Plakobranchus ocellatus
van Hasselt, 1824

Order: SACOGLOSSA
Superfamily: ELYSIOIDEA
Family: Plakobranchidae

DISTRIBUTION

Found throughout the tropical Indo-West Pacific.

PHOTO

Animal on right with its parapodial flaps pulled open to show the longitudinal ridges which line the inside of the "parapodial cavity". The bright green colour is caused by the countless chloroplasts kept there by Plakobranchus.
Osterland Reef,Great Barrier Reef, August 1987. Length 5cm. PHOTO: Bill Rudman.

RELATED TOPIC

Solar-powered Sea Slugs

Plakobranchus ocellatus is a common inhabitant of shallow coral reef pools and lagoons. It is seldom seen as it is well-camouflaged, half buried in the coral sand. Although to my knowledge it has never been studied by algal physiologists, it deserves their attention. As I show in the photo above, it stores huge numbers of bright green chloroplasts in ridges hidden from view beneath the parapodial flaps. It may seem puzzling that this slug has evolved a method of storing chloroplasts, but at the same time shades them from the sunlight which would allow them to photosynthesise and provide nourishment for the slug. I have a theory, which is totally untested, that Plakobranchus deliberately shades the chloroplasts from the bright tropical sun. Most chloroplasts have an optimal light intensity at which they photosynthesise. They can be "burnt out" if exposed to too much light. By covering the chloroplasts with the parapodia, Plakobranchus rather than cutting the chloroplasts off from light, is just reducing the light levels to a better level for photosynthesis. I would love someone to test this hypothesis... Bill Rudman.

References:
• Hasselt, J.C.v. (1824). In: Andre Férussac. Extrait d'une lettre du Dr. J.C. van Hasselt au Prof. van Swinderen sur mollusques de Java (traduit de l'Allgem. konst en letterbode, 1824, nos. 2,3,4) Tjuringe (ile Java) le 25 Mai 1823 (1). Bulletin des Sciences Naturelle et de Geologie, 3: 237-248.
• Jensen, K.R. (1992). Anatomy of some Indo-Pacific Elysiidae (Opisthobranchia: Sacoglossa [=Ascoglossa]), with a discussion of the generic division and phylogeny. Mal. Soc. London, 58: 257-96.

Authorship details
Rudman, W.B., 1998 (December 11) Plakobranchus ocellatus van Hasselt, 1824. [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/placocel

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