Such a high number of juveniles has never been recorded in any of the populations from other regions. Juvenile specimens were reported but never at

abundances exceeding 10% of all the individuals sampled (e.g. Ryan 1956, Rychter 1999, Roche & Torchin 2007, Fowler et al. 2013). Even though both Roche & Torchin (2007) and Fowler et al. (2013) regard juveniles as specimens with CW < 2.5 mm, their data should be taken into account, because according to López-Greco & Rodríguez (1999) and Luppi et al. (2004) acquiring maturity is a long process. Roxadustat clinical trial Moreover, functional, gonadal and morphometric maturities are not always synchronised and can be reached at different stages of growth. In addition, our particular method of collection, the bottom dredge, could have contributed to the large abundance of smaller individuals as this method traps small, cryptic specimens hidden among other organisms (e.g. blue mussels or macrophytes). A high abundance of smaller individuals may indicate the reproductive success of R. harrisii in the Gulf of Gdańsk, and as a consequence, explain the demographic expansion of the target population. According to Gonçalves et al. (1995), R. harrisii larvae are produced from April to September in temperate areas. In the Gulf of Gdańsk, ovigerous females of R. harrisii were found

between June and October, just like find more the population inhabiting Finnish coastal waters ( Fowler et al. 2013). Compared to other studies in the southern Baltic Sea (i.e.

the Dead Vistula River or the Vistula Lagoon), females in the Gulf of Gdańsk appear to produce egg masses earlier and retain them later than other populations ( Turoboyski 1973, Rychter 1999, Normant et al. 2004). While the differences may result from the application of a diversity of sampling regimes (i.e. dredging instead of traps), this extended reproductive period could be due to several environmental factors. In the Gulf of Gdańsk, R. harrisii experiences much more stable sea surface temperatures as compared to the Dead Vistula River or the Vistula Pregnenolone Lagoon, which are shallower areas that undergo rapid temperature changes ( Majewski 1972, Kondracki 2002). These fast temperature changes have been shown to impact the zooplankton communities in the Dead Vistula ( Paturej & Kruk 2011). Many crab species, including R. harrisii, exhibit sexual dimorphism with males attaining larger sizes than females – this has been observed in R. harrisii populations in the Dead Vistula River and the Odra Estuary ( Normant et al. 2004, Czerniejewski 2009). However, in the Gulf of Gdańsk population and other populations inhabiting Finland (introduced) and Louisiana (native), there were no significant size differences between the sexes ( Fowler et al. 2013). The biggest male found in the Gulf of Gdańsk was smaller than the biggest males from other populations inhabiting Polish waters ( Table 2).