Bizarre behavior of Dendrodoris denisoni

June 23, 2006
From: Dong Bum Koh

Dear Bill,

We observed this bizarre behavior of Dendrodoris denisoni during night diving in Jeju island. When we found it the  mantle seemed fully inside out, and it only slowly returned to its  original posture.

Locality: Hwa Soon , 4m, Jeju Is.. Korea, Pacific, 19 Dec.2004, Sandy. Length: Approx.35mm. Photographer: Sang Yul Shin.

Could you let me know the meanings of behavior?

Best regards,
Dong Bum Koh

drkoh@seasee.co.kr

D B Koh, 2006 (Jun 23) Bizarre behavior of Dendrodoris denisoni. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/16945

Dear Koh,
I have seen this behaviour in some dendrodorids when I have been trying to preserve them, but not in the field. As I discuss on the Preservation Fact Sheet, most nudibranchs need to be narcotised or 'put to sleep' before they are preserved, otherwise their muscles contract and they become a shapeless ball which is impossible to dissect. Animals without a solid skeleton to which the muscles can be attached, use the fluid in their body cavity to keep their general shape, but when unnatural contractions occur, the blood can be forced into strange places, and so result in strange shapes. Dendrodorids like D. denisoni seem to have little elasticity in their mantle skin - in fact the mantle seems to lack a muscle layer, so when the mantle contracts, the fluid is forced into the more elastic foot which results in the look you have described and illustrated.

I am not sure why it has happened 'in the wild', but it is possible that the Dendrodoris has been 'mouthed' by a fish and then spat out. Fish often take potential prey into their mouth for testing before chewing or swallowing, and it is possible a fish found it distasteful and spat it out, but in the process digestive or salivary secretions may have irritated its mantle. Another possibility, which I have seen in aquarium situations, is that the slug has come into contact with the feeding tentacles of a sea anemone, and this is its reponse to being stung.

Whatever the reason, some good has come of the situation, because you have taken an excellent photo of a dendrodorid head [see close-up photo], which is quite different from the head of most other dorids [see Dorid head Fact Sheet ]. Dendrodorids do not have a 'snout' or a pair of oral tentacles. They have a pair of flaps which run from the foot up on each side of the mouth to form a little 'box' around the pore-like mouth. Dendrodorids do not have any radular teeth, so they feed by secreting digestive juices out on to the sponge they are feeding on. The sponge tissue is then half digested into a 'sponge soup' which the dorid can then suck in. However, this can't take place in the open sea or the juices would be quickly diluted by the surrounding sea water. Unfortunately we can't actually see what's happening when they are feeding, but I think these flaps are there to form a seal around the mouth and the sponge, to enclose the digestive processes which are occurring outside the body.

Best wishes,
Bill Rudman

Rudman, W.B., 2006 (Jun 23). Comment on Bizarre behavior of Dendrodoris denisoni by Dong Bum Koh. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/16945

Factsheet

Dendrodoris denisoni

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