Cratena peregrina and reproductive organs

December 1, 2006
From: Dominique Horst

Hi Bill,

Hope you spent nice holidays in Europe.

On the following pictures of Cratena peregrina, a kind of hole is visible on their right side after the second rank of cerata. Following your explanation about a similar structure on Hypselodoris tricolor (message #17001) , I've deduced that this correspond to reproductive ducts.
Do you agree on this ?

Locality: Cap d'Antibes, 8 m, France, Mediterranean sea, 24 August 2006, rocks on sand. Length: 20 mm. Photographer: Dominique Horst.

Many thanks for your continuous help on this forum.
Kind regards,
Dominique

dominique.horst@wanadoo.fr

Horst, D., 2006 (Dec 1) Cratena peregrina and reproductive organs. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/18453

Dear Dominique,
Yes this is the genital opening. If you look just in front of the next row of cerata you will see a small grey spot (near a reddish mark). This is the kidney opening. The anus is further back amongst the next groups of cerata. This may seem a strange place to have all these body openings, but it is a historical record of how they have evolved from snails. Most higher animals [worms, crustacea, vertebrates etc] usually have a head end with a mouth and a tail end where the other openings are situated. But if you think of a snail, its shell has only one opening, so all its internal organs have to open together at the head end. How all the openings got there is still debated by malacologists, but during the evolution of sea slugs, the shell was discarded and the animal was free again to become bilaterally symmetrical, with a head at one end and a 'tail' at the other. In the process, some of the these openings began migrating down the right side of the body towards the 'tail'. In aeolids, they haven't got far, with the genital opening, anus and kidney opening still on the right side of the body near the front. In the dorids however, the anus and the kidney opening have moved to the back of the body, with the gills, and only the reproductive opening has stayed up the front on the right side.

Just as our coccyx bone lets us know our ancestors had tails, the position of a slugs reproductive opening lets us know they evolved from snails.

Best wishes,
Bill Rudman

Rudman, W.B., 2006 (Dec 1). Comment on Cratena peregrina and reproductive organs by Dominique Horst. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/18453

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Cratena peregrina

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