Hemigrapsus takanoi in the Eastern Scheldt:

I often visit sites in the Eastern Scheldt and while there, I always check what lives in the littoral zone. In summer 2005, I found the entire upper littoral zone was colonised with a 2-4 cm large crab I had never seen before. The shore crab I was used to, Carcinus maenas, was only to be found in the lower littoral zone and had entirely disappeared from the upper zone. This is what the strange animal looks like:

Hemigrapsus takanoi from Neeltje Jans, The Netherlands 

Cédric d'Udekem d'Acoz solved my problem: the crabs belong to the recently introduced Hemigrapsus takanoi, originating from Japan. The crabs were presumably introduced with Asian oysters and first observed at the Dutch coast in 2000. The animals are serious competition for our native Carcinus maenas. I placed some specimens in my marine aquarium and soon ended up with an ovigerous female:

Ovigerous female of Hemigrapsus takanoi in my aquarium

Last November, my aquarium was full of these zoea larvae, most probably from Hemigrapsus takanoi (see Wear 1970, below):

Zoea larva of Hemigrapsus takanoi

Some literature:

Asakura, A. & Watanabe, S. (2005) Hemigrapsus takanoi, new species, a sibling species of the common Japanes intertidal crab H. penicillatus (Decapoda: Brachyura: Grapsoidea). Journal of Crustacean Biology 25(2): 279-292.

d'Udekem d'Acoz, C., M. Faasse (2002) De huidige status van Hemigrapsus sanguineus (de Haan, 1835) en H. penicillatus (de Haan, 1835) in de noordelijke Atlantische Oceaan, in het bijzonder in Nederland, met opmerkingen over hun biologie (Crustacea, Decapoda, Brachyura) [The current satus of Hemigrapsus sanguineus (de Haan, 1835) and H. penicillatus (de Haan, 1835) in the northern Atlantic Ocean, with special emphasis on the Netherlands, with remarks on their biology]. Het Zeepaard, 62(4): 101-115 [English translation available on request].

Nijland, R. & Beekman, J. (2000) Hemigrapsus penicillatus De Haan 1835 waargenomen in Nederland. Het Zeepaard 60(3): 169-171.

Wear, R.G. (1970) Life-history studies on New Zealand Brachyura. 4. Zoea larvae hatched from crabs of the family Grapsidae. New Zealand Journal of Marine and Freshwater Research. 4(1): 3-35. Downloadable at www.rsnz.org/publish/nzjmfr/1970/1.pdf

Photographs: Jan Bosselaers

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