# Variation in sperm performance and mitochondrial metabolism of Mytilus spp. from the North and Baltic Seas under different environmental scenarios

## TL;DR

The study shows how temperature and salinity affect the reproductive performance of blue mussels from the North and Baltic Seas, with distinct optimal ranges for each population.

## Contribution

The study reveals population-specific reproductive traits in Mytilus edulis and links sperm performance to mitochondrial function under environmental stress.

## Key findings

- Baltic Sea mussels have optimal sperm motility at salinity 13–17, while North Sea mussels require 21–35.
- High temperature (25°C) reduces mitochondrial efficiency and increases ROS production in mussels.
- Cold and low salinity conditions significantly impair sperm motility and fertilization success in both populations.

## Abstract

Climate change, including seawater warming and salinity fluctuations, is increasingly affecting marine ecosystems worldwide. The blue mussel, Mytilus edulis, widely distributed along the temperate coasts of the Northern Hemisphere, thrives in environments characterized by temperature fluctuations and salinity gradients. In particular, populations in the Baltic and North Seas are exposed to significant variation in these factors, which can affect the reproductive capacity of blue mussels, essential for sustainability of their populations. This study assessed the effects of varying temperature and salinity on the reproductive performance of blue mussels from the Baltic and North Seas, focusing on sperm motility, ATP content and fertilization success. Additionally, sperm mitochondrial function in Baltic Sea mussels was examined under different temperature and osmolarity conditions. The results showed that mussels from both populations tolerated seawater warming, but were sensitive to cold and low salinity, with sperm motility and fertilization success significantly impaired under these conditions. The salinity window for sperm motility and fertilization was population specific: optimal ranges were a salinity of 13–17 for Baltic Sea mussels and 21–35 for North Sea mussels. Notably, North Sea mussels were unable to reproduce at salinity 9, whereas Baltic Sea mussels were severely impaired at salinity 5. High temperature (25°C) reduced mitochondrial respiratory efficiency and increased reactive oxygen species (ROS) production, while osmolarity did not appear to be a key factor. These findings highlight population-specific reproductive traits in M. edulis and link sperm performance to mitochondrial function, providing new insights into benthic adaptation to changing coastal environments.

Summary: Salinity and temperature impact the sperm performance of North and Baltic Sea Mytilus edulis, with optimal conditions reflecting the specific environmental characteristics of their distinct native habitats.