A blue shark has become the latest shark to be spotted off the Cornish coast this summer.
Blue sharks are becoming increasingly rare in Cornish waters.
Members of the Mounts Bay Pilot Gig Rowing Club said they saw a 7ft long blue shark swim under their boat as they practised near Penzance.
It is the latest in a series of apparent shark sightings off the Cornish coast this summer.
A marine expert said that blue sharks have been known to attack humans, but never in British waters.
Richard Pierce, chairman of the Plymouth-based Shark Trust, said: "Blue sharks have been here for millions of years. They are summer visitors.
"Sadly they are becoming unusual. The rate at which we are killing them off means that sooner or later we won't be seeing them."
The rowers said they were about 500 yards off the beach when they saw the creature on Wednesday, but Mr Peirce said this meant it was unlikely to have been a blue shark.
He added: "Generally speaking you wouldn't see a blue shark unless you were six miles plus - and more commonly seven or eight miles - off shore.
"It would be rare for a blue shark to be that close to shore. Porbeagles are far more commonly found inshore than blue sharks."
Series of sightings
There has been a series of suspected but unconfirmed sightings of dangerous sharks off the coast of Cornwall this summer.
Surfer Luke Goodman, 25, was in the sea off Gwenver Beach, near Sennen Cove, when a 5-6ft shark swam under his surfboard on August 13.
He believed it could have been a bull shark, an aggressive fish which has frequently attacked people in shallow water.
In early August a group of friends out shark fishing off Bude, Cornwall, saw a large fish jump out of the sea and belly-flop back.
They said they believed the 12-15ft-long fish was a Mako shark, the fastest member of the shark family and a creature which has attacked humans.
In July surfing school pupils at Gwithian beach, near Hayle, Cornwall, were evacuated from the water after a suspected Mako sighting.