These animals have discarded shells in favor of more sophisticated biological and chemical defenses. They include the true sea slugs, nudibranchs, sea hares, bubble shells and side-gilled slugs. The gills and anus are placed behind the head at the rear of the body. These animals have de-evolved from the 180º torsion of the prosobranchs. In other words, they have untwisted. In place of shells, have developed unique defenses for protection - acids, toxins, stings and colorful camouflage.

These creatures are hermaphrodites. They have both male and female reproductive abilities. This ensures that any animal of the same species will be an appropriate mate. Fertilized egg cases or coils are then laid on the substrate and they eventually hatch as trochophore larvae to begin their cycle.

Opisthobranchs have varied feeding habits. Some are herbivores. Others are specialized predators upon sponges, bryzoans or hydroids.

Order Notaspidea

These are the side-gilled slugs that have a set of feathered gills between the foot and the mantle on the right side of the body. They make themselves distasteful to predators by secreting glandular acid. They are thought to prey on sponges or tunicates.

Family Pleurobranchidae

Orange Gumdrop
(Berthellina citrina)

This is a nocturnal, bright orange, orange-yellow slug. It is found crawling around on ledges and cliffs. By day, it hides under rocks. True to its family, it secretes an acid when disturbed. This is a common species in Hawaii.


These Opisthobranchs are the Nudibranchs, and they prey on fixed organisms such as sponges, hydroids, etc. and even hunt down other nudibranchs. Their fascinating colors are often a warning that they are foul tasting, toxic or venomous. They are unusual in that they recycle toxins found in their prey - irritants found on sponges or the nematocysts or stinging cells of hydroids or others. These nematocysts are stored in their own body, armed and ready to fire. Some rely on camouflage or nocturnal activities for protection. The most important suborders are the Dorids and the Eolids.


Their main characteristics are the circlet of feathery gills at the anus and the sensory tentacles, rhinophores, at the front. Doridacea are the largest group of nudibranchs.

Goldlace Nudibranch
(Halgerda terramtuentis)

This animal is covered with ridges and bumps that crisscross with an irregular network of gold lines. It has white rhinophores and gills speckled with black. It is found in and around caves at 15 to 100 feet. They are endemic to Hawaii.

Red-Spotted Nudibranch
(Chromodeoris sp)

This is a medium sized nudibranch, mottled red and white with scattered red dots mostly around its dorsal center. These dots are fringed with white and sometimes form 2 loose rows. Its margin is white with a gold edge. It blends in well with its habitat.

Spanish Dancer
(Hexabranchus sangueiueus)

This is the largest and most conspicuous nudibranch in Hawaii. At night it is usually a blotchy pinkish red, while by day it is often pink to deep crimson. Its rhinophores and gills may have yellow or gold highlights. The mantle is bordered with white and yellow. This will flare out in a colorful extension if disturbed, an effort to make it look larger. It swims by flexing its body and undulating its outspread mantle similar to a Spanish dancer's skirt; hence its common name.

The small red Imperial Shrimp can sometimes be found living on the surface of this nudibranch. The Spanish Dancer lays a conspicuous egg casing that resembles beautiful pink or red roses usually located on a rocky substrate. These casings have the same sponge-derived toxins within them as the adult nudibranchs do. An Egg-eating Nudibranch (Favorinus japonicus) may be found within the egg casing. It feeds exclusively off the eggs of other sea slugs. It devours the egg mass from the inside out and often its own egg coils may be found within the Spanish Dancer's larger egg casing.

Tom Smith's Nudibranch
(Glossodoris tomsmithi)

This nudibranch is creamy white with porcelain white dots on its back. It has a ruffled margin rimmed in yellow. Rhinophores are blue-black as are the feathery circle of gills.

Trembling Nudibranch
(Chromodoris vibrata)

This is a very beautiful nudibranch from the family Chromodorididae. As with many in its family, its gills are constantly shaking and trembling. Its body is yellow with several white spots and rimmed in dark purple. Its gills and rhinophores are tipped with purple.

White-Margin Nudibranch
(Glossodoris rufomarginata)

This nudibranch will gather in small groups under ledges or on vertical faces to feed on a common gray sponge (Cacospongia sp). Its white egg cases are often nearby. It has a tan dorsal surface, speckled with orange-brown. It has a conspicuous white border around its body with an orange-brown ridge. Its gills and rhinophores are brown and white.

Family Phyllidia

This is a group of tough, firm-bodied dorids. Leaf-like secondary gills under the mantle have replaced the usual circle of gills. Dorsally there are brightly colored bumps and ridges. These animals are typically poisonous.

Varicose Phyllidia
(Phyllidia varicosa)

This nudibranch has a black body covered with pale blue ridges with rows of orange or yellow tipped bumps. It is also known as the Scrambled Egg Nudibranch. It preys on sponges (Hymeniaeidon sp) that contain a chemical poisonous to fishes and crustaceans. If bothered, it will secrete a toxic and very pungent mucus. This is the most common member of the family in Hawaii.

Pustulose Phyllidia
(Phyllidiella pustulosa)

This creature is found around areas of heavy coral growth. Its black body is studded with pinkish tubercles. It feeds on poisonous sponges and concentrates the toxins into its tissues for its own defense. It has bright colors to warn potential predators that it is poisonous.


These animals lack gills and are usually long and frilly, bearing pairs of feathery protuberances along the body. Eolids usually prey on hydroids, corals or other stinging animals and incorporate these stinging capsules (nematocysts) into their own body for protections. Some may also incorporate into their own tissue symbiotic algae (zooxanthellae) from their prey to augment their food supply and enhance their color.

Blue Dragon Nudibranch
(Pteraeolida lanthina)

This is a long, frilly, Chinese dragon-like creature with blue, purple and greenish colors. Different kinds and numbers of zooxanthellae probably account for the wide variety of colors. Sometimes their stinging cells are strong enough to be felt by humans. This animal is called the Purple Frilly locally.

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