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Related Articles: Abalone, Prosobranchs, Nudibranchs, Mollusks, Algae Control

/The Conscientious Marine Aquarist

The Stomach-Footed Mollusks, Class Gastropoda, Subclass Opisthobranchia, Sea Slugs

by Bob Fenner

Aplysia californica... Coldwater...

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.

Aplysia californica, the California Sea Hare. Found off the two Californias... Baja and the U.S.... coldwater (NOT tropical). To fifteen inches, several pounds. At right, one being mis-offered as a tropical in a South Carolina retail store. Below: one off of San Diego, intertidal egg masses (resemble tangled spaghetti, can be of many colors) and large individual left high (and moist) with a minus-tide event. http://www.seaslugforum.net/factsheet.cfm?base=aplycali

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Aplysia dactylomela, the Spotted Sea Hare. 3-6 inches usually, 12 in. maximum. Shallow to 120 feet in depth. Worldwide, circum-tropical distribution. Feed on algae. Discharge purple "ink" if disturbed... may be problematical in captive conditions. Two photographed at night off of Cozumel.


Petalifera ramosa. To 2 cm. S. Japan to Indonesia. Usually found on Caulerpa. Here on the substrate in N. Sulawesi.

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. 

Chelidonura amoena, E. Indian Ocean to S. China Sea over to Hawaii. Raja Ampat 08.


Chelidonura inermis, the Striped Sea Slug. San Diego coastal image. 

Chelidonura inornata Indonesia and trop. Australia. To 4 cm. Raja Ampat 08.


Chelidonura varians Eliot 1903, THE Flatworm Eating Sea Slug! Tropical Indo-West Pacific. To seven cm. in length. This one off of Northern Sulawesi, Indonesia. Similar to Philinopsis gardineri, but with much longer rear projections.
Verticals (Full/Cover Page Sizes Available
Philine rubrata Gosliner, 1988. West, Central Indo-Pacific. To 3.5 mm... yeah, small. Fiji pic.

Philinopsis gardineri (Eliot 1903). Likely feed on Polychaete (bristle) worms rather than opisthobranchs. Similar to Chelidonura varians, but with much shorter rear projections.


Sagaminopteron psychedelicum. Psychedelic Batwing Slug. Indo-W. Pacific; incl. Hawaii and Fiji. Bali 2014

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.

Berthella martensi Pilsbry 1896, Highly variable in color, markings. Sponge feeder noted for shedding its mantle in three pieces.   S. Leyte 2013.
Pleurobranchus sp. Lembeh 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.  

Costasiella kuroshimae, Kuro Sapsucking Slug. To 7 mm/.25". Indo-West Pacific. Bali 2014

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. 

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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. 

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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. 

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Elysia (Thuridilla) picta (Verrill 1901), 1.5 inch max. Red, blue and green rhinophores; yellow head. Cozumel 2011.


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Genus Stiliger:

Stiliger ornatus Ehrenberg 1828. Trop. Indo-W. Pacific. Bali 2014

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.

Thuridilla sp. Pemuteran, Bali, Indo. 2014

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

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