Halgerda Formosa 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. We have sighted five specimens on several hundred dives.
They are a fairly large species of nudibranch growing up to 60 mm. They have a translucent white body crossed by a reticulate pattern of orange ridges. The ridges are more pronounced when they are not at full stretch. On the upper dorsal section there are blackish marks between the orange lines. There is a thin white line around the edge of the mantle and there are black dots on the foot and around the edge of the mantle. The rhinophores are white with black marks. The gills are also white and have black markings on the sides. A very attractive looking nudibranch. On the upper dorsal section ahead of the gills there are two or three holes into the body.
HALGERDA FORMOSA IN THE WILD
Halgerda formosa are rare and are they are not commonly seen. We have seen three on several hundred dives. On all occasions they have been seen during the day on exposed seaward reefs in approximately 12 meters of water. They do not seem to appreciate lights on them and head for cover very quickly. When photographing one of the specimens in the post something very unusual occurred.
As I found the specimen, so it was perched on a sponge. I took two photographs and realized I was using the wrong settings. I moved the camera and lights to the side to change settings and as I was doing this, out the corner of my eye I saw something fly about a meter above the bottom in an arc. I looked for the Halgera Formosa and it had gone. I followed the trajectory of the object that had flown though the water and found the specimen rolled up as set out below. I have no idea how it shot across such a distance, there was no current and no other diver nearby that could have contributed towards its motion. It unrolled and crawled off.
The distribution of Halgerda formosa seems uncertain, it is recorded from Reunion, Mauritius, Mayotte and Madagascar. We have seen specimens around Dar es Salaam and on Mafia Island in Tanzania. They are usually seen on coral reefs feeding on sponges.
The Halgerda Formosa 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 FORMOSA CLASSIFICATION
(unranked): clade Heterobranchia
Species: H. Formosa