New research on egg coiling direction
July 30, 1999
From: I.S. Roginskaya & V.A. Grintsov
Dear Bill!
Do you remember last year I asked you about the direction of motion the nudibranchs are carried out while producing spiral egg-masses (March 5,1998 and (March 12,1998)? And your answer:
" However the direction in which the nudibranch coil is spiralled is the same throughout the world". (starting from the centre of the spiral and coiling to the left?)
Recently my colleague and co-author Vladimir Grintsov from Sevastopol (Ukraine, Black Sea) performed the interesting experimental studies of the egg-laying of an alien doridacean Doridella obscura Verrill 1870 (=Corambe batava Kerbert, 1886, according to C. Swennen and R. Dekker,1995). This small nudibranch, known as native to the Atlantic coast of North America, had been introduced accidentally to the Black Sea in the veliger stage via ships' ballast water discharge in the 1980s and 1990s.
The spiral egg masses of this unique northern hemisphere species long ago were indicated as dextral, i.e. clockwise directed (Franz, 1967; Perron and Turner,1977; Roginskaya and Grinsov, 1995). The recent data of V.Grintsov are remarkable: all the 14 specimens of D. obscura, he observed in the aquarium invariably started the egg-laying from the largest coil of the spiral, moving leftward towards the centre. And though the final flat spiral egg-ribbon looked like dextral, in fact the spirals were sinistral. The same mode of the direction of coiling while attaching the egg-mass to the substratum had already observed Costello (1938) in Hopkinsia rosacea from Monterey Bay, USA.
I have now in my possession a lot of "pseudodextral" spawns of D. obscura (sent by Dr. Grintsov), preserved in alkohol, some with the substratum. I am sending you a picture of 4 spawn, seen from above, attached to the piece of plastic. I am afraid, these images are useless for printing , but good as evidence. I am not skilled in microphotography, so I simply placed a small Petri dish (with the egg-ribbons on the plastic in water) on the desk-scan and tested different magnifications and backgrounds. The diameter of the right, (most dense) spiral is ~4mm.
Perhaps the alien Onchidoris depressa arrived to the southern hemisphere (Rudman, March 5,1998) also starts egg-laying from the outer whorl and its sinistral spirals are in fact dextral? In the Monograph of T. Gosliner (Nudibranchia of Southern Africa, 1987) two spiral egg ribbons of Corambe sp. are dextral (fig.11b). Maybe our Doridella once arrived with ships from the southern hemisphere and were forced to change their natural clockwise direction to a counterclockwise one, beginning from the largest whorl?
Best wishes.
Irina S.Roginskaya.
P.P.Shirshov Institute of Oceanology
Moscow, Russia.
irina7@hotmail.com
Vladimir A.Grintsov.
Institute of Biology of Southern Seas,
Sevastopol, Ukraine.
Dear Irina & Vladimir,
Firstly what a great way to 'photograph' the egg masses. And of course I remember your messages. I was hoping that you would find time to share more information with us.
Thanks for the interesting information on egg spiralling. It seems we have to go out and start watching animals actually laying down their egg ribbons. It s a bit of a problem if some are 'tricking' us by starting at the outside and spiralling to the centre! Another problem we should be aware of with published photos in popular books is the not unusual occurrence of printers reversing negatives during the layout and printing process. While in the case of shelled gastropods this mistake is very obvious, because the ahell coils in the wrong direction, it would not be so obvious with bilaterally symmetrical nudibranchs and their egg-spirals.
I for one will be hunting through my field notes and certainly in the future will be taking careful note of whether egg ribbons are being spiralled from the centre our or form the outside in. Perhaps we can say that if an egg spiral is incomplete in the centre (as in the three on the left in your 'photo') we can predict that the egg spiral was started at the outside. What do you think?
Anyone else's observations are welcome.
Bill Rudman.
Rudman, W.B., 1999 (Jul 30). Comment on New research on egg coiling direction by I.S. Roginskaya & V.A. Grintsov. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/1128Related messages
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