Sea Hares at North Shore City, New Zealand

May 16, 2000
From: Joel Cayford

There has been an explosve outbreak in Sea Hare/ Sea Slug populations along parts of East Coast Bays beaches at North Shore City, Auckland, New Zealand. They range in size from small to the size of a hand palm, and emit the dye when disturbed. No sea birds will touch them.

A local scientist claims their presence is due primarily to sewage bacteria being present in the sea water. This is a controversial matter as the local Council is strenuously rehabilating local sewer networks. Each week it tests beachwater for enterococci. For the past few months there have not been elevated levels in the vicinity of this population of Sea Hares. (The standard is 100 organisms/100ml of water).

These beaches are adjacent to urbanised landscapes. We have had a dry summer, with little or no rain.

What is the most likely set of causes for this sudden increase in Sea Hare / Sea Slug population?

Regards,
Joel Cayford

cayford@watermagazine.com

Cayford, J., 2000 (May 16) Sea Hares at North Shore City, New Zealand. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/2391

Dear Joel,
If you could send me a photo of the Sea Hare involved I could give you a more detailed answer. I presume you are talking about a species of Aplysia, which feed on certain species of sea weed, but it is also possible you have a population of Bursatella leachii, another type of Sea Hare, which usually feed on blue-green algae that form a layer over the bottom.

In both cases it is unlikely anything in the water column could directly affect the size of their populations, although increased nutrients might help the algae they feed on to flourish. However there is a much simpler explanation for a Sea Hare population explosion - it's normal!

For some background information on Sea Hares have a look at the Sea Hare Page. Population explosions are quite a normal feature of Sea Hare biology and it's usually when they all start dying simultaneously that people begin to mistakenly blame pollution. Rather than repeat what I have said before, have a look at my answer to an earlier message from Annie Troutman on a population explosion of Aplysia dactylomela in Florida last year. Also have a look at the messages about
A. brasiliana which is doing the same this year.

If you have a look at the page on Mass Mortality you will find links to a number of other pages and messages on the subject.

Basically, there are some years when all conditions are favourable. There is plenty of food, good larval settlement, and calm weather. Most of the juvenile Sea Hares survive settlement from the plankton and grow to maturity. At this stage they become rather obvious on the shore and when they begin to breed and die their dead and dying carcases become even more obvious as they wash up on the tide line. By coincidence there is a population explosion in at least one part of Sydney Harbour at the moment. The photo of multiple mating in Aplysia dactylomela, which I recently posted, was of animals collected from just such a 'population explosion'.

You also mention that 'no seabird will touch them'. This is not because they are diseased but because they have chemical defences to protect themselves from predators. Have a look at the pages on attack & defence and chemical defence.

Best wishes,
Bill Rudman.

Rudman, W.B., 2000 (May 16). Comment on Sea Hares at North Shore City, New Zealand by Joel Cayford. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/2391

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