A digest of elasmobranch tapeworms.

This review brings together decades of work on elasmobranch tapeworms. The field has advanced significantly over the past 20 yr, with an emphasis on the discovery and description of novel taxa, and the establishment of phylogenetic frameworks for individual orders and their interrelationships. Tapeworms parasitizing elasmobranchs represent 9 orders and include 977 species and 201 genera-over 250 species and 50 genera are new within the last 2 decades. The 9 orders are treated individually, highlighting recent assessments of phylogenetic relationships informed by molecular sequence data. All but the "Tetraphyllidea" are monophyletic. Although much remains to be learned about their interrelationships, existing phylogenetic hypotheses suggest that elasmobranch tapeworms have played a key role in the evolution of the cestodes of essentially all other vertebrate groups. The apical organ is a defining feature (i.e., a synapomorphy) of a clade consisting of acetabulate taxa and Litobothriidea. Novel hook amino acid composition data support the independent origin of hooks in the various groups of hooked tapeworms. Cestode records exist for representatives of most of the major groups of elasmobranchs, however skates (Rajiformes) and catsharks ("Scyliorhinidae") are particularly neglected in terms of species sampled. The majority of tapeworm species are extremely host-specific exhibiting species-specific (i.e., oioxenous) associations with their hosts. Rapid advancements in elasmobranch taxonomy, with over 300 of the 1,200 species appearing new in the past 20 yr, signal the need for careful attention to be paid to host identifications; such identifications are best documented using a combination of specimen, photographic, and molecular data. Above the species level, many cestode taxa are restricted to host orders, families, or even genera. Documentation of these affiliations allows robust predictions to be made regarding the cestode faunas of unexplored elasmobranchs. Trypanorhynchs are the notable exceptions. Life cycles remain poorly known. Recent applications of molecular methods to larval identifications have reinvigorated this area of research. Tapeworms are more diverse in elasmobranchs of tropical and subtropical waters, but they occur globally not only at the poles and in deep waters, but also in freshwaters of South America and Southeast Asia. The cestode faunas of batoids are much more speciose and complex than those of sharks. The faunas of deeper water sharks are particularly depauperate. The tapeworms of elasmobranchs and their hosts are now among the most well documented host-parasite systems in existence. This system has not yet reached its potential as a resource for investigations of basic ecological and evolutionary principles.