Behaviour and mortality of benthic crustaceans in response to experimentally induced hypoxia and anoxia in situ

Crustaceans are sensitive to hypoxia and are therefore useful indicator organisms for oxygen depletions. We used the experimental anoxia generating unit in the northern Adriatic Sea to artificially induce and document hypoxia and anoxia on a small scale (0.25 m2 per deployment). Behavioural responses and mortalities were documented for 9 crustacean species typical for the sublittoral soft bottom of the Gulf of Trieste. All species showed a similar succession of atypical responses, albeit at different thresholds. The first reaction to declining dissolved oxygen (DO) was avoidance by climbing to more oxygenated (higher) positions. The animals left their shelters, altered their activity patterns, and exposed themselves to a higher risk of predation at mild hypoxia (2 to 1 ml l–1 DO). Moderate hypoxia (1 to 0.5 ml l–1 DO) triggered changes in interand intraspecific interactions, resulting in aggregations of up to 27 individuals at the highest elevations. At severe hypoxia (0.5 to 0.01 ml l–1 DO), sublethal responses such as discarding of camouflage (Ethusa mascarone) were recorded, and 54% of all crustaceans died. Anoxia left most remaining individuals immobile and moribund until death. Almost all responses were related to DO thresholds—hydrogen sulphide (H2S) had not yet evolved. Pisidia longimana, Galathea spp., and Macropodia spp. were the most sensitive; Pilumnus spinifer and Ebalia tuberosa were the somewhat more tolerant species. Only Nepinnotheres pinnotheres survived prolonged anoxia as well as high H2S concentrations.