Hermilius youngi Kabata 1964
Creators: name: Boxshall, Geoff
Publisher: name: Zenodo
Year: 2018
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Maxillopoda
Siphonostomatoida
Caligidae
Hermilius
Hermilius youngi

description: Hermilius youngi Kabata, 1964
(Fig. 66)
Material examined. 4♀♀ from Neoarius graeffei (Kner & Steindachner, 1867) (TC17597) 26 June 2016, 2♀♀ QM Reg. No. W53121, 2♀♀ NHMUK 2017.340–341; 1♀ from (TC17624) 27 June 2016, NHMUK Reg. No. 2017.342.
Site on host. Gill filaments.
Differential diagnosis. Cephalothorax well developed with lateral zones folded ventrally (Fig. 66A) and supported by prominent ribs and with membrane along outer margins. Cephalothorax flexed ventrally in fixed specimens. Frontal plates with marginal membrane lacking lunules. Fourth pedigerous somite distinct but concealed by genital complex. Genital complex large and dorsoventrally flattened, with wide wing-like lateral extensions not reaching to base of abdomen (Fig. 66B,C); about 1.2 times longer than wide, with rounded posterolateral corners. Abdomen indistinctly subdivided by constriction at mid-length (Fig. 66C), about as long as wide. Caudal rami longer than wide, with setiform caudal setae. Antenna comprising short unarmed first segment, lacking any process, longer second segment (Fig. 66D) and well developed subchela forming large apical hook with conspicuous accessory hook; subchela with single seta and small process on proximal part. Tine of postantennal process vestigial; papillae on vestigial process bisensillate; no papilla found on adjacent body surface. Maxillule with long slender posterior process. Maxilla with robust calamus and short stumpy canna fused to basis (Fig. 66E). Maxilliped very slender. Sternal furca with medium-length, more or less parallel tines (Fig. 66F). Leg 1 with hirsute vestigial endopod; spines 1 to 3 on distal exopodal segment well developed, as long as (spine 3) or markedly longer than segment (spines 1 and 2); seta 4 short and naked: posterior margin with 3 plumose setae. Leg 2 biramous, with 2-segmented rami: outer spines on exopodal segments aligned close to longitudinal axis of ramus; proximal outer spine on third segment similar in length to spine on second segment. Leg 3 with 2-segmented exopod (Fig. 66G); first segment bearing straight spine but lacking inner seta; compound distal segment armed with outer spine laterally, and with 3 spines and 2 plumose setae around distal margin (Fig. 66G); endopod 2- segmented; first segment with elongate velum and inner seta; compound distal segment with 5 plumose setae. Leg 4 lobate (Fig. 66H), broader near apex; armed with large plumose seta laterally, plus 3 vestigial spines around apex. Mean body length of female 3.95 mm, range 3.75 to 4.13 mm (based on 4 specimens).
Remarks. Kabata (1964a) distinguished this species from its congeners using a combination of features of the antenna, leg 4 and genital complex. The flattened genital complex of H. youngi has wing-like lateral expansions which terminate some distance anterior to the base of the abdomen, so the most posterior part of the genital complex is only a little wider than the abdomen itself. In H. longicornis the wing-like expansions extend right to the base of the abdomen. Leg 4 of H. youngi was not figured in Kabata’s original description because he was unable to observe it clearly (Kabata, 1964a). Study of the new material reveals that it is an irregularly cylindrical lobe armed with 3 reduced setal elements distally plus a well developed seta laterally, which probably represents the outer protopodal seta. It is very different from leg 4 in other Hermilius species. The reduced setation of both rami of leg 3 also helps to distinguish H. youngi from its congeners; the compound distal exopodal segment carries a total of 4 spines and only 2 setae, while the compound distal endopodal segment carries only 5 plumose setae instead of the usual 6.
Kabata (1964a) found one ovigerous female on each of two catfish hosts; he gave Neoarius australis as the type host and Netuma australis as a second host. The species name australis within the Ariidae is from the original combination Arius australis Günther, 1867, and its current valid name is Neoarius graeffei according to Eschmeyer (1998) and Bailly (2015). It seems likely that both Neoarius australis and Netuma australis, which were collected on different occasions, could be referable to Neoarius graeffei. The new material from Moreton Bay, the type locality, was also collected from Neoarius graeffei, the type host.
description: Published as part of Boxshall, Geoff, 2018, The sea lice (Copepoda: Caligidae) of Moreton Bay (Queensland, Australia), with descriptions of thirteen new species, pp. 1-172 in Zootaxa 4398 (1) on pages 139-142, DOI: 10.11646/zootaxa.4398.1.1, http://zenodo.org/record/1202953