Dear Feedback,
A recent article in Porcupine! (16:12-13) by David Dudgeon is a critique of my previous article (Porcupine! 15: 7-8) and raises some interesting questions about the relationship between snails and crabs in Shap Sze Heung stream (hereafter referred to as SSHS). Normally, when a critique of a journal or newsletter article is written, the author of the original paper is sent a copy of the critique and invited to comment so that both the critique and the responses to it can be published together. Sadly, this did not occur and the reader must now go back to volume 16 of Porcupine! to read the comments by Dudgeon on the article by Dickman in volume 15.
To give a quick review of the situation, it went something like this. In the fall of 1995 about 180 first year Ecology and Evolution students at HKU spent one morning combing over a section of SSHS and the remarkable thing, in my opinion, was the absence of any snails in their collections of animals from the stream They found crabs, fish, shrimps, caddisflies, mayflies midge larvae and much more, but not a single snail. Their observations piqued my curiosity as to why this stream had no snails. There was periphyton growing on a number of the rocks in sunny stretches, so a few snails grazing on the attached algae might be expected, as they are found in most other streams in Hong Kong. So why are there no snails in Shap Sze Heung stream?
The first hypothesis was that the snails may be seasonal. To test this hypothesis I returned to the stream in the summer and spring and still found no snails. The second hypothesis was that the levels of calcium in the stream might be so low that snails would be unable to form functional shells. Atomic absorption analyses of the SSHS sediments that I sampled were carried out for numerous elements including calcium. These analyses revealed that calcium was low but not too low for thin-shelled snails to exist. The other elements tested did not appear to be unusually high or low when compared to other streams in Hong Kong where snails were found.
I hypothesized that perhaps the crabs in the stream could consume thin-walled snails. To test this hypothesis a simple manipulative experiment was performed. Crabs from the stream were removed and placed in an aquarium containing commonly-occurring snails taken from the Pokfulam Reservoir stream. Within one day nearly all of the snails had been broken open and the contents consumed by the few crabs placed in the aquarium. This observation gave rise to the hypothesis put forward in my Porcupine! article, that low calcium means that only thin-shelled snails are likely to occur in SSHS and the crabs in SSHS were capable of consuming thin-shelled snails.
In his critique of this study David Dudgeon claims that other parameters might be responsible for the absence of snails in the stream. He suggested two possibilities: food was limiting and/or sodium or some other critical ion was limiting. As mentioned above, if food was limiting then why is periphyton growing on most of the stones in the many sunny reaches of the stream? If sodium or other elements were critical then why were they present at similar levels reported for streams containing snails? The fact that Dudgeon & Corlett stated in Hills and Streams (1994) that Cryptopotamon feeds on leaf litter was overridden by a simple manipulative experiment indicating that it may be an opportunist and it is certainly capable of feeding on thin-shelled snails (sorry David).
In 1996 K.T. Chan completed his M.Phil thesis at HKU. According to David, Mr. Chan observed Brotia hainanensis, a thick-shelled snail, in streams with calcium of 1.2 mg per litre. This information was not available to me when I published the 1995 observations from SSHS. It is possible that under low calcium concentrations the shell of Brotia hainanensis might prove thin enough for Cryptopotamon to break open.
So what does all of this prove? Not that manipulative experiments were not carried out - they were carried out. It proves that manipulative experiments are difficult to construct in such a way that all the critical parameters are tested. For example, if sodium or some other ion is critical to snail egg development or larval development it may not be detected in standard cage experiments unless the entire snail life cycle was examined. What I think all of this proves is that science often progresses incrementally when testable hypotheses are published. Such hypotheses eventually are tested by others who are interested in using them or questioning them. This is as it should be. David provides a very interesting hypothesis - that food or some other critical ion is limiting and that is why there are few if any snails in SSHS I hope that one or more Porcupine! readers will some day decide to answer the question: "Why are there no snails in Shap Sze Heung Stream?".
In the last issue of Porcupine! there is an article by Gray Williams which is well worth reading. He assures students that presenting their results at a departmental seminar is a worthwhile way to elicit critical comments by staff and students and that they should welcome such critical comments because they often improve the final product. I think the same can be said about publishing one's hypotheses. One takes a risk in publishing, as one does in presenting a seminar. But it's taking the risk that makes doing science exciting.
MIKE DICKMAN
P.6
Dear Feedback,
The bumblebee picture in Porcupine! number 16 is VERY misleading! Can we have a note in the next issue pointing out that the local one looks nothing like that?
RICHARD CORLETT
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