Thecacera picta from Great Barrier Reef

May 3, 1999
From: Jacquie Sheils

Dear Bill,

Just a note to let you know that Thecacera picta is very common where I work at Hardy Reef, about 80km NE of the Whitsunday Islands.

We see them regularly at depths of about 20m on sand and rubble slopes at the edge of Hardy Reef. At times there appear to be aggregations of them converging on one area, no doubt for sexual purposes!

Jacquie Sheils

jsheils@mackay.net.au

Sheils, J., 1999 (May 3) Thecacera picta from Great Barrier Reef. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/827

Dear Jaquie,
I would love some photos of this animal from the Whitsundays as I don't know of any records of Thecacera picta from that region.

Concerning 'breeding' aggregations. There is a lot of debate about why we find aggregations of Sea Slugs. What we do know is that most mature Sea Slugs will mate, or attempt to when they meet. I guess this is a survival strategy for animals which can't see each other and may only cross paths infrequently. It is possible that in species with very specialised diets, aggregations occur because they are all attracted to a colony of their favourite, but perhaps, uncommon food.

Most Sea Slugs hatch as free-swimming veliger larvae which spend some time in the plankton before settling down on the bottom and growing into crawling slugs. In many cases, it appears that chemicals from their adult food trigger the larvae to stop swimming and to settle down. Having planktonic larvae ensures that, in most species, each generation is spread far and wide.

Some species however have what is called 'direct development' where the slug no longer has a free-swimming larval stage and hatches straight from its egg mass as a tiny crawling slug. In these species, the egg masses are often laid on, or very near, their food, which means that the young often start life in an aggregation and usually stay together until the food runs out. These species often grow very quickly, so unless observations are made very frequently, it may appear that a group of slugs has just 'moved in' when in fact they have always been there but have grown to visible size very quickly.

It's a bit difficult to make a guess at whether, or why, Thecacera aggregates. I suspect it feeds on a bryozoan (lace coral) colony but as with many Sea Slugs, until we know more about their basic natural history it's difficult to say anything sensible about them.

Any photos and/or observations on the feeding, breeding, egg-laying etc of Thecacera, or any Sea Slugs for that matter, would be very welcome.

Best wishes
Bill Rudman.

Rudman, W.B., 1999 (May 3). Comment on Thecacera picta from Great Barrier Reef by Jacquie Sheils. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/827

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