Hatschekia pholas ( Wilson, 1906 )

Hatschekia pholas ( Wilson, 1906) View in CoL

Syn: Caetrodes pholas Wilson, 1906

Material examined

22 ♀♀ and 1♂ from gills of Arothron stellatus (Anonymous, 1798) ( TC17188 ) collected off Peel Island , Moreton Bay on 15 January 2016 : 16 ♀♀ QM Reg. Nos. W55139 View Materials , 6 ♀♀ and 1♂ NHMUK Reg. Nos. 2022.217- 223. The male was found in amplexus with a female .

Supplementary description of female.

Total body length excluding caudal rami ranging from 1.21 to 1.51 mm, with a mean of 1.38 mm (n = 5). Body ( Figs. 1A–C View FIGURE 1 , 12A View FIGURE 12 ) dorsoventrally depressed, comprising large anterior cephalothorax and short trunk bearing genitoabdomen posteriorly. Cephalothorax broader than trunk anteriorly, narrower posteriorly, longer than wide (690 x 650 μm); dorsal cephalothoracic shield with evenly convex frontal margin and produced posteriorly into flattened plate-like extension concealing anterior part of trunk in dorsal view ( Fig. 1C View FIGURE 1 ): shield supported by T-shaped subsurface chitinous frame. Trunk about 1.4 to 1.5 times longer than wide (850 x 560 μm); with greatest width about at level of insertion of third legs; tapering slightly from level of fourth legs towards posterior extremity; rear margin with rounded posterolateral lobes extending beyond rear margin of genitoabdomen: trunk with paired dorsolateral swellings just posterior to rear margin of dorsal cephalothoracic shield ( Fig. 1B View FIGURE 1 ). Genitoabdomen ( Fig. 12B View FIGURE 12 ) wider than long excluding caudal rami, comprising fused genital and abdominal somites; bearing paired genital apertures dorsally. Widest proximal part of genitoabdomen apparently telescoped within rear part of trunk. Caudal rami about 1.8 times longer than wide; armed with 5 naked setae of different lengths; apical seta fused at base, shorter than ramus; lateral seta located about at about 65% of lateral margin. Fertilised females carrying pair of spermatophores and complex chitinous coupler adhering to dorsal surface of genitoabdomen ( Figs. 13 View FIGURE 13 , 14A View FIGURE 14 ). Mean number of eggs per egg sac = 30.6 (range 26 to 37, n = 5).

Rostrum with paired lateral rostral processes ( Fig. 12C View FIGURE 12 ).Antennule ( Fig. 12C View FIGURE 12 ) indistinctly segmented: segmental setation pattern 9, 5, 4, 1, 13 + ae; 2 unequal setae located on antero-dorsal surface of first segment. Antenna ( Fig. 12D View FIGURE 12 ) 3-segmented, comprising very short unarmed coxa, robust tapering basis, and unsegmented endopod forming subchela: subchela bearing claw at a right angle to proximal swollen part. Mandible ( Fig. 12E View FIGURE 12 ) stylet-like, bearing row of 5 minute, marginal teeth subapically. Maxillule bilobed ( Fig. 12F View FIGURE 12 ): inner lobe armed with 2 short setae and outer lobe with 2 longer setae. Maxilla ( Fig. 12G View FIGURE 12 ) subchelate: 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 minute seta and bifid tip.

Swimming legs 1 and 2 biramous; members of each leg pair joined by slender interpodal bars ( Figs. 1A View FIGURE 1 , 12H View FIGURE 12 ). Leg 1 ( Fig. 12I View FIGURE 12 ) with fused sympod armed with outer and inner setae: exopod indistinctly 2-segmented and slightly curved; proximal segment bearing outer distal spine; distal segment bearing 3 setae on apical margin and 1 short seta distally on inner margin: endopod unsegmented; lacking seta on proximal part; distal part armed with 2 apical setae, inner slightly longer than outer, plus minute setal vestige at inner distal corner. Leg ornamented with curved rows of minute spinules: 5 on sympod, and 2 each on exopodal segments 1 and 2, and 3 on endopod. Leg 2 ( Fig. 12J View FIGURE 12 ) with fused sympod armed with outer seta: exopod distinctly 2-segmented; proximal segment long, bearing slightly curved outer distal spine; distal segment bearing 1 long seta on apex and 1 short seta either side: endopod unsegmented; lacking seta on proximal part; distal part armed with 1 short outer seta and 1 long inner seta on distal margin. Leg ornamented with curved rows of minute spinules: 2 on sympod, 3 on first exopodal segment and 2 on second, and 3 on endopod. Leg 3 located dorsolaterally on trunk at about 50% of length ( Fig. 12A View FIGURE 12 ), represented by two setae originating directly on trunk surface. Leg 4 located dorsolaterally on trunk at about 80% of length ( Fig. 12A View FIGURE 12 ), represented by single seta originating directly on trunk surface.

