates to maintain position or move in the water column. Swims away rapidly when disturbed (up to speeds of 18 in/sec [45 cm/sec]), typically shedding the mucous sheet used for feeding. May form large aggregations at the water's surface.

feeding ecology and diet

Feeds on zooplankton that it collects with a large, delicate mucous sheet that is produced by glands along the edge of the wing plate. Traps prey by slowly sinking in the water column and entangling planktonic particles as it descends. Mucous web and entangled food items ingested by the proboscis. Predators have not been reported.

reproductive biology

Protandric hermaphrodite; it matures and functions first as a male and then as a female. Mating behavior has not been reported. Eggs are spawned in mucous strings that may extend to 1.6 ft (0.5 m) in length.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Resources

Books

Lalli, Carol M., and Ronald W. Gilmore. Pelagic Snails: The Biology of Holoplanktonic Gastropod Mollusks. Stanford, CA: Stanford University Press, 1989.

Other

"Sea Slug Forum." July 10, 2003 [July 27, 2003]. <http://www.seaslugforum.net>.

Katherine E. Mills, MS