Parasites and Navanax inermis

August 16, 2007
From: Kevin Lee

Bruce Wight's recent parasite post reminded me of some infestations I've witnessed on Navanax. I was was wondering if Navs, when afflicted with parasites, purposely "expose" themselves, [see middle photo] in the hope that a fish or crab might come by and clean off the offenders. I watched this Nav for five minutes and, all the while, it had its mantle flared out.

All three fotos were taken in La Jolla Canyon.

Locality: La Jolla, 65 feet, California, USA, Eastern Pacific. Upper: 03 Sep 2005. Middle: 27 July 2007. Lower: 15 Oct 2006.  Sandy bottom. Length: 7 cm. Photographer: Kevin Lee.

Kevin Lee

Kevin L., 2007 (Aug 16) Parasites and Navanax inermis . [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Kevin,
I have looked at these photos rather hard and I think there is quite a bit of damage to the posterior end of the animals in the upper two photos.  There are definitely copepods crawling over the skin but I think they are probably normal commensals.

In the upper photo there are two animals, the one on the right looks intact, but the one at the back seems to have the mantle organs missing - the shell, the gill, etc just aren't there. In the middle photo the 'flaring' bit at the top of the photo, with the copepods on it, is the foot, so the animal is upside down. The whitish 'sausage'-shaped structure in the middle is almost certainly the mucus gland section of the reproductive system, but that should be obscured by the large gill. Both the gill and the dorsal part of the posterior shield containing the shell appear to be absent. I think any 'itchiness' caused by the copepods is the least of these animal's concerns. My guess is there is something in the Canyon with a liking for the back end of Navanax. If you see an animal like this again try a bit of gentle poking and prodding to see if it is all there.

Thanks for the lower photo. One thing that does seem to differentiate Navanax from other aglajids are the elaboration of the pads on either side of the mouth which house the sensory bristles. In Navanax they have become curved ridges which are so clearly visible in your photo.

Best wishes,
Bill Rudman

Rudman, W.B., 2007 (Aug 16). Comment on Parasites and Navanax inermis by Kevin Lee. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from


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