Nudibranchs in this genus are large and red, and are characterized by gill branches that protrude from six holes in the dorsum. Many different species have been named. While the consensus among most nudibranch workers nowadays is that all Indo-Pacific Hexabranchus are part of one highly variable species, it seems that only Valdés, 2002, has looked at specimens from various parts of the range to try to confirm this. While Valdés' study supported the notion that there is only one species, my observations in the Marshalls and Hawaii make me believe there is still a bit more to the story. For more information (and speculation), see the page on Hexabranchus pulchellus from Hawaii. It seems likely to me that the Marshall Islands' Hexabranchus sanguineus is not the same as the species originally described from the Red Sea, but for now we are leaving it under that name. In the Marshalls, Hexabranchus specimens tend to hide by day but will often come out of their holes at night to wander in the open. They are not commonly found in the Marshalls, but a few have been seen at Enewetak and Kwajalein Atolls. They have been found in lagoon and seaward reef habitats from the surface--one was found still alive, just washed in on the eastern seaward beach of Enewetak--to depths of more than 50 meters on the seaward reef slope.
The animal immediately below was found under a rock on a Kwajalein Atoll seaward reef on 11 April 2010.
A similarly colored young specimen from a Kwajalein lagoon reef on 3 October 2011.
The photo below shows the wavy oral tentacles that flank the mouth on the underside of the anterior end.
A head-on view of another reveals quite a face!
This is the same individual as in the face shot above. Nice large set of gills.
They swim by dorsal-ventral bending of the body and sending waves along the unfolded spanish dancer skirt.
The individual below is the one that was washed in on the beach at Enewetak. When returned to the depths, it seemed to be fine and crawled away.
Hexabranchus sanguineus is one of the few nudibranchs with a well-known common name. It swims when disturbed, and it can flare out its normally rolled up margins to look like the flowing skirts of a dancer. Not surprisingly, it is known as the Spanish Dancer nudibranch. It swims by flexing its body up and down and paddling itself through the water.
This is by far the largest nudibranch found so far in the Marshalls. The individual below was one of four individuals observed one night (29 Sep 1989) on the leeward seaward reef slope at a depth of about 20 meters. They were not accurately measured, but by measuring them against camera gear it was obvious that each of the four exceeded 500mm in length. There were also several egg masses scattered about that night. In the 17 years since then, despite hundreds of dives in the same general area and under similar conditions, we have seen large Hexabranchus only a couple more times and never more than one at a time. You'd think something that large would not be able to hide that well.
Young specimens are very colorful, almost looking like a kind of chromodorid nudibranch.
At 8mm long, the individual directly below was one of the smaller ones we have found, beaten by the following photo of a 6mm animal.
These two shots by Christina Sylvester show another very small individual found in a Kwajalein reef quarry. Its dorsal spots are reminiscent of what we call young specimens of Hexabranchus pulchellus from Hawaii.
More from the Kwajalein reef quarries. It seems these quarries make good nurseries for this species.
Created 15 December 2006
Updated 16 July 2016
Return to phanerobranch thumbnails