Population biology of a phytophagous mite of marine and estuarine rocky shores (Astigmata, Hyadesiidae)

Abstract

The present investigation deals with the population biology of the phytophagous mite Hyadesia fusca (Lohmann), a typical inhabitant of the littoral of marine and estuarine rocky shores in Northern Europe. The study site is an artificial rocky shore located in the mesohaline zone of the Weser estuary in Northern Germany. The vertical distribution of H. fusca extends from the littoral fringe down to the middle Fucus zone (about mean tide level), but the highest population density is found within the green algae zone along a narrow band in the vicinity of the mean high water line. This tidal zone is dominated by Blidingia spp. (Chlorophyta, Ulvaceae), the preferred food of H. fusca with very high nutritional quality. It is suggested, that food supply is an important factor for the distribution pattern observed. The population density of the viviparous H. fusca showed in 1991 two peaks with the first one in June and a second, extremely high peak in September. Each peak was preceded by a rise in alga biomass and was characterised by a subsequent predomination of adults, larvae and nymphs, a change in sex ratio favouring the females, and an increase in the proportion of larvigerous females. Winter populations have a high percentage of tritonymphs, a balanced sex ratio, and a low portion of larvigerous females. It is concluded, that the two fast increases in population size were the result of (1) a diminished abiotic stress due to a thicker algal cover resulting in lower mortality, and (2) a higher level of natality due to the high proportion of pregnant females. The life history of H. fusca comprises at least two overlapping generations per year with the tritonymph as main hibernating stage.