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Plakobranchus ocellatus van Hasselt, 1824

Plakobranchus ocellatus slides across the sandy substrate.

Above: Plakobranchus ocellatus showing the large parapodia meeting at the midline that cover the
digestive gland containing myriad chloroplasts sequested from their algal food.
Kavieng Harbour, Papua New Guinea. (40 mm)

Plakobranchus ocellatus van Hasselt, 1824

Family: Plakobranchidae

Most of us would be aware of the symbiotic relationship that exists between corals, which are animals, and zooxanthellae, which are algal plant cells (dinoflagellates) that live in the tissues of the coral. This is a mutualistic relationship. The coral polyps receive the benefit of nutrients produced by the photosynthetic activity of the zooxanthellae that in return benefit from the metabolic waste products of the coral – a win – win situation. Some species of nudibranch also utilise this same method.

Kleptoplasty (literally theft of plastids) on the other hand is a radically different process. It is unique in the animal world to certain species of sacoglossan sea slugs. Here we have utilization of the actual chloroplasts from the algal cells upon which they feed rather than a symbiotic relationship with an intact plant cell.

By rupturing the cell walls of the algae it is feeding upon the sea slug can suck out the contents for digestion. Part of those contents includes chloroplasts, the plant cell organelles that carry out the process of photosynthesis whereby (using a simplified explanation) the light energy of the sun is used to convert carbon dioxide and water into sugars and oxygen. The sea slug is able to sequester the chloroplasts intact and by retaining them within specialized cells of the digestive gland obtain the benefit of the products produced. The chloroplasts survive for periods of several hours up to several months depending upon the species of sea slug. Some of those species depend upon the “farmed” chloroplasts for up to a quarter of their energy needs.

This is an amazing enough process in itself but the real eye-opener might come from examining the method by which the chloroplasts function and are maintained. The chloroplast requires “instructions” from the genes of the actual plant cell nucleus. This would seem to suggest therefore the transfer of algal nuclear genes to a molluscan host. Recent research has shown that the sea slug does not at the same time absorb these plant genes for those “instructions” to be given so some other, as yet undiscovered method must be at work.

Some of the humble and simple sap-sucking sea slugs are actually proving to be remarkable and complex creatures, evolving one might say, almost as hybrid plant-animals.

Plakobranchus ocellatus is one such sacoglossan sea slug. It is a long-term retainer of functional chloroplasts (up to ten months) and has a body form suitable for maximizing their efficiency. It has a wide and flattened body with parapodia that can be opened or closed to regulate the quantity and intensity of light falling upon the huge numbers of chloroplasts contained in the ridged digestive gland beneath. This ability has led to it and its relatives being dubbed Solar Powered Sea Slugs. It has been recorded feeding on a variety of of algal species. Mostly observed living on shallow sandy substrates into which it likes to burrow. Recorded growing to 70 mm in length. Distribution covers the entire Indo-Pacific region from East Africa to the Hawaiian Islands.

Plakobranchus ocellatus starting to burrow under the sandy substrate.

Above: Plakobranchus ocellatus starting to burrow
under the sandy substrate

Plakobranchus ocellatus grazing on algae.

Above: The sap sucking sea slug Plakobranchus
grazing on algae to obtain the chloroplasts
that provide it with an ongoing source of nutriment.

(Photos and text by David Mullins)