Environmental tolerances of the beach-hopper Orchestia gammarellus (Pallas) (Crustacea:Amphipoda)

Orchestia gammarellus (Pallas) will tolerate temperatures from 0°C to c. 30°C but dies in < 1 h at 35°C. Both small and large animals survive > 96 h in water (aerated) between 1 and 60%(.) salinity. In distilled water and 100%(.) brine death occurred after 8–24 h. In fully saline water, pH reduction to c. 3·3 is tolerated. Highly alkaline (pH 11–13) conditions in low salinity media are rapidly fatal. Ammonium concentrations (in fully saline water) some five orders of magnitude higher than normal for seawater are tolerated, reflecting the enhanced ammonium concentration encountered in rotting weed beds. Cl⁻ ion concentration equivalent to that in normal seawater will not support O. gammarellus in the absence of Na⁺ ions. High NO₃⁻ concentrations (> 100 mg NO₃⁻ litre⁻¹) are fatal in < 24 h. A large range of NaCl concentrations is tolerated (one to two orders of magnitude dilution) by immersed animals. Animals will live longer in air, however, when fed low Na⁺ food than when immersed in seawater diluted to an equivalent ion strength. Ca²⁺ and K⁺ ions cannot substitute for Na⁺. Orchestia gammarellus will, however, tolerate wide variations in the ratio of Na⁺:K⁺ ions (i.e to a shift from 30:1 to 0·3.1) in hypo-osmotic media. At 10°C it will survive c. 5 h of anoxia. It will penetrate into strongly reducing conditions (− 318 mV) even with access to better oxygenated conditions. In nature, behavioural avoidance probably ensures that O. gammarellus does not encounter many of these parameters at acutely limiting values. That this species is so tolerant, however, bears on the evolutionary adaptability of talitrid amphipods to non-marine conditions. Desiccation sensitivity and Na⁺ ion dependency are the species' most important barriers to life on land.