Hatschekia Poche, 1902

Genus Hatschekia Poche, 1902 View in CoL

Diagnosis: Female body comprising cephalothorax, trunk and genitoabdomen bearing caudal rami. Cephalothorax typically with well defined dorsal shield supported by subsurface chitinous frame, usually consisting of median longitudinal bar and paired lateral bars of varying lengths. Cephalothorax incorporating first pedigerous somite but ventrally located first legs typically positioned at junction with trunk, posterior to rear margin of dorsal cephalothoracic shield. Frontal margin of cephalothorax with median rostrum with or without paired rostral processes ventrally; frontal region crenulate in some species. Trunk comprising second to fifth pedigerous somites: shape and relative size of trunk variable, typically subcylindrical or slightly dorsoventrally flattened with linear to weakly convex lateral margins. Trunk rarely with paired dorsal processes anteriorly, but commonly with paired processes or lobes posterolaterally. Anterior part of trunk often narrow, forming “neck” region, or with second pedigerous somite defined by constrictions. Genitoabdomen comprising genital complex and abdomen; capable of being partly telescoped within trunk. Genital apertures located dorsolaterally on anterior part of genitoabdomen and paired copulatory pores located ventrally. Caudal rami with 5 or 6 setae.

Antennule typically indistinctly 5-segmented with setation pattern 10, 5, 4, 1, 13 + ae, or reduced. Additional segmental fusions resulting in 3- or 4-segmented antennule in some species. Some setal elements on basal segment modified as robust spines in some species. Antenna 3-segmented: first segment unarmed; second segment typically with pitted surface ornamentation; third segment forming terminal claw with or without seta proximally. Mandible stylet-like, lying within oral cone, armed with up to 7 teeth. Maxillule bilobate, typically with 2 setal elements per lobe. Maxilla 4-segmented: comprising unarmed basal segment, long second segment bearing setal element proximally on medial margin; third segment short with distal seta; fourth segment small, terminating in bifid claw and usually armed with fine seta. Maxilliped absent. Legs 1 and 2 biramous with 1- or 2-segmented rami: members of leg pair connected by interpodal bars on ventral surface of somite. Interpodal bars ornamented with paired processes in some species. Setal formula of legs 1 and 2 typically:

Protopod Exopod Endopod

Leg 1 1 – 1 1 – 0; 3 to 6 0 – 0/1; 2 to 5

Leg 2 1 – 0 1 – 0; 3 to 5 0 – 1; 2 to 5

Interspecific variation restricted to proximal endopodal segment and distal segments of both rami. Legs 3 and 4 reduced, located on lateral margins of trunk. Leg 3 typically a small lobe bearing 1 to 3 setae. Leg 4 typically represented by single seta. Leg 5 absent.

Male body comprising cephalothorax and post-cephalothoracic trunk. Cephalothorax incorporating first pedigerous somite and bearing dorsal shield as in female. Trunk comprising second to fifth pedigerous somites plus genital somite, and abdomen, all fused. Trunk bearing paired genital apertures posteriorly on ventral surface; genital apertures closed off by opercular plates formed by sixth legs. Posteriormost part of trunk derived from abdomen and bearing paired caudal rami posteriorly. Caudal rami typically more elongate than in respective female.

Antennule as in female, showing sexual dimorphism in some species. Antenna 3-segmented: first segment short, unarmed; second segment typically bearing strong process extending distally; third segment armed with 1 or 2 setae and bearing terminal claw (bearing accessory process). Mandible, maxillule and maxilla as female. Maxilliped absent. Legs 1 and 2 biramous with 1- or 2-segmented rami as in female. Legs 3 and 4 reduced, as in female except leg 3 represented by 1 to 5 setae. Leg 5 absent.

Remarks

The updated diagnosis presented here incorporates new data on the tagmosis obtained using CLSM. The root cause of the uncertainty surrounding the correct terminology for the posteriormost region of the female body has been the lack of any trace of leg 5 because the position of this leg is the usual marker for the fate of the fifth pedigerous somite. The ventral longitudinal trunk muscles (VLM) of H. pholas are visible in Fig. 1 View FIGURE 1 and the pattern of intermediate attachments to the body wall, as visualized by the interruptions in the striated trunk muscle bundles, clearly shows that the fifth pedigerous somite is incorporated into the trunk. In ventral view ( Fig. 1A View FIGURE 1 ) each of the paired VLM forms broad attachments to the body wall that mark the planes of the original pedigerous somite 2/pedigerous somite 3 articulation ( Fig. 1A View FIGURE 1 , ped2/3), and the original pedigerous somite 3/pedigerous somite 4 articulation ( Fig. 1A View FIGURE 1 , ped3/4). Within the fourth pedigerous somite the VLM bundle tapers posteriorly towards a narrow attachment marking the plane of the original pedigerous somite 4/pedigerous somite 5 articulation (ped4/5). Only a single bundle of muscle fibres continues posteriorly through the fifth pedigerous somite and this inserts on the anterior rim of the posteriormost region of the body. The posteriormost region of the female body is, therefore, a genitoabdomen, comprising the genital somite and abdomen. Contraction of this last muscle pair would help to “telescope” the anterior part of the genitoabdomen inside the rear end of the trunk.

In adult males the paired genital apertures are carried at the rear end of the trunk, indicating that the genital somite is incorporated into the trunk. The posteriormost part of the male body carries the caudal rami and represents the free abdomen.