The name says it all. It has six gills – Hexabranchus, and is blood-coloured – sanguineus. This species was originally described from the Red Sea. Specimens from that region are not red and white mottled as in the images posted here but are a deep and uniform “blood” colour.
This nudibranch, also known as the Spanish Dancer because of the display it puts on when disturbed, is unusual among the dorids in having six separately inserted gills. These are arranged posteriorly on the dorsum in a circle around the anus and each one can contract into its own “pocket” for protection. However, it has recently been considered a phanerobranch and not a cryptobranch because those gill “pockets” do not close over to completely cover the contracted gills. With the cryptobranchs, the separate branches of the gill are joined at the base forming a single entity that retracts completely into a common pocket beneath the mantle.
The six gills referred to in its scientific name is a generalisation as the number can vary either side of that number, notwithstanding that it is also possible that a fish may take a bite and nip off one or more.
In juvenile specimens it is interesting to note that the surface of the mantle in-between the six separately inserted gills is often quite translucent and that their pockets have a slightly raised rim that seem to join up forming a ridge. Rudman, in 2006, has suggested that it is worth investigating whether this is indicative of a remnant gill pocket, noticeable in juveniles, that has been lost in the evolution of the species and asks: “Does that make Hexabranchus a very special cryptobranch, or does it suggests that it is a link between the two groups, indicating that the phanerobranchs evolved from the cryptobranchs?”
– Thompson, T. E. (1972). Observations on Hexabranchus from the Australian Great Barrier Reef (Gastropoda : Opisthobranchia). The Veliger 15 (1), 1-5.
– Francis, M. P. (1980). Habitat, food and reproductive activity of the nudibranch Hexabranchus sanguineus on Tongatapu Island. The Veliger 22 (3), 252–258.
– Rudman, W. B., (1999, March 31). Hexabranchus sanguineus (Ruppell & Leuckart, 1828). [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/hexasang and associated messages.
– Wägele, H., and Willan, R. C. (2000). Phylogeny of the Nudibranchia. Zoological Journal of the Linnean Society 130, 83–181.
– Valdes, A. (2002). How many species of Hexabranchus (Opisthobranchia : Dorididae) are there? Molluscan Research 22, 289-301.
David A. Mullins – July 2020