Heteropsyllus

Position of Heteropsyllus within Harpacticoida

Heteropsyllus is a highly controversial genus of marine harpacticoids. Previously, it was most often attributed to the Cletodidae family (Lang 1948). Recently, it has usually been referred to the Canthocamptidae (Nam & Lee 2006). However, among cantocamptids, this genus is distinguished by plesiomorphic characters, like the presence of a free mandible exopod, a free maxillule exopod, the presence of three maxillary endites on the, and the presence of more than one seta on the syncoxa and endopod of the maxilliped.

The most characteristic difference is the character of dimorphism in the P3 structure, where males of Heteropsyllus have a slightly modified outer seta on the last segment of the endopod. In particular, this character was one of the main reasons for the isolation of Heteropsyllus by Kornev and Chertoprud (2008) into the separate monotypic family Heteropsyllidae Kornev & Chertoprud, 2008. However, it is currently unknown how the P 3 male endopod evolved. Thus, the absence of the characteristic division of the distal segment of P3 endopod into pseudosegments, which is present in cantocamptids, may, on the contrary, turn out to be apomorphic rather than plesiomorphic. The same type of transformation can be observed in the family Laophontidae Scott T., 1904. The most basal group Esolinae Huys & Lee, 2000, with genera such as Archilaophonte Willen, 1995, Esola Edwards C., 1891, and others, already has a well-defined division of the distal segment of the male P3 endopod; however, most genera within Laophontinae Scott T., 1904 have an already undivided distal segment (Willen 1995; Huys & Lee 2000). In their phylogenetic analysis, Huys & Lee (2000) also characterize the three-segmented male P3 endopod as plesiomorphy and the two-segmented endopod as apomorphy.

In our opinion, the division of the male P3 endopodite into pseudosegments, as well as the presence of a strongly modified outer seta into apophysis, is characteristic of the common ancestor of a large number of families in Cletodidimorpha Lang, 1948 sensu Kornev & Chertoprud, 2008, excluding the families Tetragonicipitidae Lang, 1944, Normanellidae Lang, 1944 and Cletopsyllidae Huys & Willems, 1989. Already within the various groups of Cletodidimorpha, secondary fusion of pseudosegments and reverse transformation of the modified outer seta probably occurred. It is quite possible that these are neotenic transformations, as in the family Laophontidae (Huys & Lee 2000). Such transformations can be traced even within the genus Heteropsyllus, where H. spongiophilus has the most pronounced dimorphism, with a large modified apophysis, and H. coreanus has a small, slightly displaced thin apophysis.

Inside the Canthocamptidae, Heteropsyllus occupies one of the basal branches. We consider it highly likely that Heteropsyllus is a basal branch of the subfamily Hemimesochrinae. Thus, Huys and Thistle (1989) state that Heteropsyllus, unlike typical Hemimesochrinae, does not possess pineapple-setae; however, as can be seen in this description, highly similar setae are found on the distal segment of the female antennule and on several segments of the male antennule. There is also much in common between Hemimesochrinae and Heteropsyllus in the structure of the mouthparts. The maxillae have similarly shaped setae on endites, which, at first, are evenly thin, but then sharply taper to a fine tip. They are brought together by the characteristic structure of one of the setae on the middle endite of Heteropsyllus, which, in the species of the genus Mesopsyllus, has an even more modified appearance. The maxilliped claw in H. spongiophilus and M. glacialis has four large, long spines, although a similar structure is not observed in other Heteropsyllus species.

Heteropsyllus differs from typical Hemimesochrinae in the absence of dimorphism in the structure of the P4 endopodite. However, for complete clarity, qualitative descriptions of the morphology of other genera of Hemime- sochrinae, such as Hemimesochra Sars G.O., 1920 and especially Hanikraia Huys, 2009, are required.