Subclass Opisthobranchia, Sea Slugs, Largely are missing or have reduced shells. Have other (chemical, biological defenses), camouflage. Comprise five living Orders: Anaspidea (Sea Hares), Cephalaspidea (Bubble Shells and Headshield Slugs), Notospidea (Side-Gilled Slugs), Nudibranchia (Nudibranchs) and Sacoglossa (Sap-Sucking Slugs). Opisthobranch means "gills behind" where most of these gastropods have their gills and anus situated (as opposed to the Prosobranch snails with their anus and gills up front). Opisthobranchs are hermaphroditic, both functional females and males in one body, though cross-fertilization is the rule. Laid eggs in clusters or bands hatch out to pelagic trochophore larvae which metamorphose into veligers, finally settling down as miniatures of their parents. There are some 2,000 species worldwide, and about 150 of these can be found in Hawaiian waters. 

Order Anaspidea, Sea Hares: Named for their rabbit ear-like projections called rhinophores (used for taste, current detection), and two anterior-projecting oral tentacles. Some can swim in the water column. Herbivorous, eating Red, Green, Brown Algae and Seagrasses. Can be dangerous in captivity if disturbed to the point of releasing fish-repelling purple "ink"...

Family Aplysiidae: In old Hawai'i sea hares were called Kualakai and some were cooked in an imu wrapped in ti leaves... and consumed.

Order Cephalaspidea, the Headshield Slugs: Worldwide, often have shells (often reduced, internal), often with operculums. Burrowing marine forms mainly that eat bristleworms, other worms, foraminiferans, bivalves, snails. Named for wedge-shaped heads that many use to plow through the sand, generally living just below its surface. 

Philinopsis gardineri (Eliot 1903). Likely feed on Polychaete (bristle) worms rather than opisthobranchs. One in N. Sulawesi.

Family Bullidae: Bubble Snails/Headshield Slugs.

Bulla striata, the Striate Bubble. Tropical West Atlantic. 1- 1 1/2"

Order Notospidea: Sidegill Slugs. May have a shell inside the body, outside, or absent entirely. The group is named for the location of their large plume-like feathery gills (between their mantle and foot on the right side of the body). Nutritionally their grazing carnivores, preying on sponges, sea squirts and other bottom-dwelling invertebrates, even fishes, with their large radulas and wide jaws. They are rarely preyed upon due to the release of self-produced sulfuric acid when bothered.

Pleurobranchus sp. Lembah Strait 08.

Order Sacoglossa: Sap-Sucking Slugs. Specialized feeders on algae, mainly seaweeds in shallow waters (Common name for having a radula of one row of piercing teeth for suctorial feeding on algae... whose chloroplasts they sometimes retain and use for solar-powered sources of sugars). Shelled and shell-less/slug-like. Elysia, Alderia, Berthelina. 

Elysia (Tridachia) crispata, the Lettuce Sea Slug (not a Nudibranch), may be brown, green or yellow in general coloration. Needs live rock, algal growth for food. Take care to screen pump intakes as this animal is often sucked into them. To 1 1/2 inches in length. Aquarium and Bonaire photos.

Bigger PIX: The images in this table are linked to large (desktop size) copies. Click on "framed" images to go to the larger size.

Elysia diomedea, Mexican Dancer. Bears black and yellow lines on the frilled parapodial folds. Sea of Cortez to Panama. To 2 inches in length. This one off of Cabo San Lucas.

Elysia expansa. W. Australia, Indonesia. To 3 cm. Here in N. Sulawesi.

Elysia ornata (Swainson 1840), known to science as the Ornate Sap-Sucking Slug. This genus is often sold to aid in the control of marine aquarium algae overgrowth, particularly Bryopsis. Much easier to control such outbreaks by limiting nutrients, providing competitors for light, food, using more generalized herbivores. To about an inch and a half in length.

Genus Thuridilla: 13 species, 11 Indo-Pacific.

Thuridilla bayeri.  Madagascar to Marshall Islands. Indonesia. To 2 cm. One the size of a ladybug/bird or eraser head in Raja Ampat.

Thuridilla lineolata.  Indonesia. To 2 cm. N. Sulawesi pic.

Thuridilla undula, Gosliner 1995.  Blue bodied, with black and orange markings. S. Sulawesi pic.

Order Nudibranchia- Naked-gill sea slugs

Bibliography/Further Information:

The Sea Slug Forum (great online scientific resource)

South west Indian ocean Seaslug: http://seaslugs.free.fr/index.htm