Are nudibranchs indicators of global warming?

May 13, 2001
From: Richard Willan

Dear Bill,
I read your recent message recording Hypselodoris kaname from the Poor Knights Islands, northern New Zealand, with interest. This tropical Indo-Pacific nudibranch represents the third species to have been recorded from the Poor Knights, as this group of islands is popularly known, this year (the others are Cadlinella ornatissima and Protaeolidiella juliae)(see Protaeolidiella reference below), all three of them were recorded between September 1999 and April 2001. It has been known for a century that tropical Indo-Pacific animals reach the north of New Zealand as larvae drifting on the east Australian current, that they arrive during the years the water temperatures are hottest and that they settle at the Poor Knights if they are going to settle anywhere. The Poor Knights seem to be a "stepping stone" to the New Zealand mainland. Shelled molluscs have been some of the animals that have been used as demonstrations of this pattern because their taxonomy is relatively well known and people frequently collect them. Nowadays nudibranchs are noticed just as frequently as shells used to be collected. Divers have been visiting the Poor Knights regularly since the early 1970's, so these nudibranchs would probably have been mentioned if they had turned up previously.

Perhaps nudibranchs, like corals, are bioindicators for global warning? But unlike corals that are long-lived and respond to global warming by "bleaching" the zooxanthellae out of their tissues, nudibranchs are short-lived and so they would seem potentially even more sensitive bioindicators of global warming when populations of Indo-Pacific species become established in marginal areas where they never existed before. It would be interesting to know if other people have noted expansions of range, like the cases cited above, in recent times. Furthermore, I would encourage divers in northern New Zealand to see if such tropical nudibranchs form permanent populations. If three tropical species have been recorded already, one can bet there will be more.
Incidently, I agree with you that Ian Skipworth's chromodorid is Hypselodoris kaname and that H. koumacensis is a junior synonym. For your information, this species also occurs in eastern Australia; in 1985 Carol Buchanan photographed a specimen that was found on a fish trap that had been set in 100 metres off Coffs Harbour. It is typical of H. kaname in every way, but it could not be named at the time because the specimen was not preserved.

• Willan, R.C. (2001) First record of the aeolid nudibranch Protaeolidiella juliae (Burn, 1966) in New Zealand. Poirieria, Journal of the Auckland Museum Conchology Section, 27: 31-33.

Richard

richard.willan@nt.gov.au

Willan, R., 2001 (May 13) Are nudibranchs indicators of global warming?. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/4326

Thanks Richard,
Perhaps you could see if the divers who found these other two species would be willing to post their photos on the Forum so we have a visual record of their interesting find.

Your comments about global warming are interesting. The biggest problem with finding biological evidence of global warming is the lack of any baseline data. Divers have certainly been visiting the Poor Knights for some years but I suspect in earlier times most of them were more interested in hunting fish and collecting shells etc than in photographing nudibranchs, so we can't really say whether thse recent tropical finds are evidence of global warming or part of a fluctuating cycle which sometimes allows tropical larvae to reach northern NZ and survive.

You are familiar with Long Reef in northern Sydney. It is a relatively short reef but it sticks out from a headland that juts out into the sea, like an arm waiting to 'catch larvae floating past. I ran a search on the species of the cowry genus Cypraea which we had in our collection from Long Reef. In total there are 25 species represented, 24 of which are tropical in distribution. Anyone seeing the list would think that Long Reef was a shell-collecting paradise in the tropics. But no, it is a fairly rich temperate water reef with a smattering of temporary tropical species. At any time you might find one species of tropical Cypraea, but usually there would be none. The list is based on over 120 years or more of collecting, but without a scientific sampling program over the same period we have no way of knowing whether
Long Reef is becoming more tropical or not. I had thought of comparing the collection dates of all the specimens but I that would reflect the relative activity of collectors over time rather than give us any information on when cowry species were actually present. It is certainly an interesting topic.
Best wishes,
Bill Rudman

Rudman, W.B., 2001 (May 13). Comment on Are nudibranchs indicators of global warming? by Richard Willan. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/4326

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