Sap-sucking slugs, headshield slugs, sea hares & polyclad flatworms. Part 20 of my documentary, "Mucky Secrets", about the fascinating marine creatures of the Lembeh Strait in Indonesia. Watch the full 90-minute documentary at    • Mucky Secrets (full) - The Marine Cre...  

The sap-sucking slug (Sacoglossa, sacoglossan) Elysia sp. is not a nudibranch. It does not have gills as such but breathes through two leafy flaps called parapodia that run most of the length of its body. The rhinophores on its head have a semi-tubular form. It feeds by sucking the fluid from green algae, and the chloroplasts it contains give the body a bright green colour which fades if the slug goes short of food. Behind the rhinophores it has tiny photo-receptors for eyes. The white spots are raised glands that can secrete a repellent white substance.

Headshield slugs (family Aglajidae, superfamily Philinoidea, clade Cephalaspidea) lack tentacles and most retain a small thin internal shell. They also have parapodia, which are wrapped up and around the body. Many excrete mucous to help them burrow into the substrate, and the headshield prevents sand entering the mantle cavity. The Gardiner's headshield slug (Philinopsis gardineri) feeds on polychaete worms. And the pleasant headshield slug (Chelidonura amoena) feeds exclusively on acoel flatworms that infest hard corals and sponges. Small, dark eyespots on the front of its head give it very primitive vision.

Like the striated frogfish, the ragged sea hare (Bursatella leachii) is camouflaged with long papillae that help it disappear on a seabed strewn with algae. Sea hares (family Aplysiidae, superfamily Aplysioidea, clade Aplysiomorpha) get their name from the overall body shape and the long pair of rhinophores on the head, which are tubular, and give it an acute sense of smell. It also has a second pair of tentacles at the sides of the mouth and it gobbles up the thin layer of cyanobacteria that coats the seabed. Below the rhinophores it has a pair of tiny eyes. If it is disturbed it can release a noxious mixture of white opaline and purple ink. Recent research has shown that this sticks to the antennae of predators such as lobsters and dulls their senses. The bright blue eyespots covering the body are more vivid here than in populations in other parts of the world.

Ragged sea hares and the similar but smaller lined sea hare (Stylocheilus striatus) sometimes form huge swarming aggregations comprising hundreds or even thousands of individuals of varying size. They tumble over each other, devouring cyanobacteria and defecating as they stampede across the sea floor. In an aggregation they are an easy target for predators. Pufferfishes and predatory sea slugs have been seen to pick them off one by one. They breed quickly and have even been sold into the aquarium trade as "sea bunnies" for eating unwanted algae and providing food for other tank inhabitants with their larvae. It is said that inhabitants of some of the Cook Islands and Austral Islands collect and eat swarms of these sea hares, discarding the toxic internal organs. It is a mystery why sea hares aggregate like this. They have been observed to all mate, spawn and die at the same time.

Although they resemble sea slugs, polyclad flatworms (Polycladida) are quite different. The ruffled periphery of the glorious flatworm, Pseudobiceros gloriosus, forms a pair of pseudotentacles reminiscent of nudibranchs' rhinophores. Occasionally flatworms leave the seabed to swim and when they do, they are a spectacular sight.

There are English captions showing either the full narration or the common and scientific names of the marine life, along with the dive site names.

Thanks to Chris Zabriskie (http://chriszabriskie.com) for the music track, "Air Hockey Saloon" and to Purple Planet (http://www.purple-planet.com) for the music, "Mountain Breeze (pad)". These tracks are licensed under a Creative Commons Attribution 3.0 Unported license.

Thanks to the staff and keen-eyed divemasters of Two Fish Divers (http://www.twofishdivers.com), for accommodation, diving services and critter-spotting.

I have more scuba diving videos and underwater footage on my website at:
http://www.bubblevision.com

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Full list of marine life and dive sites featured in this video:

00:00 Sap-sucking Slug, Elysia sp., Hairball
00:48 Headshield Slug, Philinopsis gardineri, Makawide
01:16 Headshield Slug, Chelidonura amoena, Aer Perang
01:37 Ragged Sea Hare, Bursatella leachii, Hairball
02:55 Lined Sea Hare, Stylocheilus striatus, Retak Larry
04:09 Glorious Flatworm, Pseudobiceros gloriosus, Retak Larry