Halgerda wasinensis - colour variation

January 7, 2004
From: Bill Rudman

While looking at Gavin Bushell's photos of Halgerda wasinensis I suddenly realised they are a sort of 'Rosetta Stone' linking typical Halgerda wasinensis with a number of problematical animals which lack the black background colour, and have had a tortured presence on the Forum as 'perhaps' a strange form of Halgerda formosa. A recent example of this is Marina Poddubetskaia's message from Mayotte.

We normally think of H. wasinensis as having a black mantle and a network of yellow ridges, but as Gavin Bushell's photos show the colour is more complex than that. The black is made of two components. The first component is a large black spot in each 'valley' between the yellow ridges. The second component is a tough fibrous layer which encloses the visceral mass internally. In many species of Halgerda this visceral envelope is black. It is visible through the translucent body wall and gives this species its usually very dark background colour. In some animals the black spot on the skin would appear to enlarge to fill the whole space between the yellow ridges while in other animals the black internal envelope covering the viscera lacks black pigmentation or else the body wall is sufficiently thick to prevent the colour showing through the body wall. In these instances the animal appears white with scattered black spots. If this interpretation is correct then the animal I misidentified as Halgerda punctata in an earlier publication (Rudman, 1978) is almost certainly this species, as are most of the problematic specimens at present on the Forum under H. formosa, including photo 1, photo 2, photo 3, photo 1, photo 1.

Halgerda wasinensis can be defined externally as having a mantle covered with a reticulate pattern of thin yellow ridges and often with a strong background colour of black caused by the dark internal visceral envelope showing through the translucent body wall. Usually there is a rounded black spot in each 'valley' between the yellow ridges, and around the mantle edge there are many black spots. In some animals black spots are completely absent from the mantle.

The upper two-thirds of the rhinophore club is black, the lower third white,, and there are black spots, sometimes forming a continuous line along the posterior side of the white rhinophore stalk. The translucent white gills have scattered black spots and markings. The black sometimes appears as a continuous line but in smaller individuals it is present as a black tip. There are many black spots in a band around the edge of the white foot.

If this interpretation is correct then this species occurs throughout the Indian Ocean from eastern Africa to the Similan Ids in the Andaman Sea and Christmas Island, just south of Jakarta, Indonesia.

• Rudman, W.B. (1978) The dorid opisthobranch genera Halgerda and Sclerodoris from the Indo-West Pacific. Zoological Journal of the Linnean Society, 62: 59-88.

Bill Rudman

Rudman, W.B., 2004 (Jan 7) Halgerda wasinensis - colour variation. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/11828

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