Halimeda sea slug

Dear Eirik,
Do you know where the Halimeda came from, or is this like your earlier find of Tritoniopsis elegans, a lucky bonus from your aquarium shop?

From your drawings and detailed description I am pretty sure I can identify your animal as Elysiella pusilla. It is not actually a nudibranch, but it is an opisthobranch sea slug belonging in the ORDER Sacoglossa. If you have a look in the Species List at the names listed under Sacoglossa you will find the diversity and variety of shape in the sacoglossans is nearly equal to the nudibranchs, even if only a few are as brightly coloured. What is more remarkable is that all the sacoglossans are herbivores feeding by sucking cell sap from algae. Elysiella pusilla, like most sacoglossans is very specialised, feeding on only one or a small group of algae, in this case in the genus Halimeda. This genus of seaweeds are unusual because they have a calcareous outer skeleton. Because this makes the plant effectively rigid, it is broken into chains of jointed segments so that it can move in the currents without breaking.

The plant grows by adding new segments at the tip of each 'branch'. These new and growing segments are softer and more lightly calcified than the older segments, and Elysiella is usually found, well-camouflaged, on these younger segments. Interestingly, most new growth in Halimeda occurs at night, and at this time it concentrates toxic chemicals in these growing regions to deter predators. It doesn't seem to deter Elysiella.

Best wishes,
Bill Rudman.