Chondracanthus parvus

Chondracanthus parvus is a parasitic copepod that parasitises the smooth-cheek sculpin, Eurymen hyrinus, by attaching itself to the inner side of the fish's operculum (the flap covering the fish's gills). Chondracanthus parvus belongs to a family of parasitic copepods known as the chondracanthids, which contains 160 species, all of which are parasites of marine fishes. Phylogenetic studies of the chondracanthids indicate that these copepod have consistently co-evolved with their hosts, and their phylogeny closely reflects the evolutionary history of the fish that they infect. Such parasites are like heirlooms of the evolutionary past and phylogenetic studies conducted on these living markers can in turn shed light on the evolutionary history of their hosts.

Picture from Ho et al. (2006).

References:

Paterson, A.M. and Poulin, R. (1999) Have chondracanthid copepods co-speciated with their teleost hosts? Systematic Parasitology 44:79-85.

Ho, J-s., Kim, I-H., and Nagasawa, K. (2006) Copepod parasites of the fatheads (Pisces, Psychrolutidae) and their implication on the phylogenetic relationships of Psychrolutid genera. Zoological Science 22:411-425.