Ovotestis Posterior hermaphrodite duct

Sea slug anatomy. (Illustration by Christina St. Clair)

Crop

Nerve Oviduct Vagina 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 nudi-branchs 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 nudi-branchs as it crawls along the substrate; it follows the trail of slime left by a nudibranch, sneaks up onto the prey, and sucks

This individual of the Chromodoris willani species has a tiny emperor shrimp (Periclimenes imperator) riding on it. The emperor shrimp does not appear to harm its host, and may help to keep the host clean. (Photo ©Tony Wu/www.silent-symphony.com. Reproduced by permis-

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 cov-

This species Hypselodoris bullocki is very common on tropical reefs throughout Asia; its body has a wide range of color variation, though all have the same basic body shape. These four specimens are mating. Nudibranchs are hermaphroditic, and often congregate like this for mass breeding. (Photo ©Tony Wu/www.silent-symphony.com. Reproduced by permission.)

This species Hypselodoris bullocki is very common on tropical reefs throughout Asia; its body has a wide range of color variation, though all have the same basic body shape. These four specimens are mating. Nudibranchs are hermaphroditic, and often congregate like this for mass breeding. (Photo ©Tony Wu/www.silent-symphony.com. Reproduced by permission.)

ering. 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.

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