A digest of fish tapeworms.
Members of the following seven of a total of 19 cestode orders mature in rayfinned fishes (Actinopterygii): Amphilinidea (mainly in acipenseriforms and osteoglossiforms; 8 species in 6 genera), Bothriocephalidea (in several freshwater and marine fish groups; 129/47), Caryophyllidea (mainly in cyprinids and suckers, also in some catfishes; 117/42), Haplobothriidea (exclusively in bowfin; 2/1), Nippotaeniidea (in osmeriforms and perciforms; 6/1), freshwater Onchoproteocephalidea (mainly in catfishes 194/55), and Spathebothriidea (in several freshwater and marine fish groups; 6/5). Updated information on species diversity, host associations, interrelations and geographical distribution is provided for every group. The existing phylogenetic hypotheses suggest that tapeworms colonized ray-finned fishes several times and form several independent lineages. From a total of 461 fish tapeworms only 92 species are exclusively marine. So, freshwater species dominate the assemblage. No general patterns in host use can be observed at the level of fish definitive hosts because cestodes of fishes occur in not closely related host groups. Nevertheless, only three fish orders host almost three fourth of all tapeworms of fishes, namely Siluriformes (36% of all cestode species), Cypriniformes (22%) and Perciformes (16%). Nearly two thirds (61%) of fish tapeworms have a strict (oioxenous) specificity and one third (33%) is stenoxenous. The highest proportion (8%) of euryxenous species is among the bothriocephalideans, including one of the most opportunistic fish helminth, the invasive Asian fish tapeworm (Schyzocotyle acheilognathi), which has been reported from more than 200 fish species and axolotl, snakes and birds. Tapeworms of fishes are more diverse in the temperate zones of the northern hemisphere (Nearctic and Palearctic regions with 26% and 23% of all species) than in the tropics, with the exception of onchoproteocephalideans in South America (together with a very few other tapeworms represent 29% of all species). However, this may reflect a lower sampling effort in tropical regions and the southern hemisphere. Caryophyllideans dominate in the Australasian (58% of all species, but only 12 species found) and Nearctic (56%; total number=102) realms, and together with ncoproteocephalideans in the Ethiopian (both 41%; n=39), Palearctic (both 37%; n=90) realms, and with bothriocephalideans in the Oriental realm (34%; n=36). The Neotropical region is dominated by onchoproteocephalideans (in fact almost exclusively members of the family Proteocephalidae; n=112), which represent as many as 95% of all species reported. The major part of the undescribed diversity of fish tapeworms regarding the total number of species can be expected in the latter region, but this concerns only a single cestode family, Proteocephalidae (order Onchoproteocephalidea).