Spatial variability in the population structure of the shore crab Carcinus maenas (Crustacea: Brachyura) in a shallow-water, weakly tidal fjord

The spatial vanabllity in population structure of the shore crab Carcinus maenas was studied In a shallo\v-water fiord in the lsland of Fyn, Denmark The fjord compnses an enclosed area whlch is geomorphologically divlded into a shallo~v cove (mean depth 2 m) , known as Keitinge Nor, and a deeper narrow area closer to the open sea, known as Kerteminde Fjord Tidal range IS small (ca 20 cm) Sex-ratio favoured males at most of the sltes sampled wlth the exception of the inland slope between the shallow Kertinge Nor and the deeper Kerteminde Fjord whlch apparently was a spawnIng site Crab denslties were much higher in Kertemlnde Fjord than In the shallower Kertinge Nor Ventral carapace colour varled between green and orange-red with the proport~on of the green morphotype Increasing wlth increasing distance from the open sea Analysls of the population slze structure ldentlfied 3 differentiated areas for both male and female crabs Sltes closer to the mouth of the fjord were charactenzed by a numerical dominance of small crabs, whereas crabs in Kertinge Nor were clearly of a much larger size Sltes In Kerteminde Fiord were characterized by a wider range of slzes Some crabs in Kerteminde Fjord carried epiblonts on the carapace, but those were absent in crabs from lceltinge Nor, moreover, a few crabs In Kertinge Nor carried Myt~ lus eduhs byssus threads indicating that epibionts had fallen from the carapace5 Thus, population structure cha~acterlstlcs such as colour morphotype proportions, size structure and occurrence of eplbionts are In accordance wlth the occurrence of a hlgh degree of environmental vanability In the area decreasing towards the mouth of the fjord and the open sea