Type genus. Eurythenes S.I. Smith in Scudder, 1882.
Generic composition. Monotypic.
Description. See Stoddart & Lowry (2004).
Remarks. During the past two decades, Lowry and his collaborators have split the old family Lysianassidaeelevated to the rank of superfamily Lysianassoidea—into a large number of new families. In their studies, a largely phenetic approach was applied, in the sense that each group of taxa sharing distinctive characters was automatically assigned to a separate family or genus, without attempting to unravel the deep interrelationships between taxa. In one case, they even rejected taxa definitions based on phylogenetic analyses (Lowry & Kilgallen 2014: discussion on Pseudorchomene). In the course of this division process, Stoddart & Lowry (2004) erected the monotypic family Eurytheneidae for the genus Eurythenes. Recent works such as Havermans et al. (2010, 2011), d'Udekem d'Acoz & Havermans (2012) and Corrigan et al. (2014) have shown that an important part of traditional or semiphenetic classifications of lysianassoid amphipods are not supported by molecular phylogenies. Therefore, the validity (and the composition) of the family Eurytheneidae remains an open question, as indeed for many other lysianassoid families. Future molecular phylogenies involving a larger number of lysianassoid taxa and a higher number of genetic markers will probably help to solve such issues, but for the time being, with limited data at hand, the family Eurytheneidae is accepted. The question of its delimitation is another issue to be solved. Should the Eurytheneidae remain restricted to Eurythenes as proposed by Stoddart & Lowry (2004) or should the family be expanded to include other genera? In a recent multigene phylogenetic study including 15 scavenger lysianassoid taxa, Corrigan et al. (2014) have shown that Eurythenes forms a clade together with the genera Cyclocaris, Paralicella and Stephonyx. While these genera exhibit significant morphological differences, they also share similarities, such as a reduction of coxa 1, which might be synapomorphies. To visualise the differences between these genera and Eurythenes, see for instance the illustrations of Cyclocaris given by G.O. Sars (1900), Lowry & Stoddart (2011) and Horton & Thurston (2014), those of Paralicella by Barnard & Ingram (1990), and those of Stephonyx by Diffenthal & Horton (2007), Narahara et al. (2012) and Lowry & Kilgallen (2014). If further molecular studies based on a larger number of taxa confirm the existence of a cluster of genera surrounding Eurythenes, it would be logical to expand the definition of Eurytheneidae to include these additional genera. This would imply the relegation of the monotypic family Cyclocarididae Lowry & Stoddart, 2011 (type genus: Cyclocaris) to the rank of junior synonym of Eurytheneidae Stoddart & Lowry, 2004.