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Summary
This study examines effects of water temperature on sperm swimming velocity, longevity, respiration, and fertilization success in the free-spawning serpulid polychaete Galeolaria caespitosa. Temperature is expected to influence metabolic rates (and swimming speeds) of sperm, and may affect sperm swimming through the mechanical effects of changing water viscosity. To separate the physiological and mechanical effects of water temperature, viscosity and temperature were manipulated independently. Changes in water temperature significantly influenced fertilization success and duration of sperm activity. In the range 11–31°C peak fertilization rates (>80%) were observed at 21°C, the typical summer water temperature in the native habitat. Both lower and higher temperatures resulted in significantly lowered fertilization rates. Longevity experiments demonstrated a trade-off between sperm velocity and duration of activity: at 21°C high sperm velocity declined sharply within 4 h, reaching zero within 8 h; at 11°C sperm maintained reduced velocity for over 12 h. Thus, temperature influenced sperm longevity and fertilization success by directly affecting sperm swimming velocity. Both physiological and viscosity-induced components of temperature were shown to affect sperm swimming velocity and corresponding fertilization success in G. caespitosa. Lower water temperature also resulted in significantly lower respiration rates of G. caespitosa sperm. However, viscosity alone did not significantly affect sperm respiration, suggesting that sperm may not be capable of compensating for velocity changes due to increased viscosity. Studies examining sperm metabolism and physiological compensation are needed to understand how broadcasting organisms achieve high fertilization rates in a wide range of ambient temperatures.
