Phyllodesmium magnum - trailing observation

Dear Jean-François,
Thanks for this interesting observation. We are building quite a list of species on the Trailing page. This is the second record we have of this aeolid, Phyllodesmium magnum, trailing. At such a size it must be quite a spectacular sight. The other record is Binyamin Koretz's [message #14655] from the Red Sea.

As I have discussed elsewhere, we aren't really sure why this form of close trailing - head on tail - occurs. Those species of aglajids such as Navanax inermis, which feed on other opisthobranchs, certainly follow mucus trails in search of prey, and at times end up eating their own kind. Related species, such as Chelidonura varians, feed on flatworms, but also exhibit this trailing behaviour. I can understand that following mucus trails of your own species could be useful in finding a mate to breed with or even a source of food, but I can't see why some species have developed this extreme behaviour of not even losing touch with their 'friend'. Perhaps finding a mate is so difficult for these species that they can't afford to literally lose touch.

You ask how opisthobranchs normally find partners. Although many species use their chemosensory rhinophores to find their food, I know of no studies to show they can find partners by waterborne chemicals, except for the fairly recent discovery of a 'love drug' in Sea Hares which I mentioned a few days ago [message #17419] which seems to attract sea hares to others of their own kind who are egg-laying. It's possible that there are other waterborne signals for other opisthobranchs but at present it seems they meet either by chance, usually as an aggregation at a food item. We clearly have much to learn.

I am not sure if you kow that Phyllodesmium magnum is a solar-powered slug, obtaining much of it nutreint requirements from the microscopic plants [zooxanthellae] in its tissues. I have included a close-up inset in the photo alongside. The brown specks you can see in the skin are the zooxanthellae.

Best wishes,
Bill Rudman