Wonderful to see these amazing creatures! Five European Brook Lampreys - Lampetra planeri. After 3 to 7 years of living blind in burrows in the mud, they metamorphose into sighted adults, swim upstream, then party and die. This is the clearest shot of them showing 4 of the group. They are some of our most primitive vertebrates: they have a cartilage skeleton like sharks but lack gill covers and jaws. In this shot you can see the row of seven gill holes behind the eye. They pick up stones with their sucker mouths and move them around to build a nest for their eggs. I watched entranced for nearly an hour while they hefted stones around between brief, energetically squirmy bouts of mating.