Halgerda tessellata is a dorid nudibranch. As with other nudibranchs, they are shell-less marine gastropod mollusks. To make up for the lack of protection from not having a shell, they are usually highly poisonous. They are rare in Tanzanian waters and we have sighted two specimens on several hundred dives. They derive their name from the word tessellate, which means shapes that fit together with no spaces in between. Their contrasting coloration makes them a stunning sight close up as can be seen in the images.
They are a fairly small species of nudibranch growing up to 50 mm. Across the dorsal part of the body is a pattern of raised orange ridges which may be more reddish in some specimens. Between the ridges is black with a large number of white dots. The gills are white and have black markings on the sides. The rhinophores are yellow on the base and black on the tips. The rear of the mantle has a black stripe down it. A very attractive looking nudibranch.
HALGERDA TESSELLATA IN THE WILD
Halgerda tessellata are rare and are not commonly seen. We have seen two on several hundred dives. On all occasions they have been seen during the day on exposed seaward reefs in approximately 15 meters of water. They do not seem to appreciate lights on them and head for cover very quickly.
The distribution of Halgerda tessellata is from the East Coast of Africa across to Australia. They are usually seen on coral reefs feeding on sponges.
The Halgerda tessellata feed exclusively on sponges. They may absorb poisons from the sponges, which provide them with protection.
As with all nudibranchs they are hermaphrodites. They are unable to self fertilize and must mate with another member of their species. To mate, two approach each other and deposit sperm packages into each other. A specialised sexual organ on the right side of the body means that mating is done in a head to tail fashion right side facing . Both specimens then lay eggs which are typically laid in a spiral gelatinous mass, which is attached to a substrate.
HALGERDA TESSELLATA CLASSIFICATION
(unranked): clade Heterobranchia
Species: H. tessellata