Enjoying spring in Orkney, SCUBA’ s science writer Karen Boswarva contemplates the adaptations of an abundant bi-valve
OCEANSCIENCE
Mussel powers
Enjoying spring in Orkney, SCUBA’ s science writer Karen Boswarva contemplates the adaptations of an abundant bi-valve
It’ s been an absolute delight to grab the snorkel and dive kit after work to make the most of the long, calm, and sunny evenings of late spring. I’ ve visited a couple of new places lately, collecting data to help build a picture of the habitats and species within them. They are not your typical sites, one being a coastal lagoon, the other, a narrow channel between the sea and a small tidal inlet.
The lagoon is a single sheltered basin that covers an inland area of approximately 796 hectares. Freshwater drains in from the land which surrounds the basin, bar one narrow inlet that connects it to the sea. As the tide floods, fresh seawater enters the basin, mixing up the freshwater to form a salinity gradient.
These natural processes have created a unique environment that supports diverse ecosystems, where sheltered marine, brackish, and freshwater species can thrive. The site I dived was farthest from the inlet, in an area of calm, clear water fringed by serrated wrack( Fucus serratus) and beaked tassleweed( Ruppia maritima) – an aquatic plant. The first time we dived here – in February – the ice cracked under our boots and with a water temperature of 2 ° C, the persistent brain freeze was almost unbearable!
In comparison, the narrow and shallow channel is situated under a bridge. There is a bay one side and a small muddy inlet on the other. Beneath, a torrent of water flows. Less than a metre deep, fresh sea water floods in as the tide rises, and warmed brackish water rushes back out to sea as it ebbs. It’ s far too shallow to dive, so the snorkel comes out instead. I wasn’ t sure what to expect as my mask submerged but it was incredible! I was met by a luscious array of mixed seaweeds and hydroids that danced back and forth in front of my eyes, while large clusters of sponge and bryozoans encrusted the substrate.
The star of the show was the abundance of blue mussels, Mytilus edulis.
They can form dense beds increasing biodiversity by providing habitat for other marine organisms. Blue mussel beds are considered priority habitats and are often listed as a conservation importance within Marine Protected Areas.
In the lagoon, the mussels were small in size, yet densely covered every available surface; wrapped around the tassleweed and wrack, clustered on boulders, and peppered on the seabed like gravel. Under the bridge they were much bigger, forming deep beds that extended out to the intertidal. They looked very different but were in fact the same species.
I was instantly curious as to how they could adapt and thrive in such different environmental conditions and went digging for information. I hope you find it as interesting as I did. Blue mussels are osmoconformers, their internal fluids remaining isotonic to the environment. This means they can maintain a steady salinity, even in fully fresh water, for short periods of time. Reducing or closing the valves,( a process called valve-gape) regulates the rate of filtration and protects the mussel from long-term exposure to fresh water and harmful conditions including chemicals and pollutants. Measuring the differing gape angles and speed of valve-gape is a non-invasive method used to study blue mussel behaviour.
Studies have shown that larger mussels react faster, opening and closing more rapidly. This suggests a trade-off between effective feeding and predation risk. The mussels in the lagoon may therefore be smaller due to a combination of factors including: reduced food availability, freshwater / chemical influence, and increased predation risk. The end result is a reduced gape angle for longer periods that in turn limits feeding and subsequent shell growth. Fascinating! �
The beautiful Orkney foreshore near the Broch of Gurness, looking across to the island of Rousay
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