Mike Langman, 58, was walking with his wife, Tanya, when he spotted a pod of dolphins and quickly grabbed his camera.
They began leaping from the water and Mike noticed one of them looked odd and had something trailing from its side.
And after Mike zoomed in on the picture he realised it was a lamprey, an ancient parasite, attached to the side of one of them in the sea off the coast of Torquay, Devon.
He said: "I kept an eye on the pod. I was drawn to a very active pair of dolphins continually jumping clear of the water, unlike the rest of the group.
"Eventually I managed to take a series of photos of the pair, which wasn't easy as knowing where they would jump next was guesswork.
"I could see while photographing that it was almost certainly an immature dolphin (calf) and presumably its parent.
"Arriving home and downloading the photos onto my computer I was astonished to see a large lamprey attached to the dolphin.
"As a keen naturalist I knew that lampreys were parasitic creatures attaching themselves with grippy 'teeth' to their host, to feed on them.
"I also remember that lampreys were ancient, horrific looking creatures which I thought were mostly bottom dwelling and slow swimmers, so I was astonished to see one attached to the side of a fast moving dolphin."
Lampreys are a type of jawless fish which have a toothed, funnel-like sucking mouth, and they feed by boring into the flesh of other fish to suck their blood.