Population dynamics and salinity tolerance of Hyadesia fusca (Lohman) (Acarina, Sarcoptiformes) from brackish water rockpools, with notes on the microenvironment inside Enteromorpha tubes

Abstract

The salt water mite Hyadesia fusca has been recorded from Scandinavia for the first time. It is entirely restricted to the green alga Enteromorpha spp. in the littoral zone or in rockpools. The algae serve as substratum, food and breeding room but they also give rise to violent fluctuations in abiotic environmental parameters. However, oxygen availability and hydrogen-ion activities do not differ very much inside and outside the algal thalli. The fluctuations recorded are between 2 and 12 g O₂·10^-7 cm^-2 min^-1 and between pH 8.5 and 10.1. H. fusca dominates the fauna of many brackish water rockpools. During breeding in June and July more than 900 animals, mostly larvae, may be found on 0.1 g Enteromorpha dry weight. In winter an average of 13 hibernating animals were found per 0.1 g algae. Copulation follows after a long precopula with the male riding on the female. The newborn larvae are often found near the rhizomes inside the algal tubes. The mite is very tolerant of salinity variations. In laboratory experiments more than 4 weeks of survival, copulation, breeding and larval growth were found to be almost equally successful in salinities between 0.5 and 40‰S. Hyadesia fusca is a characteristic member of the rockpool ecosystem. It is often very abundant, one of the few Enteromorpha grazers of of the ecosystem and it serves as food for fishes in the system. Together with some better known crustaceans this mite because of its tolerance and survival capabilities must be considered one of the hardiest members of the unstable rockpool ecosystems.