Hatschekia difficilis Kabata, 1991

Boxshall, Geoffrey A. & Bernot, James P., 2025, The Hatschekiidae (Copepoda: Siphonostomatoida) of Moreton Bay, Queensland, Australia, with a key to species reported from Australian waters, Zootaxa 5716 (1), pp. 11-65 : 41-42

Hatschekia difficilis Kabata, 1991

Material examined: 1 ♀ from gills of Scarus ghobban Forsskål, 1775 ( TC16969 ) collected off Amity , North Stradbroke Is., Moreton Bay on 11 January 2016, QM Reg. No. W55137 .

Supplementary description of female

Total body length excluding caudal rami 1.63 mm.Body( Fig.10A )cylindrical, comprising anterior cephalothorax and elongate trunk tapering posteriorly to minute triangular genitoabdomen. Cephalothorax oval, about 1.3 times wider than long (185 x 234 μm). Dorsal cephalothoracic shield supported by T-shaped subsurface chitinous frame with long median longitudinal bar extending to about 80% of shield length. Trunk about 6.6 times longer than wide (1.45 x 0.22 mm); with greatest width in posterior half; trunk tapering strongly towards posterior extremity appearing triangular with rounded apex (in dorsal view). Pair of subspherical swellings present posterolaterally, just anterior to level of genital apertures ( Fig. 10A, B ). Genitoabdomen fused with trunk and comprising fused genital-double and abdominal somites; bearing paired genital apertures dorsally. Caudal rami about twice as long as wide (23 x 11 μm); armed with 6 naked setae of different lengths; lateral seta located about at middle of lateral margin. Both egg sacs containing 9 eggs.

Rostrum bearing paired lateral rostral processes ( Fig.10C ).Antennule ( Fig. 10C ) short, indistinctly 5-segmented: segmental setation pattern 10, 5, 4, 1, 11 + ae; 2 unequal setae located on dorsal surface of first segment. Antenna ( Fig. 10D ) 3-segmented, comprising short unarmed coxa, robust tapering basis, and distal subchela: surface of basis ornamented with minute pits; subchela with swollen and thickened base drawn out into curved claw. Mandible ( Fig. 10E ) stylet-like, bearing row of 4 marginal teeth subapically. Maxillule bilobed ( Fig. 10F ): both lobes armed with 2 setae, setae on outer lobe more robust and slightly longer than on inner lobe. Maxilla carried on pedestal produced into curved posterior flange (arrowed in Fig. 10G ); coxobasis armed with single inner seta proximally on basal part; subchela comprising long segment armed with slender seta at inner extremity and distal claw with bifid tip.

Swimming legs 1 and 2 biramous; members of each leg pair joined by slender interpodal bars ( Fig. 10H ). Transverse band of weakly sclerotized integument present between interpodal bars. Leg 1 ( Fig. 10I ) with fused sympod armed with outer and inner setae: exopod distinctly 2-segmented; proximal segment with outer distal spine; distal segment bearing 3 long setal elements around apex and 3 reduced setae along inner margin: endopod distinctly 2-segmented; proximal segment unarmed; distal segment armed with 3 unequal setae around apex and 2 reduced setae on inner margin. Leg ornamented with curved rows of minute spinules: 2 on sympod, 3 on exopodal segment 1, and 1 each on distal segments of both rami. Leg 2 ( Fig. 10J ) with fused sympod bearing outer seta; exopod 2- segmented; with articulation between segments offset; proximal segment bearing short outer spine on extended outer distal tip of segment; distal segment bearing 2 unequal setal elements on apex and 1 reduced seta distally on inner margin: endopod distinctly 2-segmented; proximal segment unarmed; distal segment armed with outer seta, 3 unequal setae around apex, and 1 rudimentary inner seta. Leg ornamented with curved rows of minute spinules: 1 on sympod, and 3 on exopodal segment 1 and endopodal segment 2, and 2 on exopodal segment 2. Leg 3 located laterally on trunk at about 28% of length ( Fig. 10A ), represented by small posterolaterally-directed lobe armed with 1 short and 2 long setae ( Fig. 10K ). Leg 4 not observed.

