Dana, 1852
Diagnosis
Globose species, sexual dimorphism slight (H. inflata Habitus 1, H. inflata Habitus 3). Rostra undeveloped. Dorsal margin almost straight, female height : length ratio greater with posterior and ventral margins curved like a sector of a circle. In ventral aspect, carapace bilaterally symmetrical about both longitudinal and lateral axes (H. inflata Habitus 2, H. inflata Habitus 4). Frontal organ a stubby, fat structure about twice the length of the limb of the first antenna (H. inflata 1, H. inflata 3). First antennae lacking sexual dimorphism. Second antennae are different insomuch as the male endopodites carry hook appendages which differ little in size between left and right. The a seta of first endopodite podomere reduced in both sexes (H. inflata 2, H. inflata 4). The caudal furca with seven pairs of hook setae.
Remarks
There are two similar species which have been consistently confused. There has been the persistent use of a junior synonym, H. brevirostris , for both of them. Angel, 1982 redescribed the smaller species H. pelagica Claus which had been synonymised with H. inflata (= brevirostris ) Dana by numerous other authors. He was able to demonstrate that, where the two species co-occurred, there was circumstantial evidence of character displacement which affected carapace size in the juveniles instars which co-occur bathymetrically, whereas the adults showed depth segregation. The species most likely to be encountered in UK waters is the larger, more northerly species.
Distribution
This is a neustonic-shallow mesopelagic species, which is recorded from all oceans. It is rare in UK waters, usually being restricted to latitudes <40°N. Its occurrence in our waters may result from advection within mesoscale eddies. 4 (R.R.S. Discovery Map).
Type specimens
None designated; status of original material uncertain.
Type locality
Original material from 23°S 41°10'W.