Sea Slugs: Strange, Violent Sex, Heads That Regrow Bodies and Sea Hare Orgies

Home | Category: Sea Slugs, Nudibranchs, Star Fish and Unusual Sea Life


20110307-NOAA  sea slug seagoddes_100.jpg
sea slug
Sea slugs are strange creatures that have changed little in a 100 million years. Some species have sensory horns and most have exterior gills on their tails. Some of those found in coral reefs are quite colorful. The term sea slug can mean a nudibranch (See Below) or various other opisthobranchs. The term is also used to describe other kinds of reduced-shell gastropods and even cucumbers. [Source: Fred Bavendam, Smithsonian magazine, August 1989]

Columbia University's Eric Kandel won a Nobel Prize in medicine for his studies of dopamine and the brains of sea slugs. Blind and deaf, sea slugs can sense light and dark but mostly they locate prey through touch and by following their chemical trail. Using a hooklike-tongue called a radula, it places the prey in its mouth and swallows it hole.

Sea slugs are more often seen crawling across benthic (ocean bottom) substrates. Their lack of a shell makes sea slugs more mobile. They move around with a fleshy "foot" similar to that on seas shells, which advances them forward through muscular contraction or the movement of hairlike cilia on the bottom of the foot. The upperside of their long soft bodies is covered with waving delicate tentacles that come in variety of brilliant colors.

Sea slugs have a lifespan of up to one year in the wild bit only one month in captivity. Their short lifespan in captivity is attributed to the difficulty in replicating their diet in the wild due their toxicity and danger to other organisms. Some people keep sea slugs in aquariums but typically the practice is only recommended for advanced aquarists because of the difficulty involved with rearing supporting organisms such as hydroids, sponges and byrozoans. [Source: Megan Jones, Animal Diversity Web (ADW)]

Websites and Resources: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); Fishbase; Encyclopedia of Life; Smithsonian Oceans Portal ; Monterey Bay Aquarium ; MarineBio; Websites and Resources on Coral Reefs: Coral Reef Information System (NOAA) ; International Coral Reef Initiative ; Coral Reef Alliance ; Global Coral reef Alliance ; Global Coral Reef Monitoring Network


Opisthobranchs are a subclass of mollusks and gastropod that includes sea hares, nudibranchs, bubble shells and pteropods. They are essentially snails that dispense with their shells when they become adults. Some can swim but most move along the bottom of the ocean. Most are less than 10 inches in size. There are about 4,000 species of opisthobranchs of which 3,500 are nudibranchs and sea slug species.

Glossodoris symmetricus
Opisthobranch is now regarded as an informal name for a large and diverse group. The term is no longer considered to represent a monophyletic group (a group of organisms that share a common ancestor). Opisthobranch means "gills behind" (and to the right) of the heart. In contrast, Prosobranch means gills in front (of the heart). Opisthobranchs are characterized by two pairs of tentacles and a single gill behind and to the right of the heart. With the lack of a heavily mineralized shell, there has been very little fossil record of the group. However, molecular clock studies have suggested that Opisthobranchia emerged as early as the Carboniferous Period (359 to 299 million years ago) during the late Paleozoic Era.

Opisthobranch characteristics and evolutionary trends include the reduction or loss of the shell, the elaboration of the head, foot or mantle, and the acquisition of chemical defenses. are shared by most opisthobranch taxa. The loss of shell in the group is an example of parallel evolution and has occurred on multiple independent occasions. With Opisthobranchs, there is no marked distinction between head and mantle. The tentacles, situated close to the mouth, are used for orientation. Behind them are the rhinophores, olfactory organs which often have complex forms. The middle part of the foot is the sole, used for locomotion. The sides of the foot have evolved into parapodia, fleshy winglike outgrowths. Their eyes are simple pit-cup eyes with a lens and cornea capable of detecting light and the passage of shadows but not of producing a coherent image.

