Re: Bursatella leachii & sewage

May 30, 2000
From: Richard Willan

Dear Bill,

I have unravelled the mystery of the opisthobranchs eating "bacteria" in Auckland Harbour. You are right it is part of a natural process.

In late summer every year, larvae of the woolly sea hare Bursatella leachii settle from the plankton onto the filamentous blue-green alga Microcoleus lyngbyaceus (formerly Lyngbya majuscula) throughout harbours and estuaries in northern New Zealand. They grow very rapidly on this diet to about 90 mm; they can double their size in a week! So they soon become highly conspicuous and form aggregations centered around patches of their food and they crawl from patch to patch in groups or lines elephant-file. As the seawater temperature begins to fall in autumn these Sea Hares mate and they they all die en masse in late autumn like salmon do. Stylocheilus longicauda can be present in these feeding/breeding aggregations too, but they are always less abundant than Bursatella leachii. I studied this natural cycle in Auckland Harbour for my PhD thesis some years ago (reference below).

This autumn the water has been warmer than usual in New Zealand (like Australia) and the aggregations larger, so the mass deaths following spawning have been very noticeable and caught the attention of the media.

The matter of these sea hares eating "bacteria" can be easily explained. Formerly Microcoleus lyngbaceus was classified as a member of the Cyanophyta, but that higher classification has now changed (to Cyanobacteria) to reflect the fact that this "plant" has little in comon with other algae. Obviously someone in the media picked up the bacteria part of Cyanobacteria, and wrongly concluded the Sea Hares were eating bacteria. Like you, I know of no opisthobranch that eats bacteria proper.

Reference: Willan, R.C. (1979) The ecology of two New Zealand opisthobranch molluscs. Unpublished PhD thesis, University of Auckland, 278 pp.

Best wishes,
Richard Willan.

richard.willan@nt.gov.au

Willan, R., 2000 (May 30) Re: Bursatella leachii & sewage. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/2473

Dear Richard,
Thanks for the background on the life history of Bursatella in the Auckland region. As you will see from John Buckeridge's message a couple of days ago, the media appear to have unfortunately misinterpreted the information they were provided with.

I note you are using the name Stylocheilus longicauda for the lined Stylocheilus sometimes occurring in northern New Zealand. Could I refer you to the page in the Forum where I discuss why this is a misidentification, and the name we should use is Stylocheilus striata.

Best wishes,
Bill Rudman.

Rudman, W.B., 2000 (May 30). Comment on Re: Bursatella leachii & sewage by Richard Willan . [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/2473

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