Effects of salinity on respiration and nitrogen excretion in two species of echinoderms

Abstract

The 30-d survival limit of Eupentacta quinquesemita and Strongylocentrotus droebachiensis is 12–13‰ S. The activity coefficient (1 000/righting time in seconds) of stepwise acclimated sea urchins declined from 16.3 at 30‰ S to 3.5 at 15‰ S. Oxygen consumption rates (QO2) of both species held at 30‰ S and 13°C were highest in June and lowest in December. During the summer, when environmental salinity is most variable in southeastern Alaska, the QO2 of both species held at 30, 20 and 15‰ S varied directly with salinity. Perivisceral fluid PO2 varied directly with acclimation salinity in sea urchins, but not in sea cucumbers. Perivisceral fluid oxygen content of acclimated sea urchins was significantly lower at 15 and 20‰ S than at 30‰ S due to reduced PO2 and extracellular fluid volume at the lower salinities. The QO2 of both species varied directly with ambient salinity during a 30-10-30‰. semidiurnal pattern of fluctuating salinity. No change occurred in the average QO2 of either species over a 15-30-15‰. semidiurnal pattern of fluctuating salinity. Sea urchin perivisceral fluid PO2 declined as ambient salinity fluctuated away from the acclimation salinity in both cycles and increased as ambient salinity returned to the acclimation salinity. Total nitrogen excretion of stepwise acclimated sea cucumbers declined significantly from 30 to 15‰ S, but there was no salinity effect on total nitrogen excretion in sea urchins. Ammonia excretion varied directly with salinity in stepwise acclimated sea cucumbers (67–96% of total nitrogen excreted), but there was no salinity effect on ammonia excretion (89–95% of total nitrogen excreted) of sea urchins. Urea excretion did not vary with salinity in sea cucumbers (2–4% of total nitrogen excreted) or sea urchins (2–9% of total nitrogen excreted). Primary amines varied inversely with salinity in sea cucumbers (2–30% of total nitrogen excreted), but did not vary with salinity in sea urchins (2–4% of total nitrogen excreted). The oxygen: nitrogen ratio of both species indicated that carbohydrate and/or lipid form the primary catabolic substrate. The O:N ratio did not vary as a function of salinity. Both species are more tolerant to reduced salinity than previously reported, however, rates of oxygen consumption and/or nitrogen excretion are modified by salinity as well as season.