Sea urchin. (Photo: Stock File)

Sperm competition in the sea could be affected by ocean acidification

A pioneering new study has shown the impact of climate change on global seawater conditions could change the rules of sperm competition for many important marine species.

This research, carried out by scientists from the University of Exeter and published in Scientific Reports, revealed that increasing ocean acidification, brought about by manmade carbon emissions, reduces sperm performance in a species of sea urchin, slowing down sperm in future ocean conditions.

Interestingly, they found that different males were affected to different extents and that this could change the outcome when rival male ejaculates competed to fertilise a batch of eggs in the sea.

“The majority of marine species including sea urchins reproduce by releasing their sperm and eggs directly into the seawater. Reproduction is often the most sensitive life stage to environmental stress, so it is really important to understand how these changes in ocean chemistry will affect this essential process,” explained Dr Ceri Lewis, a Marine Biology expert from the College of Life and Environmental Sciences at the University of Exeter.

The oceans absorb about one third of the carbon dioxide produced through human activities, and this is changing the seawater chemistry. The acidity of seawater has increased by approximately 25 per cent since the industrial revolution with further change projected over the coming century unless emissions are dramatically cut.

The Exeter research team measured sperm performance in current and simulated future (ocean acidification) conditions using the same techniques employed by infertility clinics to look at how healthy human sperm are.

They also ran competitive fertilisation trials where males were paired up to compete to fertilise a batch of eggs in each seawater condition with the paternity of offspring analysed to identify ‘winning’ males.

They found that ejaculates containing a greater number of swimming sperm and ejaculates containing faster swimming sperm were more successful in current ocean conditions. But crucially in conditions of ocean acidification, the number of actively swimming sperm in an ejaculate became much less important in securing fertilisation under competition.

In several cases the male urchins that won the sperm competition battle in current ocean conditions were not the winners under future conditions.

“Understanding the traits that determine the winners and losers in future conditions helps us unravel the impacts of change for populations of these important species,” Dr. Lewis concluded.
 

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