Helminth fauna of the black goby Gobius niger L. (Gobiiformes: Gobiidae) from the Finnish Archipelago, Baltic Sea: Molecular and morphological data.

Black gobies (Gobius niger) from the Finnish Archipelago, Baltic Sea, were screened for helminth infections in summer 2020. Helminths were identified morphologically and/or molecularly. Altogether 26 novel sequences were generated and analysed using maximum likelihood estimation. Morphological and phylogenetic analyses based on mitochondrial genes revealed the presence of 8 species belonging to the Digenea (Diplostomum mergi Lineage 3), Cestoda (Bothriocephalus scorpii), Nematoda (Contracaecum rudolphii A, Cucullanus sp. and Hysterothylacium aduncum), and Acanthocephala (a putative new species of Corynosoma, Corynosoma semerme and Neoechinorhynchus sp.). Phylogenetic and comparative sequence analyses revealed that the putative new acanthocephalan species is closely related to C. neostrumosum described from the Caspian seal, Pusa caspica, in the Caspian Sea. The black goby represents a new host record for four parasite species (Diplostomum mergi Lineage 3, Contracaecum rudolphii A, Corynosoma semerme and Corynosoma sp.). The Finnish Archipelago is a novel locality record for three species (Corynosoma sp., Diplostomum mergi Lineage 3 and Bothriocephalus scorpii).

An ancient alliance: Matching evolutionary patterns of cartilaginous fishes (Elasmobranchii) and chloromyxid parasites (Myxozoa).

Myxozoa is a group of endoparasitic cnidarians covering almost 2600 species but merely 53 species, mostly from the genus Chloromyxum, have been reported from sharks, rays, and skates (Elasmobranchii). Elasmobranchs play a key role in the study of evolutionary trajectories of myxozoans as they represent ancestral vertebrate hosts. Our study provides new data on Chloromyxum spp. from 57 elasmobranchs, covering 20 species from geographical regions and host groups not previously investigated, such as Lamniformes and Hexanchiformes, the most basal phylogenetic shark lineage. In total, 28% of elasmobranchs were infected with Chloromyxum spp., indicating high diversity. Of the seven distinguished species, six are formally described based on morphological, morphometric, and genetic (18S rDNA) data. Comprehensive co-phylogenetic analyses and ancestral state reconstruction revealed that parasite and host phylogenies are clearly correlated, resulting in a distinct phylogenetic separation of chloromyxids from selachid (shark) vs. batoid (ray and skate) hosts. Species infecting the most ancient elasmobranchs formed a sublineage, branching off in the middle of the Chloromyxum sensu stricto clade. Our findings indicate that chloromyxids likely invaded an ancestral elasmobranch prior the time of divergence of shark and batoid lineages. Our analyses did not show a clear phylogeographic pattern of Chloromyxum parasites, probably due to the cosmopolitan distribution and migratory behaviour of many elasmobranch hosts, but geographical sampling must be extended to confirm or refute this observation. This study provides a complex view on species diversity, phylogeny, evolution, host-parasite co-phylogeny, and the phylogeographic origin of Chloromyxum species from elasmobranchs. Our results highlight the importance of adding missing data from previously un- or undersampled geographical regions and host species which results in a more accurate estimate of myxozoan biodiversity and a better understanding of the evolution of this parasite group in their hosts and in the different oceans of our planet.