Physiological constraints on the life cycle and distribution of the Antarctic fairy shrimp Branchinecta gaini

Maturation to adulthood and successful reproduction in the Antarctic fairy shrimp, Branchinecta gaini, must be completed within a physiologically challenging temporal window of ca. 2.5 months in the southern Antarctic Peninsula. Although adults show considerable metabolic opportunism at positive temperatures, little is known of their tolerance of two physiological insults potentially typical to pool life in the maritime Antarctic: sub-zero temperatures and salinity. B. gaini are freeze-avoiding crustaceans with temperatures of crystallisation (T(c)s) of -5 degrees C. No antifreeze proteins were detected in the haemolymph. Adults osmoregulate in relation to temperature, but rapid mortality in saline solutions of even low concentration, indicate they cannot osmoregulate in relation to salinity. Survival of ice encasement at temperatures above their T-c was found to be pressure but not time dependent: at severe inoculative ice pressures, there was little immediate survival and none survived after 48 h below -2 degrees C; at mild inoculative ice pressures, immediate survival was ca. 100% at -3 degrees C, but <20% after 48 h. There was no significant difference in survival after 1 and 6 h encasement at -3 degrees C. Observations of ventilation suggest that it is not low temperature per se, but ice that represents the primary cryo-stress, with ventilatory appendages physically handcuffed below the freezing point of pool water. Both sub-zero temperatures and salinity represent real physiological constraints on adult fairy shrimp.