, 2011) In addition, this region is subject

, 2011). In addition, this region is subject RGFP966 manufacturer to the inflows from the River Vistula, which frequently change the physical and chemical parameters of the water, especially after the spring floods ( Buszewski et al. 2005). Harris mud crabs collected in the Gulf of Gdańsk were characterised by a similar carapace width range as the specimens collected in other regions, such as the Dead Vistula, the Vistula Lagoon or the Odra Estuary. Rychter (1999)

made very similar observations for the Vistula Lagoon population (Rychter, 1999, Normant et al., 2004 and Czerniejewski, 2009). The size distribution of the Gulf of Gdańsk population exhibits a normal pattern, but worth noting nonetheless is the presence of a large number of juveniles. In 2009–2010, juveniles were dominant in the samples, their abundance exceeding 31% of the total number of individuals collected. Such a high number of juveniles has never been selleck compound recorded in any of the analysed populations from other regions. Juvenile specimens have been reported but never at abundances exceeding 10% of the sampled individuals (Ryan, 1956 and Roche and Torchin, 2007). On the one hand, the presence of such a large number of juveniles may be due to dredging instead of using baited

traps, but on the other it may indicate a demographic expansion of the population under scrutiny. During dredging, small specimens are trapped between other material, whereas in the case of baited traps, only larger individuals, actively looking for food are usually caught (Miller

1990). Temperature in the Gulf of Gdańsk exhibits seasonal variations up to 20 °C (Piliczewski 2001). This factor appears to be significant in determining the occurrence of R. harrisii, many as observed in other poikilothermic organisms living in the temperate zone ( Schmidt-Nielsen 1997). On the other hand, the greater depth range of the Gulf of Gdańsk gives the crab the opportunity of remaining in its preferred temperature longer than in shallow waters. This has been confirmed by seasonal studies demonstrating that the species migrates to other depths in response to changes in the water temperature. The abundance of R. harrisii increases in the summer months and decreases in the winter. A similar pattern was observed in the Dead Vistula and the Odra Estuary population ( Turoboyski, 1973 and Czerniejewski, 2009). The changes in distribution are likely to be caused by the mud crab’s habit of overwintering, when the animals bury into the bottom sediment or hide between the shells (authors’ own observations) and remain inactive. This behaviour probably accounts for the apparent absence of crabs in their natural habitat during winter ( Turoboyski, 1973 and Czerniejewski, 2009). On the other hand, at water temperatures within the 18.1–19.1 ° C range no specimens were recorded either.

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