More than 100 freshwater pearl mussels have died after they were illegally removed from a river in the Highlands.
The molluscs were found on the upper reaches of the River Oykel near Lubcroy, Sutherland, on Thursday.
It is believed they were removed some time over the course of the last two months.
Constable Dominic Corcoran, wildlife crime officer, said: "Freshwater pearl mussels play a crucial part in sustaining a healthy and natural ecosystem in our rivers - that is why they are fully protected by legislation.
"Anyone disturbing, killing or possessing them is committing a criminal offence and incidents like this are extremely disappointing to everyone who works to conserve this protected species.
"We appreciate that this is a fairly wide timescale but I would urge anyone who believes they may have seen suspicious activity around the River Oykel in recent months to let us know.
"Whoever is responsible for removing them would have needed to spend some considerable time to remove this number of mussels, so please think back and let us know if you believe you can assist."
The freshwater mussels are protected under legislation, making it illegal to disturb, remove, kill or possess them.
They are similar in shape to common marine mussels but grow larger and can live for more than 100 years, making them one of the longest-lived invertebrates.
Mussels may very occasionally bear a pearl.