Sea Slugs Versus Nudibranchs

Sometimes the terms “nudibranch” and “sea slugs” are used interchangeably but that is not necessarily correct. Nudibranchs are a subset of sea slugs. While all nudibranchs are sea slugs, not all sea slugs are nudibranchs. “Sea slug” does not represent a specific taxonomic group but refer to different clades ( groups of organisms believed to be evolutionary descendants of a common ancestors).in the class of Gastropoda (snails and slugs including those in freshwater and on land). [Source: Wikipedia]

A Sea slug is a shell-less marine mollusc which is typically brightly coloured, with external gills and a number of appendages on the upper surface.. Sea slugs are gastropods — sea snails (marine gastropod mollusks) that over evolutionary time have either completely lost their shells, or have seemingly lost their shells due to having a greatly reduced or internal shell. The name "sea slug" is applied to nudibranchs, as well as to a paraphyletic set of other marine gastropods without obvious shells

Nudibranchs are shell-less marine mollusc of the order or clade Nudibranchia. Nudibranch means “naked gill,” a feature that separates them from other kinds of sea slugs. Most nudibranchs are not large, they are often very eye-catching with their brilliant colors. A number of other taxa of marine gastropods — many of them "sea slugs" — are easily mistaken for nudibranchs.

Types of Sea Slugs

Sea slug

Numerous families and groups of gastropods within the informal taxonomic group Opisthobranchia are called "sea slugs".The main types of sea slugs (marine slugs) are nudibranchs, sidegill slugs, bubble snails, algae sap-sucking sea slugs, and sea hares. The term "sea slug" is perhaps most often applied to nudibranchs. Some non-nudibranch sea slugs are as brilliantly colored as nudibranchs

The name "sea slug" is also often applied to the sacoglossans (clade Sacoglossa) — the so-called sap-sucking or solar-powered sea slugs which are frequently a shade of green. Sap-sucking sea slugs are often brightly colored like nudibranchs. Elysia picta is known as the painted elysia. They are green with vivid bands of orange, blue, and neon green. The brown lined elysia (Elysia subornata) has a bright green body with a thin brown line running along the very edge of their wing-like structures. [Source: Chelsea Blanchet, Animal Diversity Web (ADW) /=]

Another group of gastropods that are often labeled as "sea slugs" are the various families of headshield slugs and bubble snails within the clade Cephalaspidea. Headshield slugs like the Chelidonura varians use their shovel-shaped heads to dig into the sand, where they spend most of their time. The shield also protects sand from entering the mantle during burrowing. The lettuce sea slug (Elysia crispata) has lettuce-like ruffles that line its body. This slug, like other Sacoglossa uses kleptoplasty, a process in which the slug absorbs chloroplasts from the algae it eats, and uses "stolen" cells to photosynthesize sugars. The ruffles of the lettuce sea slug increase the slug's surface area, allowing the cells to absorb more light.

Sea hares, clade Aplysiomorpha, have a small, flat, proteinaceous internal shell. They too are called sea slugs as are the clades Thecosomata and Gymnosomata — small pelagic gastropods known as "sea butterflies" and "sea angels". Many species of sea butterflies retain their shells. These are commonly known as "pteropods" but are also sometimes called sea slugs; especially the Gymnosomata, which have no shell as adults. There is also one group of "sea slugs" within the informal group Pulmonata:

Some species of acochlidian sea slugs have made evolutionary transitions to living in freshwater streams and there is at least one evolutionary transition to land. One very unusual group of marine gastropods that are shell-less are the pulmonate (air-breathing) species in the family Onchidiidae, within the clade Systellommatophora. Peronia indica is a species of air-breathing sea slug withing this group.

Sea Slug Feeding

Sea slugs are specialized carnivores that feed on sessile (fixed in one place) animals that are not heavily exploited by other taxa — such as sponges, corals, bryozoans, or hydroids. Some species are cannibalistic and will feed on other sea slugs. Typically, sea slugs practice stenophagy, meaning that different species have a specialized diet of one or two kinds of food. [Source: Animal Diversity Web (ADW)]

20120519-NudibranchFrilled_nudibranch.jpg Their lack of a shell also makes more vulnerable to attacks from predators. Some bright colored sea slugs feed on poisonous sea anemones, ascidians, hydroids and sponges and direct the poison through their digestive system into special sacs at the tips of their brightly-colored "fingers" called cerata. The bright colors are believed to be a means of informing other creatures they are poisonous. Sometimes the toxins kill, but mostly they taste really bad or are extremely irritating. Cerata grow back.

