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Invertebrates

Terrestrial shrimps or hallucinating ecologists?

by Sukhmani Kaur Mantel

Returning from an evening of fieldwork on a warm September night, I stopped short; there in front of me was a Macrobrachium shrimp walking over a footbridge that crosses a tributary of Tai Shing Stream in Shing Mun Country Park! It had apparently climbed a 50 cm precipice to get there. The ground was completely dry and the shrimp was facing upstream as I came across it. Startled by my close scrutiny of it with my headlamp, it turned around and scuttled into a "safer" position on a lower ledge of the footbridge. It waved its chelae around a bit and then after about two minutes it suddenly walked across the ledge, jumped headlong into the water and disappeared downstream. I wondered if these were the ramblings of a deranged individual or a phenomenon so rare that I had not come across it in my year and a half of night observations?

After talking to my co-workers, I learned that two other ecologists (Sze-man Cheung and Rita Yam) have observed similar behavior for two species of freshwater shrimps, Macrobrachium hainanense and Caridina cantonensis, in other streams. Strangely enough, Rita has witnessed a Macrobrachium in a tree!

I have always assumed that many decapod crustaceans were capable of climbing over waterfalls since diadromous crustaceans migrate from the sea to freshwaters. A quick literature search confirmed that some shrimps are positively rheotactic and they migrate upstream by navigating through flowing water or in the splash zone to get over natural or artificial obstacles (Lee & Fielder, 1979; Fiévet, 1999). I am, however, unaware of any reports of shrimps walking completely outside of water and, especially, climbing trees. Since I donít know how often this event occurs or, more importantly, WHY, I am requesting any readers who have come across this behaviour in person or in literature to contact skmantel@hkusua.hku.hk with any comments. Oh, and next time you go strolling by a stream, be wary of where you step.

Bibliography

Fiévet E. (1999). An experimental survey of freshwater shrimp upstream migration in an impounded stream of Gaudeloupe Island, Lesser Antilles. Archiv für Hydrobiologie 144: 339-355.

Lee C.L. and Fielder, D.R. (1979). A mass migration of the freshwater prawn, Macrobrachium australiense Holthuis, 1950 (Decapoda, Palaemonidae). Crustaceana 37: 219-222.

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