Previously thought to be widely spread throughout Australia and the Indo-West Pacific but now known to occur only in New South Wales, eastern Australia.
UPPER: Feeding on sponge, Clathria aceratoobtusa (Microcionidae), Long Reef, Sydney, New South Wales, Australia. August 1986. LOWER: Bright red colour form on sponge. Coffs Harbour region, New South Wales, Australia. December 1990. AM C164554. PHOTOS: Bill Rudman
This species is often found crawling over rocks and algae but even though it is very conspicuous, its shape and texture makes it easily mistaken for a small patch of inedible distasteful sponge. Most species of Rostanga feed on orange or reddish-orange sponges, and they appear to retain the sponge's colour pigments in their skin to give them an almost perfect colour match with their food.
The mantle is orange-red with a close covering of colourless translucent caryophyllidia. On the basal mantle epidermis is a scattering of dark brown specks and patches, the density of brown markings varying in different specimens. In some specimens there is some white pigmentation between the rhinophores, and in most animals large whitish mantle glands are clearly visible forming a submarginal row around the mantle. Each large gland is usually surrounded by small white specks and sometimes small white secondary glands. The rhinophore stalk is translucent clear and the club is dark-brown to black with some white specks on the lamellae. The terminal knob on the rhinophore club is a translucent straw colour. The gills are watery orange with some brown dusting. The mantle is ovate and the visceral hump between the gills and rhinophores appears more raised in stationary than in moving specimens. The mantle is densely covered with caryophyllidia. The rhinophore stalk is short and inflated, and the ovate club has approximately ten lamellae arranged almost horizontally, with only a slight downward slope posteriorly. There is a rounded terminal knob to the rhinophore club. There are approximately 12 gills arranged in a close upright circle around the anal papilla. Each gill is tripinnate but the pinnules are relatively short with few secondary branches. Towards the tip of each gill the pinnules become progressively shorter to produce a rounded tip so that the complete gill circlet has a crenulate edge.
The egg ribbon consists of a thick rigid jelly containing a coiled string of large bright orange eggs. It consists of one to two and one half coils. The embryos pass through a reduced veliger stage with a cap-shaped shell before hatching as benthic juveniles fourteen or fifteen days after oviposition.
It is found at low tide on rock platforms crawling over rock and algal turf in the upper eulittoral zone and is often in association with an encrusting bright red poecilosclerid sponge Clathria (Ophlitaspongia) aceratoobtusa (Carter, 1887) (Fam: Microcionidae).
• Rudman, W.B., Avern, G. (1989) The genus Rostanga (Nudibranchia: Dorididae) in the Indo-West Pacific. Zoological Journal of the Linnean Society 96(3): 281-338.
Rudman, W.B., 2002 (February 5) Rostanga arbutus (Angas, 1864). [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/rostarbu