Hexabranchus pulchellus Pease, 1860

Hexabranchus is a tough group. Specimens can be found throughout the Indo-Pacific, and there is considerable variation between populations in different areas. These differences led to a number of different names being applied to variations. In recent years, however, the general consensus has been that all these are different variations of the same species, a view substantiated Valdes (2002), who examined the internal anatomy of specimens from wide-ranging sites in the Indo-Pacific and could find no consistent morphological differences. While that is certainly evidence that there is only one Indo-Pacific species, I think this is one of those times we have to go beyond internal anatomy and look at the whole organism. In Hawaii, there are two quite distinct varieties of Hexabranchus, the animal figured on this page and Hexabranchus aureomarginatus. The latter is characterized by opaque white blotches dorsally and a distictly yellow mantle margin. If the forms just differed in coloration, I would be more tempted to accept the suggestion that they are simply different color forms. However, the egg masses of the two species are very different in appearance, and there appear to be differences in behavior. The combination of these consistent differences convinces me that there are two species in Hawaii. And if we have two species in Hawaii, that casts some doubt, for me at least, about whether all the Hexabranchus throughout the rest of the Indo-Pacific are the same. Fortunately, Hexabranchus is a popular photo subject and there are many photos of it on the web. Cory Pittman has used the web to perform an analysis of the distribution of color forms and has come up with a breakdown that divides Hexabranchus into a number of species. I find the analysis rather compelling. Of course, this kind of analysis can be based only on the external features visible in color photographs, and would be difficult to use for variably colored species. I think the final judgement on how many Hexabranchus species there are will need to wait for a study that analyzes multiple characters: anatomy, external morphology, life history, and DNA. In the meantime, this page will use the names used on Sea Slugs of Hawaii.

This species is common in Hawaii and grows to a larger size than H. aureomarginatus. It is most often found exposed on the reef at night, although small specimens may be found exposed during the day in tide pools. The first photo below was taken in a tide pool at Makapuu Point, Oahu. The egg masses of this species can be seen on Sea Slugs of Hawaii.

Juveniles usually bear spotting, which disappears as the animal grows.

Hexabranchus is given the common name Spanish Dancer for its behavior of unrollling its wide mantle margin and undulating through the water as it swims.

Often you can see one or two of these little commensal shrimp, Periclimenes imperator, living on the body of a large Hexabranchus.

Links:
Sea Slugs of Hawaii
Sea Slug Forum

References:

Pittman, C. (undated) Speculation on the Taxonomy of Hexabranchus. Sea Slugs of Hawaii.

Valdes, A. 2002. How many species of Hexabranchus (Opisthobranchia: Dorididae) are there? Molluscan Research 22:289-301.

Created 20 April 2009

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