Remarks

The only previous report of H. difficilis is the original description which was based on three females collected from the same host species, Scarus ghobban , at Green Island, Queensland ( Kabata, 1991). There are some differences between Kabata’s description and the new material, but the key similarities, such as the presence of subspherical swellings located posterolaterally on the trunk just anterior to the genital apertures and the presence of 3 setae on leg 3, allow confidence in the identification. The new material has a more elongate trunk than the type material, but body proportions in hatschekiids are notoriously unreliable as taxonomic characters ( Jones, 1985; Kabata, 1991). The body length of two type specimens was given as 1.24 mm and the length to width ratio of the trunk is 3.4:1 (measured from Figure 41 in Kabata (1991)) compared with 1.63 mm and 6.6:1, respectively, in the new material. I consider that this difference is due to the state of contraction of the material. Kabata (1991) described a transverse welt on the trunk just behind the cephalothorax but this welt, which is inferred to result from contraction of the longitudinal trunk muscles, is not expressed in the new material, which appears to be more relaxed. These differences between contracted and uncontracted specimens are of a similar magnitude to those illustrated by Kabata (1991) for a different hatschekiid species, Prohatschekia awatati . Kabata figured two females of P. awatati one of which was contracted: the body length of the contracted specimen was 0.77 mm (measured from his Figure 1 ) and the length to width ratio of the trunk was 1.2:1, whereas the uncontracted female had a body length of 1.23 mm and the L:W ratio was 2.0:1.

The segmentation of legs 1 and 2 is the same in both sets of material, as is the setation of both rami of leg 1 but in the new material the distal endopodal segment of leg 2 carries a seta on the outer margin and a rudimentary seta on the inner margin; neither of which was observed by Kabata (1991). Other differences include the presence of two setal elements on the inner lobe of the maxillule (compared to only one found by Kabata (1991)), and the setation of the antennule. The reported differences in both of these limbs are attributed to the difficulties encountered by Kabata (1991) in observing his material. The presence of lateral processes on the rostrum was not mentioned by Kabata (1991) but these are present and are well developed ( Fig. 10C ). Similarly, the pedestal which carries the maxilla in Hatschekia has a well developed cuticular flange along its posterior margin in the Moreton Bay material, which was not mentioned by Kabata (1991). This is an unusual character state which is useful in species discrimination.

Examination of a sample of Hatschekia from a Scarus ghobban collected by Jean-Lou Justine in New Caledonia (Fish No: JNC2515) revealed the presence of two females of H. difficilis , although a second as yet undescribed species of Hatschekia was present in greater numbers on this fish host. This New Caledonian find constitutes the first record of H. difficilis from outside Australian waters.

Figures

FIGURE 1. Hatschekia pholas (Wilson, 1906), female, Confocal Laser Scanning Micrographs (CLSM). A, habitus, ventral view showing insertions of ventral longitudinal trunk muscles (VLM) marking boundaries between pedigerous somites (ped)2/3, ped3/4 and ped4/5; B, habitus, lateral view showing VLM insertions; C, habitus, dorsal view showing dorsal cephalothoracic shield extending over anterior part of trunk. Scale bar 500 μm.

FIGURE 10. Hatschekia difficilis Kabata, 1991, female.A, habitus, dorsal; B, posterior extremity of trunk and genitoabdomen, ventral; C, right rostral process and right antennule; D, antenna; E, mandible; F, right maxillule, ventral; G, maxilla, with arrow showing flanged pedestal; H, interpodal bars, ventral; I, leg 1; J, leg 2, K, leg 3. Scale bars: A, 300 μm, B–D, G–J, 50 μm, E, F, K, 25 μm.