The name says it all. It has six gills – Hexabranchus. The family and genus hold six separate species. The species Hexabranchus sanguineus (sanguineus – blood-coloured) was originally described from the Red Sea. Specimens of that species from that region are not red and white mottled as in the images posted here but are a deep and uniform “blood” colour. Hexabranchus sanguineus is now known to be a complex of five species across the broad Indo-Pacific.
These nudibranchs, also known as the Spanish Dancer because of the display they put on when disturbed, is unusual among the dorids in having six separately inserted gills. Actually the number can vary. These gills are arranged posteriorly on the dorsum in a circle around the anus and each one can contract (not completely retract) 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.
– Tibiriçá, Y., Pola, M., Pittman, C., Gosliner, T. M., Malaquias, M. A. & Cervera, J. L. (2023). A Spanish dancer? No! A troupe of dancers: a review of the family Hexabranchidae Bergh, 1891 (Gastropoda, Heterobranchia, Nudibranchia). Organisms Diversity & Evolution. Published online: 20 June 2023, https://doi.org/10.1007/s13127-023-00611-0
David A. Mullins – July 2020 (Revised July2023)