Distribution and swimming behaviour of Neomysis integer (Peracarida: Mysidacea) in response to gradients of dissolved oxygen following exposure to cadmium at environmental concentrations

Abstract

Neomysis integer (Peracarida: Mysidacea) is a dominant member of the hyperbenthos of the upper reaches of European estuaries. They are highly dynamic environments that experience, amongst other things, large-scale fluctuations in dissolved oxygen (DO) concentration. The present study reports the tolerance and swimming responses of N. integer to hypoxia, together with the modifying effects of environmentally realistic cadmium concentrations (0 to 1.0 μg Cd2+ l-l). The tolerance (ET50, or time taken for 50% of mysids to show an effect) of N. integerto hypoxia was ca. 5 min at 20% saturation (59.4 μmolO2 l-l); however, a 7 d pre-exposure to 0.5 μgCd2+ l-1 reduced the ET50 to ca. 3 min. Mysids did not avoid areas of severe hypoxia (<10% saturation) and made excursions of short duration (ca. 30 s) into such areas. Following cadmium exposure, however, mysids did avoid areas of hypoxia. When challenged with a current velocity of 6 cm s-1 (normal for their natural habitat), mysids showed positive rheotaxic behaviour, but as DO concentration was progressively decreased, fewer mysids swam forward into the current and more maintained position. Fewer cadmium-exposed mysids showed positive rheotaxis than control mysids, and decreasing DO concentration had no significant effect on the proportion of mysids exhibiting this behaviour. The results are discussed in terms of how DO concentrations might affect mysid distributions in the natural environment and how cadmium exposure might disrupt these responses.