Re: Variations in Hypselodoris carnea

Dear Colin,

It certainly looks as though the brown background colour in your upper photos is from the internal digestive gland and the yellowish band around the mantle edge could possibly be the result of mantle gland contents.

Concerning the yellow sponge which your animal looks to be feeding on. I am afraid I can't see any structural details which I can use. Amost all species of Hypselodoris feed on species of Dysidea, or related genera, and like the sponge in the lower photo the colony has cone-like shapes all over and a radiating pattern and network of structural fibres which are usually quite obvious. Although at times the colonies can be reasonably large and thick, often they consist of a very thin layer over the substrate, or over another sponge colony. It's possible in this case that the sponge the slug is eating is a fine layer over the yellow sponge which we can't really see, but I really can't say. It would be interesting to see what the sponge cover looked like just above the slug? If you have a photo showing just a bit more above the slug it might be useful. As to learning more about sponges, I'm afraid there is not an easily available general book, and no easy way for amateurs - or non-sponge specialists - to identify most sponges. This is for a host of reasons, not the least being that there is still a lot of research needed to sort out sponge taxonomy. Another big problem is that sponge colonies are often very variable in shape and even colour and so species can often only be identified by cutting sections through the tissue for microscopic examination. In sponges with skeletal spicules, we need to examine not only the shape of the spicules, but how they are arranged in the sponge colony. Most sponges eaten by chromodorids present even greater problems because they lack spicules, so we are left with examining the arrangement of the fibrous network to determine species and families. My advice is to try and become familiar with your local sponge fauna even if you can't actually name species. If you find slugs taking a particular interest in particular sponges, keep taking photos of both the sponge and the slug. It is proving an invaluable help.

Thanks very much,
Bill Rudman