We compared the hyposalinity tolerances of black prickleback (Xiphister atropurpureus) and penpoint gunnel (Apodichthys flavidus) that were collected in an intertidal area during a low-tide near Bamfield, British Columbia. We found black pricklebacks completely removed from the ebbing tide where they may be exposed to hyposaline conditions for up to five hours. Conversely, penpoint gunnels were found in larger tidepools or subtidal areas where they are less likely to be exposed to hyposaline conditions. The tolerance of each species was determined by measuring oxygen consumption (µmol·g·h) and counting opercular beats (per minute) in full-strength (∼30 ppt) and dilute seawater (∼6 ppt). Black pricklebacks (N = 10) consumed oxygen at a significantly lower rate (P = 0.001) in dilute seawater when compared to full-strength seawater, whereas there was no significant difference in consumption rate by penpoint gunnels (N = 10). The rate of opercular beats for both species significantly decreased in dilute seawater. Black pricklebacks showed a greater decrease in breathing rate, and opercular beating stopped completely in nine of the ten dilute seawater trials for periods ranging from 10 – 60 minutes. The lower oxygen consumption and breathing rate of black pricklebacks suggests that this species may have the ability to depress metabolic activity to remain in the intertidal zone during an ebb tide. These findings indicate that physiological adaptations may be a factor in the habitat portioning between black pricklebacks and penpoint gunnels.
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