Sea slugs are slow-moving creatures. Many feed on corals, sponges and sea anemones. Some attacking sea slugs rear up like striking cobras. Some of these grab sea anemones with their relatively large jaws and are sucked into the anemone's tube, where they take their time consuming its tentacles.

Many sea slugs feed on coral polyps. The color of sea slugs often closely resembles the color of the coral they feed on. This give them camouflage against potential predators. One species found in the Great Barrier Reef sprouts legs that look like coral branches and feeds on algae living in its tissues much like coral does.

Opisthobranchs Reproduction

All adult opisthobranchs are hermaphroditic. Many mate by formulating copulating chains with a number of individuals lined up head to tail. The first in line is a female and the last is a male with those in the middle acting as both male and female. Eggs hatch free-swimming larvae that disperse and later metamorphose into juveniles without shells and eventually adults.

Among one species, the Navanax, mating couples take turns being male and female. David Attenborough wrote: "First one behaves as a male, extracting a long tentacle-like penis from a pore in its head and pursuing the other by following its trail of mucus. When it eventually catches up, the pursued lifts its hind end, allowing the pursuer to insert its penis in genital pouch in the rear. The two move along in tandem. After about ten minutes, they separate and change roles.”

“ Hermissenda “ seas slugs are hermaphroditic cannibals that can mate as a female, mate as a male, eat its own species or be eaten by its own kind.

Sea Slug’s Mating Ritual Ends with a Stab Between the Eyes

Sea hare eggs

The hermaphroditic sea slugs Siphopteron makisig possess both male and female sex organs — which including a a double-ended penis with a penile stylet and penile bulb that is 1.27 millimeters in length. When pairs come together to mate, they stab each other between the eyes their a needle-like penile stylets and exchange prostate fluid. This tactic was described as "just weird" by a researcher who co-authored a 2013 study about the odd behavior, published in the journal Proceedings of the Royal Society B. Scientists are uncertain as to why exactly the slugs target this body area for stabbing, but they suspect that the hormonal injection may serve to increase the possibility of successful fertilization. [Source: Mindy Weisberger, Live Science, February 14, 2023]

This kind of behavior — stabbing a partner during sex— is called “traumatic insemination.” It has been documented in other organisms such as bed bug but sea slug are reportedly the first to target between the eyes on the head. Associated Press reported: In its native Pacific waters, the sea slug Siphopteron makisig looks tiny and delicate, like a bud of colored glass. But in reality this slug is a mirror-image mating machine. Like most sea slug species, S. makisig is a hermaphrodite, endowed with both male and female reproductive organs that it uses at the same time during mating. But unlike other sea slugs, it tops off trysts with unusually targeted stabbing. [Source: Eva Van Den Berg, Associated Press, February 15, 2022

The sex starts normally enough. To fertilize eggs developing in each slug’s female parts, the other slug deposits sperm with its penis. Actually, only half of its penis, which has two prongs: one that delivers sperm with its bulbous end, and the other tipped with a syringe-like stylet (and sometimes called hypodermic genitalia). During the sex act, each slug stabs the other with the stylet, which delivers prostate fluid likely bearing hormones. Evolutionary biologist Rolanda Lange says the fluid may “increase the fecundity of a sea slug’s own sperm, or inhibit that deposited by previous partners.”

S. makisig is than 6.5 milimeters (a quarter inch) long, with yellow and red markings on its translucent white exterior. It slug lives on sand beds at ocean depths from about 20 feet to about 90 feet and commonly nestles within microalgal formations. The mollusk has been observed in waters off the Philippines, Australia, and Indonesia.

Sea Slug Chops Off Their Heads and Regrow Their Bodies

Scientists in Japan have discovered that two species of sea slugs can rip off their heads and regenerate new bodies, including the heart and other vital organs, in less than a month. Scientists theorize that the slugs, Elysia marginata and Elysia atroviridis, do this to rid themselves of internal parasites. [Source: Annie Roth, New York Times, National Geographic June 10, 2021] reported: “The findings, published in March 3021 in the journal Current Biology, describe Elysia marginata and Elysia atroviridis sea slug heads detaching and crawling away from their bodies. Within hours, the researchers say these disembodied heads started munching on algae again as though nothing had happened. Per the Times, the researchers think the sea slugs’ grisly strategy may be a way of ridding themselves of parasites. [Source: Alex Fox,, March 10, 2021]

“Susan Milius of Science News notes that there are other examples of similarly extreme regeneration in the animal kingdom, including flatworms and sea squirts. But these creatures, according to Science News, have simpler bodies. The sea slugs are regrowing vital organs such as the heart, while flatworms and sea squirts don’t have hearts to begin with.

