Researchers from The University of Western Australia have found that sperm may be more vulnerable to environmental stress than previously thought.

The scientists studied the blue mussel, which spawns its sperm and eggs directly into the ocean, to test the impact of rising ocean temperatures on sperm cells. The findings have been published today in Proceedings of the Royal Society B.

They found the temperature of the external environment, through which sperm from externally fertilising species swim on their journey to fertilise an egg, had a significant effect on the genetic contents of sperm cells.

Lead author of the study, Dr Rowan Lymbery from the UWA Centre for Evolutionary Biology, said it was generally assumed that sperm were simple DNA-delivery machines, and that the contents of their cells remained the same regardless of the environment they encountered on their way to fertilising an egg. 

“However, recent evidence shows that sperm contain important molecules called RNAs, which are the messenger products of genes and are highly sensitive to the environment outside cells,” Dr Lymbery said.

“We found that when sperm swim through warmer external temperatures, the amount of RNA from several important genes in sperm decreases.”

Dr Lymbery said that under rapidly changing environmental conditions, such as climate warming, changes to the genetic composition of sperm could have detrimental effects on embryo development.

“These RNAs in sperm are likely to be important for early embryo development, and our study suggests that ocean warming will reduce the amount of these molecules that embryos have available,” he said.

Dr Lymbery said the ocean had absorbed 90 per cent of the Earth’s warming in the past 50 years, which would have a significant impact on the future reproduction of species that inhabited it.

“This shows another way that the warming of our oceans could threaten the reproduction and survival of marine species,” he said.

Studying the reproduction of externally spawning species could also offer insight into fertility in other animals, including humans, Dr Lymbery said.

“In many animals, sperm cells undergo numerous post-ejaculatory modifications before fertilisation, however we have rarely looked at whether these include molecular changes inside the sperm,” he said.