A bed of horse mussels off the Caithness coast is the largest in Scotland, according to Scottish Natural Heritage (SNH).
Marine biologists measured the site off Noss Head, near Wick, at almost 1.5 sq miles (4 sq km).
SNH said horse mussel beds provided shelter and feeding grounds for young fish, such as cod.
The site off Caithness is among the three largest beds of horse mussels in the UK.
SNH said horse mussels were larger than blue mussels commonly found along the sea shore and could grow to between 10 and 20cm.
Crabs and starfish eat young horse mussels, but those that survive could live for more than 50 years.
SNH said spiny dogfish sharks were also recorded in a survey of the Southern Trench, a deep channel in the Moray Firth.