These are parasitic leeches. We had these identified as Stabarobdella macrothela but reviewers on iNaturalist suggest they are some other species in the subfamily Pontobdellinae. While we are not sure about the figured animals, Stabarobdella macrothela is typically parasitic on sharks and rays. Apparently after loading up with blood from their host, they drop off and spend some time digesting the meal. To feed again, they need to latch onto another host. All the specimens figured below were found on lagoon or seaward reefs at Kwajalein Atoll and were presumably in-between hosts. They were all seen on the bottom under rocks or in rubble, which makes me wonder how they manage to catch some of the animals they are supposed to be living on.
Created 1 February 2019
Updated 8 May 2023