Opisthobranchs in NZ & Global Warming

August 19, 2001
From: Wade Doak

Dear Bill
To accompany my message about Aplysia extraordinaria from New Zealand, here is some more information on opisthobrachs from the Poor Knights Islands, northeastern new Zealand. A local diver, Janine Collet sighted Roboastra arika at the Poor Knights on 31 January, 2001 at the Northern Arch. Her companion took a photo. I hope it may be sent to you. As you would know, this is our second Roboastra.

I have been diving the Poor Knights since 1963 and have written about its marine life in 4 books . Reading the discussion you had with Richard Willan, I suggest that the dramatic increase in nudibranch species at the Poor Knights is not just the result of increased diver effort. I am seeing a corresponding increase in exotic species right across the board: fishes , echinoderms crustaceans etc. I suggest it is more likely to be partly the result of climate change.

We were pretty good at distinguishing new species back in the early seventies - a team of red hot young diving marine biologists were with me in those days who now have positions in many leading Australian scientific institutions.

For example,the green Tambja sp. is pretty hard to see on its green Bugula food source. I think I was the first to notice it, while decompressing on the top of Landing Bay Pinnacle, and I published it in my book Beneath NZ Seas in 1971. If we were distinguishing nudibranchs like that I cannot believe we would have overlooked some of the gaudy species turning up in the last few years.

Wade Doak


Doak, W., 2001 (Aug 19) Opisthobranchs in NZ & Global Warming. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/5104

Dear Wade,
I hope Janine Collet can organise for the photo of Roboastra arika to be sent to the Forum. For that matter any records are of interest.

Concerning my discussion with Richard Willan. I was not suggesting that divers in the past had poorer observational skills than they have today. The problem is we have no systematically collected baseline data so we do not know if the apparent 'first appearances' of these tropical animals are in fact 'first appearances'. Many nudibranchs are notoriousy patchy in their distribution in both space and time. That is you can never rely on them being present at a certain place or at a particular time of the year. Most opisthobranch enthusiasts who have favourite places are always finding 'new animals' at their collecting spots even after years of visiting. You may be right in saying that more 'tropical' animals seem to be occurring at the Poor Knights now than 20 years ago, but unfortunately we have no way of knowing if this is really so. And even if it were so, we would have to ask whether the fauna 20 years ago was 'normal' or whether the present faunal composition is 'normal'? To know that, we would need baseline data from 50 or 60 years ago. It is interesting that records of tropical shelled molluscs from New Zealand in the 1800s were later dismissed as mislocalised specimens etc, but it is possible that in fact they were recording an earlier warm period.

The other point is that there are many more eyes out there and many more cameras recording things than ever before. While you and the hot young marine biologists were looking 20 or 30 years ago, most other divers then were only interested in what they could shoot with a spear gun, or collect and eat (paua, crayfish, sea urchins etc). As many nudibranchs have very short life cycles, sometimes only a few weeks, I am sure many visitations by transient tropical species could have occurred between visits by interested divers. From what I gather now, the Poor Knights would have diving parties visiting during much of the year, which must greatly increase the chances of animals being seen.

Unfortunately the lack of baseline data is a worldwide problem. Until we begin collecting faunal census data on a systematic and regular basis we will never be able to say with any scientiific certainty just what is happening to our fauna.

Best wishes,
Bill Rudman

Rudman, W.B., 2001 (Aug 19). Comment on Opisthobranchs in NZ & Global Warming by Wade Doak . [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/5104

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