Colour variation in Halgerda formosa

January 7, 2004
From: Bill Rudman

Yves Coze's series of photos of Halgerda formosa from Reunion Island give us a wonderful insight into the variation to be found in this species, which until recently had been known only from a colour painting accompanying its original description in 1880. As I wanted to keep the images reasonably large I have separated them into a number of separate messages [see message 1, message 2, message 3, message 4].

Anyone who has followed the recent messages on the Forum concerning species of Halgerda from the Indian Ocean will have noted that there were still many puzzles to resolve concerning just how many species existed and how characters should be interpreted.
Photo A: Pale colouration. Some yellow parallel lines, no transverse lines. 4 black spots on posterior mantle.
Photo B: Yellow reticulate ridges and secondary reticulate pattern in 'valleys'. Black spot on posterior foot.
Photo C: Yellow reticulate ridges and secondary parallel lines in 'valleys'. Some traces of secondary reticulation. 2-3 black spots on posterior foot and mantle.
Photo D, E: Yellow reticulate ridging and single secondary yellow longitudinal line in each valley. Single black spot on posterior mantle and foot.
Photo F: Parallel yellow lines distinct but few transverse ridges. Black spot on posterior Foot, no spot on mantle.
Photo G: Reticulate yellow ridges but little sign of longitudinal parallel yellow lines. One black spot on posterior foot and mantle.
Photo H: Reticulate yellow ridges and secondary longitudinal yellow lines. One black psot on postreior mantle, broad black band on posterior foot.
Photo I: Reticulate yellow ridges and secondary longitudinal yellow lines. Posterior end not visible for spots.
Photo J: Reticulate yellow ridges, but few secondary parallel lines. One black spot on posterior foot.
Feeding specimen Appears to have yellow longitudinal ridges. No transverse ridging. No black spots.

From these photos and others already on the Forum I think it is worthwhile to summarise the external characteristics of this species:
Mantle translucent white with a thin opaque white line at the mantle edge. There are scattered rounded tubercles, sometimes more pronounced in some animals than in others, and these are joined by a network of ridges which are edged in yellow or orange-yellow. There is a secondary pattern of yellow lines in the 'valleys' arranged in a parallel pattern oriented along the long axis of the body. In some cases there may be only on secondary line in each 'valley', and in other examples, the transverse ridges are absent and longitudinal parallel lines predominate. In one example [Photo B:] the secondary lines form a reticulate pattern. The obvious relationship of that specimen to the rest can be seen in another specimen [Photo C:] in which some of the parallel lines are branched and entangled.

The rhinophores have a black club, and running up the posterior side of the white rhinophore stalk is a band of black. The sparse white gills have a black line running along the inside [upper] edge. The lines on each gill join at the base. This is in contrast to the black line on the outer [lower] edge which stops near the base, so that the outer base of the gill circlet is translucent white.

There can be a few large rounded black spots on the mantle, usually at the posterior end behind the gills. In rare cases there can be one or two at the anterior end in front of the rhinophores. The white foot is edged with a diffuse band of yellow and at the posterior tip there can be one to three rounded black spots on the dorsal midline, or even a broad black band.

Yves'e photos are a valuable addition to our knowledge, and with earlier contributions from Marina Poddubetskaia, have certainly enabled me to clarify just what variability we can expect to find in this species. As I have often said, extra photos are never a waste. By being better able to define this species it means we can better understand other species with similar colour patterns. It certainly suggests that the problematic animals in Certainly animals such as those in message 5, message 6, message 7, message 8, message 9, which have been tentatively placed with H. formosa can no longer stay there. As I am posting in a message from Gavin Bushell, it seems they are a colour variant of Halgerda wasinensis.
Best wishes
Bill Rudman

Rudman, W.B., 2004 (Jan 7) Colour variation in Halgerda formosa. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/11824

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