Nudibranch egg masses - the direction they spiral

PHOTO

Upper: Onchidoris depressa near Hobart, southern Tasmania, Australia, 14 Feb 1984. AM C141235. Lower: Rostanga arbutus Egg ribbons showing large direct-developing eggs. Coffs Harbour region, New South Wales, Australia. December 1990. AM C64554. Photos: Bill Rudman.

Many opisthobranch egg masses form a spiral ribbon, and of these, most seem to spiral in an anticlockwise direction [sinistral] - that is coiling in a anticlockwise direction from the centre. However some such as Melibe australis and Melibe engeli seem to be dextral, coiling clockwise from the centre.

As you will see in the attached messages there is some discussion about whether in the northern hemisphere they spiral anticlockwise and in the southern hemisphere clockwise. As I mention above there are certainly some species that have anticlockwise coiling egg ribbons but we have not enough information to say whether this is based on phylogenetic relationships or geography. Another complication is that some species start the egg spiral from the outside while the majority start from the centre. If a ribbon is started at the outside then it will look the reverse to what it actually is.

Authorship details
Rudman, W. B., 2004 (August 2) Nudibranch egg masses - the direction they spiral. [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/eggspir

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