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

The spatial variability in population structure of the shore crab Carcinus maenas was studied in a shallow-water fjord in the island of Fyn, Denmark. The fjord comprises an enclosed area which is geomorphologically divided into a shallow cove (mean depth 2 m), known as Kertinge 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 sites sampled with the exception of the inland slope between the shallow Kertinge Nor and the deeper Kerteminde Fjord, which apparently was a spawning site. Crab densities were much higher in Kerteminde Fjord than in the shallower Kertinge Nor. Ventral carapace colour varied between green and orange-red, with the proportion of the green morphotype increasing with increasing distance from the open sea. Analysis of the population size structure identified 3 differentiated areas for both male and female crabs. Sites closer to the mouth of the fjord were characterized by a numerical dominance of small crabs, whereas crabs in Kertinge Nor were clearly of a much larger size. Sites in Kerteminde Fjord were characterized by a wider range of sizes. Some crabs in Kerteminde Fjord carried epibionts on the carapace, but those were absent in crabs from Kertinge Nor; moreover, a few crabs in Kertinge Nor carried Mytilus edulis byssus threads indicating that epibionts had fallen from the carapaces. Thus, population structure characteristics, such as colour morphotype proportions, size structure and occurrence of epibionts, are in accordance with the occurrence of a high degree of environmental variability in the area decreasing towards the mouth of the fjord and the open sea.