The distribution and ecology of 24 species of Decapoda Reptantia in the estuarine part of the rivers Rhine, Meuse, and Scheldt, in the southwestern part of The Netherlands, is discussed. A few species are common offshore, but a larger number are abundant in the seaward reaches of the estuaries at salinities over 15% Cl'. Below this value, only Carcinus maenas is abundant down to salinities of about 5% Cl'. In still lower salinities the alien brackish-water species Rhithropanopeus harrisii and the similarly alien freshwater species Eriocheir sinensis occur. This two species apparently occupy niches that were empty before the present inhabitants were imported from abroad. pg 212-214 The Shore Crab is certainly the commonest crab of the Delta area. Offshore, however, this species soon becomes rare and has already vanished a few kilometres from the inlets of the estuaries, exactly as described by BROEKHUYSEN (1936) for the tidal channels of the Wadden Sea area. Therefore Carcinus maenas is a typical species of the estuarine environment. Here it occupies a fairly large number of habitats: it is found in the small creeks of salt marshes, all over the tidal flats, on the mussel- and oysterbeds in shallow water, along the artificial "rocky shores" of the dikes, in the deeper part of the main tidal channels, and in non-tidal brackish inland waters. This variation of habitats points to a certain euryhalinity, and this is confirmed by the horizontal distribution over the estuaries. It is difficult, however, to relate the distribution of Carcinus to the salinity distribution as the species shows migratory movements. In general the following picture can be sketched. In summer, Carcinus maenas does not reach the isohaline of 5.5 7, Cl' at high tide during low river discharge (Fig. 11 a and b). Moreover, large concentrations of the species were only noted in salinities of at least 10 7. Cl' (Fig. 11 b). According to POULSEN (1922, 1949) C. maenas was still found at salinities of about 47. Cl' in the Baltic. BROEKHUYSEN (1936) mentions a similar limit for the former Zuiderzee. It is, however, a common phenomenon that marine species are able to penetrate further into the stable brackish water of the Baltic than into the unstable brackish waters of estuaries (DEN HARTOG, 1964). In winter, when high river discharges prevail, Carcinus is pushed back over a large distance in seaward direction. For instance, no Carcinus can be caught in this period in the Haringvliet, or in the Krammer-Volkerak area. In brackish non-tidal waters Carcinus also occurs, though not abundantly. It was found in some canals connected with estuaries, where salinity is fairly high, and in the Veerse Meer, which lake has a salinity varying between about 9 and 117. Cl'. These lake specimens are apparently in a rather bad condition since they only make slow movements when caught, unlike the specimens from the tidal waters which are always very lively. Most remarkable, however, is the occurrence in the brackish non-tidal creek near Schelphock on the island of Schouwen. This large creek was closed off from the sea in 1953, after the flood disaster early in that year. Owing to seepage of salt water it remained brackish and the salinity fluctuated between about 10 7. Cl' and 15 %o Cl'. Carcinus must have been able to tolerate the low salinities accompanied by low temperatures in the winter months, and even to reproduce in this body of water. The lowest salinity measured in winter was 10. 79 7. Cl although the temperature of the water was not measured at the same time, it is estimated to have been about 2' to 3' C, and at least under 5' C. In the extremely cold winter of 1962-63, temperatures fell below 0' C; in this period the salinity was about 13 % Cl'. In the other brackish non-tidal inland waters Carcinus does not occur, at least not continually. More extreme salinity conditions are probably the reason for this absence. The migratory movements of Carcinus in both the horizontal and the vertical directions are well known (BROEKHUYSEN, 1936; EDWARDS, 1958; NAYLOR, 1962). As pointed out above, Carcinus shows migratory behaviour in the upper reaches of the estuaries of the Delta area under the influence of salinity conditions. There is also a vertical migration: in summer the adults of the species colonize the extensive tidal flats and leave these flats again in the autumn. It seems that horizontally the males do not migrate as far out to sea as do the females. When the lower values are omitted, the data in the tables given by BROEKHUYSEN (1936), point in this direction. We tested the hypothesis that males and females do not differ in the distance they migrate in a seaward direction, and hence, that the ratio between females and males does not differ along the main axis of an estuary. For this purpose we used material caught in the Grevelingen in May of 1967. It was found, however, that the proportion of males became significantly larger in an inshore direction (0.05 > P > 0.02). Such a difference in the month of May could, however, also be caused by a more rapid inshore migration of the males than of the females, when both have wintered at an equal distance offshore. Therefore, this hypothesis was tested again for samples caught in February 1969. Owing to technical difficulties, it was only possible to obtain samples of crabs from the central and inner parts of the Grevelingen, and from the outer and central parts of the Oosterschelde estuary, the boundaries being arbitrary. These samples were combined and a X'-test was applied to see whether the distribution of the crabs was the same for the 82 males and the 70 females catched. Males predominated in the inner parts, females in the outer parts, and this difference in distribution proved to be highly significant (P < 0.005). It may therefore be concluded that in the estuaries of the Delta area, female Shore Crabs move in winter further out to sea than the males. This brings the females into warmer as well as saltier water. It is, however, impossible to distinguish between the effects of these two factors. As according to BROEKHUYSEN (1936) there is no correlation between sex and salinity, this difference may be the result of temperature. According to BROEKHUYSEN (1936) there is also no correlation between size and salinity. During our investigations we got the impression that the average size of carapace of a sample of crabs declined with declining salinity, but there was no significant decrease in size along the main axis of an estuary, at least not at salinities over 10% Cl'. Carcinus maenas can be found on all types of substrate, with the possible exception of very soft muds.
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