Preliminary estimation of the factors influencing the distribution of the midlittoral crab Portumnus lysianassa (Herbst, 1796)

In order to acquire information on the factors influencing the distribution of the Mediterranean endemic species Portumnus lysianassa (Herbst, 1796), which lives in the soft substratum midlittoral zone, 83 stations were selected along almost the entire Greek coast. In each sampling station, dredging was carried out, just above the rise of the surf zone, and sediment samples were taken with a core sampler. Two faunal samples were also taken by means of a quadrat from the lower midlittoral zone. The preliminary results of this study show that: P. lysianassa prefers to burrow in the rise of the surf zone; wherever the sediment becomes very coarse or very fine the crab populations decline; there is a correlation between the population abundance of the midlittoral bivalve Donacilla cornea (Poli, 1795) and P. lysianassa, and, moreover, this crab species is related to the composition of the midlittoral assemblage; also, P. lysianassa abundance is negatively correlated with the organic matter content of the sediment; in very exposed areas, D. cornea can be replaced or co-exist with the infralittoral species Donax trunculus (Linnaeus, 1758), and, in these cases, P. lysianassa can be replaced by the closely related infralittoral species Portumnus latipes (Pennant, 1777); when low temperatures or intensive wave action, as well as disturbance caused by swimmers in summer prevail, the population of this crab descends in the upper infralittoral zone.