“Oddly enough, the headless bodies can also survive for a few months, their hearts still beating as they begin to rot, reports Christa Leste-Lasserre for New Scientist. But, as Sayaka Mitoh, a biologist at Nara Women’s University in Japan and co-author of the paper, tells New Scientist, the decapitated bodies never sprout heads. “The head has the brain and teeth, or radula, which may be irreplaceable,” she says.

“In experiments, not all the sea slugs lopped off their own heads, and of those that did, about a third of them successfully regrew their bodies. Researchers also observed that the self-amputating sea slugs tended to be harboring crustacean parasites called copepods. According to New Scientist, regrowing a body from the neck down is a young slug’s game, as the older slugs in the experiment didn’t survive the separation. “This may seem like a silly choice,” Mitoh tells New Scientist. “But the old ones would die soon anyway, and they might stand a chance of surviving and regenerating a parasite-free body.”

“Per Science News, the slugs’ leaf shaped bodies and green coloration may explain how their severed heads can survive on their own. Slugs in the genus Elysia steal the green-pigmented engines of photosynthesis from the algae they eat, earning themselves the nickname of “solar-powered sea slugs,” per the Times. The slugs can keep these hijacked bits of cellular machinery, called chloroplasts, alive for weeks or months, according to Science News. The sugars that the chloroplasts manufacture out of sunlight provide the slugs with a low cost source of sustenance. Crucially for the severed slug heads, Mitoh tells New Scientist, the creature’s digestive glands are thought to be “distributed all over the body surface, including the head.”

Sea Hares

Sea hares look like shapeless blobs. Related to sea slugs and nudibranchs, they are mollusks and gastropod with a soft body. What little remains of their ancestral shell is imbedded in their skin. They live on the sea bottom and feel their way around with two long antennae that project out the animal's head, sort of like rabbit ears, the source of the animal’s name. Sea hares have a large foot and inhabit sea grass and seaweed beds and feed on fleshy algae, which they crop using a hard mouthpiece called a radula. .

The black sea hare is the world's largest gastropod. It can reach lengths of 100 centimeters (39 inches) and weigh as much as 13.5 kilograms (30 pounds). The California black sea hare (Aplysia vaccaria) can reach a length of 75 centimeters (30 inches) and weigh of 14 kilograms (31 pounds).

Most sea hares have several defenses; in addition to being naturally toxic, they can eject a foul ink or secrete a viscous slime to deter predators. Dolastatin is a drug taken from an Indian ocean sea hare that shows promise in treating skin cancer and has made it as far as clinical trials.

Sea hares produce ink like octopuses and squids. They have chemical sensory antenna which they use to analyze scents and chemical substances in the water. They help the animals find food and locate mates. Their main defense against predators is a foul taste that potential predators go out of their way to avoid. They can also eject a noxious purplish ink-like liquid that predators also despise.

Sea Hare emitting ink

Brown Sea Hares

Bonnie J. Cardone wrote in California Diving News: Brown sea hares have a reticulated color pattern that resembles that of the two-spot octopus, also found in SoCal waters. These animals aren’t always brown, sometimes they are tan, greenish-brown or red. Their coloration depends upon what they eat, which can include red or green algae or sea grasses. These large sea slugs are found from Oregon to Baja California, including the Sea of Cortez. Those living at the Aquarium of the Pacific eat kelp, nori and the algae that grow in their habitat. In some labs they are raised on romaine lettuce. [Source: Bonnie J. Cardone, California Diving News, November 15, 2017]

The brown sea hare’s resemblance to a rabbit comes from its shape and the two tentacles (rhinophores) on its head, which look a bit like rabbit ears. There is a tiny eye on each side of the head, an inch or so below the base of the rhinophore. According to A Living Bay, when moving forward the sea hare “slowly waves its head from side to side, receiving sensory input through its oral tentacles, its rhinophores and its eyes; in addition, the whole body is sensitive to touch.”

