Mating of Philinopsis gardineri

September 14, 2007
From: Harry Blalock

Hi Bill,

I came across 3 Philinopsis gardineri all apparently mating on a patch of soft sand the other day. I spent well over an hour just watching them to see what would happen. First one broke away and crawled about 10" away and then proceeded to bury himself in the sand. The other 2 kept going for the next 25 minutes, and then one left, wandered about 6" away and then buried himself as well. 10 minutes later the last one wandered a little bit away and also buried himself in the sand. There was a cocoon looking thing attached to the sand about 8" away from where they were all mating, and I was wondering if the cocoon is an egg sack for these guys. I'd appreciate any information you could share about the mating of these guys.

Locality: Lau Lau Bay, 25', Saipan, Northern Mariana Islands, Pacific Ocean, 9 September 2007, soft sand. Length: 3". Photographer: Harry Blalock.

Harry Blalock

Blalock, H., 2007 (Sep 14) Mating of Philinopsis gardineri. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Harry,

Have a look at the messages on the similarly coloured Chelidonura varians, There are a  few messages discussing mating there. My first thought would be that the mucus cocoon is an aglajid egg mass but I can see no sign of an egg string inside so the second possibility is an egg sac, perhaps empty, of a polychaete worm, such as Arenicola.

Best wishes,
Bill Rudman

Rudman, W.B., 2007 (Sep 14). Comment on Mating of Philinopsis gardineri by Harry Blalock. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

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