Re: Food of Dutch Geitodoris planata

September 2, 2001
From: Albert Koulman

Concerning the continuing discussion on the food of Geitodoris planata in the Netherlands, I do not think that Geitodoris planata will have big impact on the population of Mycale micracanthoxea. One has to take into account that the Oosterschelde is an artificial controlled ecosystem. Especially the areas with rocky substrates, these are all part of the Deltaworks to protect the Southwest of the Netherlands against the sea. There is an open connection between the Oosterschelde and the North Sea but the entrance is much smaller than it used to be and it can be closed when the water rises too much during gales. The introduction of rocky shores in the Oosterschelde has resulted in the introduction of many species. This is one of the reasons that almost every year a nudibranch species is found that was not previously reported from the Netherlands. There are for instance several places in the Oosterschelde where you can find large amounts of Electra pilosa, however nudibranchs that eat this bryozoa are very rare. We have had huge amounts of Elysia viridis in the Oosterschelde, and there were years that you could see over a hundred specimens of Janolus cristata in one dive, while the next year it would take 10 dives just to see one specimen, even though there is usually enough Bugula. The Oosterschelde is not a balanced ecosystem (yet). Maybe we can report next year the sighting of Aegires punctilucens, we have more the enough Leucosolenia here.

Still we have had no nudibranch in the Oosterschelde that had a real impact. Maybe for one year, but due to large temperature variations of the Oosterschelde (it is now 22 degrees Celsius and it will go down to 4 or less in the winter) they will disappear again. We have to wait until some new larvae will find there way into the Oosterschelde.

Albert Koulman

A.Koulman@farm.rug.nl

Koulman, A., 2001 (Sep 2) Re: Food of Dutch Geitodoris planata. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/5198

Dear Albert,
Thanks for this interesting description of the Oosterschelde habitat. I think the point that was being made was not that Geitodoris would affect the population of the sponge, but that the presence of the sponge was perhaps the reason Geitodoris was suddenly so abundant in the Oosterschelde. It is interesting that some of the other examples you mention are feeders on bryozoan species which are common fouling organisms which rapidly colonise spaces which become available. The nudibranchs that feed on these colonisers are equally opportunistic, with life cycles which have evolved to survive these conditions. They have very short life spans, needing to grow to maturity and breed, before the short-lived bryozoan colony on which they are living and feeding dies. They are often found in large short-lived populations.

This is not normally the lifecycle of a large sponge-feeding dorid like Geitodoris which again suggests that not only is the environment artificial, but the abundance of the sponge may also be unusual. Elsewhere in the world when a 'foreign' marine organism is introduced to a country, it often rapidly produces a dense population close to its point of introduction before beginning to spread more widely. Which sounds much like the situation with Mycale micracanthoxea and certainly makes Bernard Picton's suggestion that perhaps this new species of sponge is a recent arrival in Europe an idea worth considering.
A very interesting situation,
Best wishes,
Bill Rudman

Rudman, W.B., 2001 (Sep 2). Comment on Re: Food of Dutch Geitodoris planata by Albert Koulman. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/5198

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