#headshield slug

Colpodaspis thompson

Chelidonura hirundinina is a common headshield slug in the subtidal reefs of Hengchun. Their shells are completely reduced, and have those iconic swallow tail shaped mantle that's shared in this genus. Just like the hammerhead sharks, the margins of their "headshields" are lined with small sensory cilia that help them find their prey, and in the case of C. hirundinina, the menu mostly consists of acoel flatworms.

Photographer: David Hall

Kauai, Summer 2018

Tubulophilinopsis gardineri

Sagaminopteron psychedelicum

Images: Magnus Kjaergaard and Ed Bierman

Part 20, the final part of my documentary, "Mucky Secrets", about the fascinating marine creatures of the Lembeh Strait in Indonesia. 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 long-tailed sea hare (Stylocheilus longicauda) 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.