Roboastra gracilis eating Nembrotha kubaryana

February 13, 2006
From: Ross Gudgeon

Concerning message #15757:

After witnessing the Nembrotha kubaryana being eaten by Roboastra luteolineata reported by Kristin, and seeing her great shots of that meal, I thought you might find this shot interesting. It was taken on the same reef a week earlier only 20 metres from where Kristin took her pictures. At the time I took them I thought that the Roboastra gracilis (I think??) was only hitching a ride, but after seeing Kristin's shot I think he may have been out for a very large meal.

Locality: Lighthouse Bay, Exmouth, 12 metres, Western Australia, Indian, 28 January 2006, Reef. Length: 6 mm. Photographer: Ross Gudgeon

Ross Gudgeon

ross@gudgeon.id.au

Gudgeon, R.I., 2006 (Feb 13) Roboastra gracilis eating Nembrotha kubaryana. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/15816

Dear Ross,
Usually not many days pass between really great discoveries being reported on the Forum, and I think your observation fits into that category. I have often wondered how juveniles of these hunters feed, or even how relatively small species like R. gracilis get on when only large prey are present. If you look at your close-up alongside, the white patch in the lower left of the photo, is the white flesh of the the prey. It has a black skin layer and then a very white muscle layer beneath. What your photo shows is that the Roboastra is not trying to swallow the whole Nembrotha as the R. luteolineata was attempting in Kristin's message, it is just chewing out manageable bits of the animal - eating it alive. I'm surprised the Nembrotha still has its gills and rhinophores extended - perhaps it has reached a pain threshold or perhaps the Roboastra is not actually biting at the moment. I have checked out earlier observations on species of Roboastra feeding and can't find any referring to this type of 'grazing'. Other reports seem to suggest 'engulfing' the whole prey is the standard procedure. If anyone has further information it would be very welcome.

To my  knowledge, this is the first feeding record for Roboastra gracilis.  Photos showing behaviour, ordinary or strange, are always welcome. As with this shot, you never know when you might photograph something extraordinary.

Best wishes,
Bill Rudman

Rudman, W.B., 2006 (Feb 13). Comment on Roboastra gracilis eating Nembrotha kubaryana by Ross Gudgeon. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/15816

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