Changing performance characteristics of sea urchin sperm in future oceans

Chair: Vonda Cummings

Michael E Hudson (1), Mary A. Sewell (1)*

1 School of Biological Sciences, University of Auckland, Auckland, 1010, New Zealand

Background
Sea urchin sperm is held under high CO2 and low pH in the gonads; on spawning an increase in internal pH (pHi) results in sperm activation and swimming. pHi is a key regulator of sperm swimming through the regulation of sperm flagellar motors and pathways for ATP generation and utilisation. To maintain high levels of sperm swimming and, ultimately, fertilisation success, sperm must be tolerant to the environmental perturbations of ocean acidification. We hypothesised that under atmospheric CO2 conditions projected under ‘business as usual’ emission scenarios for the next 100-300 years that sea urchin sperm performance would be impaired: this was tested through measuring key performance indicators of motility and pHi.

Methods
In treatment pCO2 seawaters of pHTotal 7.99 (control), 7.70 (medium) and 7.48 (high), sperm motility parameters (speed and directionality) from computer assisted sperm analysis (CASA) and flow cytometric measurements of pHi using the fluorescent dye SNARF-1 were obtained from the same individuals (n=14) of the sea urchin Evechinus chloroticus.

Findings
Medium and high pCO2 treatments had significant negative impacts on both sperm motility and pHi. Multivariate analysis using CASA outputs and pHi showed significant differences between the control and the two treatment levels, but not between pH treatments. Mean pHi decreased from 7.52 (control) to 7.31 (high pCO2). Impaired sperm performance included a drop in overall motility from 83% (control) to 64% (high pCO2).

Conclusions
Future oceans with elevated pCO2 directly impact the performance of E. chloroticus sperm through impaired ability to both alkalinise pHi and attain high levels of motility. The impairment of pH-dependent cellular processes provides a plausible mechanistic explanation for the loss of sperm swimming and performance characteristics for species sensitive to elevated CO2; this will likely have consequences for fertilisation kinetics and reproductive success in broadcast spawners.