The sea hare’s mantle has wing-like flaps called parapodia on each side. These cover the gills and overlap at or near the center of the back. While brown sea hares have a shell, it is an internal one. The penis is on the right side of the head while the vagina opens in the mantle cavity, beneath the shell and between the parapodia.

California brown sea hares can grow to be 16 inches long and weigh five pounds. The measured length of any sea hare depends upon whether or not the body is extended, a fact that is, unfortunately, rarely noted. While 16-inch brown sea hares are uncommon, most of them are considerably larger than any other gastropod found in SoCal waters. They are second in size only to the black sea hare, the largest gastropod in the world. Black sea hares can be nearly twice as big as brown sea hares, reaching lengths of 38 inches and weighing a whopping 30 pounds.

Unlike their carnivorous nudibranch cousins, brown sea hares are herbivores. Dining on red algae (Plocamium cartilagineum) allows them to produce purple ink as well as accumulate chemicals in their skin and internal organs that make them unpalatable to most would-be predators. Among the California brown sea hare’s few predators is the giant green anemone. It is picky about what parts of the sea hare it eats, avoiding the digestive gland, which contains the chemicals, and regurgitating everything if any toxin is accidentally eaten. It will also expel a sea hare that produces purple ink. Other predators include starfish, lobsters and a much smaller close cousin Navanax inermis. All of the aforementioned creatures only eat small sea hares; large sea hares seem to enjoy a predator-free, albeit short, existence.

California brown sea hares are important laboratory animals, valuable in neurobiology. They have very large neurons, the largest in the animal kingdom, and very few of them, making it possible to identify individual nerve cells that are responsible for specific behaviors. They have been and are being used extensively in studying memory, behavior, and learning.” The California brown sea hares used for research are born and bred in captivity to eliminate the variables created when wild-caught animals are used.

Sea Hare Orgies

Adult sea hares carry both eggs and sperm. They can mate as a male or female or both, and often have mass orgies in which sea hares form long conga-line-like chains of individuals fertilizing and being fertilized.After the orange and yellow eggs are fertilized they are secreted in a thin noodlelike mass. A single sea hare may produce a mass of eggs several hundred meters long with as many as a million eggs. Sea hares die soon after they mate and spawn. After they die they turn to tough rubbery blobs.

Bonnie J. Cardone wrote in California Diving News: It is an unusual behavior that draws many divers’ attention to brown sea hares: During certain times of year (summer and autumn) they form large aggregations called mating chains or circles, which divers are prone to call orgies. The aggregations can last a few hours or even days, with animals joining and then leaving them after mating and laying eggs. Except for the animal at the beginning and end of the chain, each sea hare is, according to the Cabrillo Marine Aquarium, “…male to the one in front and female to the one behind, so each sea hare is both a mother and a father.” After mating, each sea hare lays a string of up to 80 million pale yellow or orange eggs (that astounding number is not a typo!). The gelatinous eggs are amassed in a central pile attached to the substrate or seaweed, where they release a peptide pheromone that attracts even more sea hares. It should be noted that a pair of sea hares can mate, too. [Source: Bonnie J. Cardone, California Diving News, November 15, 2017]

Sea hares need to lay lots of eggs because so many are eaten by predators. Eggs that escape this fate turn a darker color in a little more than a week and hatch in 10 to 12 days. Brown sea hares only live about a year and usually die after laying eggs. (Female octopuses die after their eggs hatch.) The larvae that emerge from the egg strings free swim for about 30 days before settling down in water about 60 feet deep. They eat so much during the next three months that their weight doubles every 10 days. All the while they are morphing into adults and growing larger as well as slowly moving into shallower water.

Image Sources: Wikimedia Commons, NOAA

Text Sources: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.

Last Updated May 2023

This site contains copyrighted material the use of which has not always been authorized by the copyright owner. Such material is made available in an effort to advance understanding of country or topic discussed in the article. This constitutes 'fair use' of any such copyrighted material as provided for in section 107 of the US Copyright Law. In accordance with Title 17 U.S.C. Section 107, the material on this site is distributed without profit. If you wish to use copyrighted material from this site for purposes of your own that go beyond 'fair use', you must obtain permission from the copyright owner. If you are the copyright owner and would like this content removed from, please contact me.