Recent evolution of ancient Arctic leech relatives: systematics of the hook-faced fish worms

Background/Question/Methods

Acanthobdellida, an elusive group of worms that parasitizes freshwater Arctic fishes, is sister to the decidedly-flashier true leeches. Their unique morphology and phylogenetic relationships have inspired much speculation; however, they are seldom collected, and their biology is not well understood. One of the two species, Paracanthobdella livanowi, has never been sequenced or included in any phylogenetic analysis. While it was initially thought that the range of Acanthobdella peledina was restricted to northern Eurasia, two reports from the 1970s recovered specimens from Alaska, but these populations have not been characterized morphologically or molecularly. In this study, we detailed the morphology of Acanthobdellida – including specimens of A. peledina from Alaska – accounting for ontogenetic variation and trajectory. We also examine the phylogenetic relationships of the order as a whole.

Results/Conclusions

We review existing literature on the natural history and systematics of the enigmatic hook-faced fish worms, Acanthobdellida. We characterize the differences between acanthobdellidan species, and revise their classification accordingly: whereas each species was previously placed within its own family, we opt to classify both in the family Acanthobdellidae on the basis that the phylogenetic and morphological differences are not sufficient to require more than one family. Finally, we find preliminary evidence suggesting that the Alaskan and Nordic populations of Acanthobdella peledina are genetically – but not morphologically – distinct, and discuss the evolutionary implications of these findings.