Iridescent nudibranch Notobryon wardi
Hitch-hiking skeleton shrimp, of the Caprella genus.
A different iridescent nudibranch Notobryon wardi
But again hitch-hiking skeleton shrimp, including juveniles and maybe eggs?
While photographing some iridescent nudibranchs I concentrated on the convoluted gills. These are semi-transparent, seemingly filamentous, and very hard to make out exactly. On looking at the resulting photos later I discovered that the gill area, which is between two sets of flattened lobes, was full of skeleton shrimp! These shrimp are highly modified amphipods, with large, powerful nippers, and a set of legs with which it holds on. This allows them to rise up and grab plankton floating by. They normally remain on algae or hydroids, which is where I have seen them before.
Looking at previous photos of iridescent nudibranchs revealed that I had captured this before, just not realised it. On subsequent dives I found more nudibranchs, some of which again had skeleton shrimp. It seems that they make themselves at home on the nudibranch, as the bottom photo shows adults and juveniles, and maybe eggs? The skeleton shrimps would benefit from their mobile host, as they are normally sessile - moving around will expose them to a more varied food source. What the nudibranch gains is not known, and I would guess it does not gain anything.
So far this phenomena has only been observed on Atlantic sea-board iridescent nudibranchs, as my False Bay photos do not reveal any, but it will be interesting to take a closer look when winter sets in and False Bay becomes cleaner and more diveable.
Thanks to Dr. Griffiths of UCT for information.
Nudibranchs of Southern Africa - a guide to the opisthobranch molluscs of southern Africa, T Gosliner, 1987.
Two Oceans, A guide to the marine life of Southern Africa, GM Branch, CL Griffiths, ML Branch and LE Beckley, 1994.