Description of male.

Total body length excluding caudal rami 343 μm. Body ( Fig. 14B View FIGURE 14 ) slightly dorsoventrally depressed, comprising anterior cephalothorax and long trunk tapering posteriorly into incorporated urosome. Cephalothorax ovoid in shape, about 1.4 times wider than long (86 x 122 μm). Dorsal cephalothoracic shield supported by m-shaped subsurface chitinous frame. Trunk about 3.0 times longer than wide (260 x 86 μm); anteriorly trunk with first two pedigerous somites weakly defined by indentations; greatest width of trunk just posterior to level of insertion of third legs; tapering posteriorly to level of genital apertures. Genital apertures located ventrally ( Fig. 14C View FIGURE 14 ); genital opercula each armed with single seta. Abdomen undivided, tapering slightly towards rear margin. Caudal rami elongate, about 2.8 times longer than wide; armed with 5 naked setae of different lengths; primary seta fused to ramus and just longer than ramus; lateral seta located at about 43% of lateral margin.

Rostrum with transverse posterior margin with small spinous process at each posterolateral corner and 3 pairs of rounded knobs on ventral surface; bearing well developed, paired lateral rostral processes extending to flange on posterior margin of proximal antennulary segment ( Fig. 14D View FIGURE 14 ). Antennule ( Fig. 14D View FIGURE 14 ) indistinctly 5-segmented: segmental setation pattern 7, 5, 4, 1, 12 + ae; proximal segment with flattened semicircular flange near distal end of posterior margin. Antenna ( Figs. 14D, E View FIGURE 14 ) comprising unarmed coxa, robust basis and distal subchela; basis bearing tapering chitinous process distally, projecting across base of subchela; subchela consisting of proximal segment bearing 1 seta plus curved distal claw bearing digitiform process proximally on concave margin. Parabasal papilla rounded and ornamented with curved array of tooth-like denticles (arrowed in Fig. 14D View FIGURE 14 ). Mandible, maxillule and maxilla as in female.

Swimming legs 1 and 2 biramous; members of each leg pair joined by slender interpodal bars ( Fig. 14F View FIGURE 14 ), each bearing 2 pairs of posteriorly directed spinous processes with rounded tips. Irregular sclerotized bar present between interpodal bars. Leg 1 ( Fig. 14F View FIGURE 14 ) with fused sympod armed with outer and inner setae: exopod indistinctly 2-segmented; proximal segment bearing outer distal spine; distal segment bearing total of 5 setal elements around apex and inner margin: endopod 2-segmented; first segment unarmed; second segment armed with long apical seta plus shorter seta on each side. Leg ornamented with curved rows of minute spinules: 3 on sympod, 3 on first exopodal segment and 2 on second, plus 1 on first and 3 on second endopodal segments. Leg 2 ( Fig. 14F View FIGURE 14 ) with fused sympod bearing outer seta; exopod unsegmented; bearing long outer spine derived from proximal segment; bearing 4 setal elements along distal and inner margins: endopod distinctly 2-segmented; proximal segment unarmed; distal segment armed with long apical seta plus shorter seta on each side as in leg 1. Leg 2 ornamented with curved rows of minute spinules: 3 on sympod, 2 each on exopodal segment, plus 1 on first and 3 on second endopodal segments. Leg 3 ( Fig. 14B View FIGURE 14 ) located dorsolaterally on trunk, represented by two setae originating on trunk surface. Leg 4 located dorsally on trunk, represented by single seta originating directly on trunk surface.

Remarks

The female of H. pholas was redescribed in modern detail by Uyeno & Nagasawa (2013) who noted several differences from the original description by Wilson (1906) and from the redescription by Pillai (1968). However, they examined Wilson’s syntype material in the collections of the United States Natural History Museum and were able to confirm that their material from the Ryukyu Islands, Japan was conspecific with the types. The Australian material conforms closely to Uyeno & Nagasawa’s (2013) redescription. The few minor differences include the apparent absence of the minute innermost seta on the distal margin of the endopod of leg 2 in the Australian material. This seta is present on the endopod of leg 1 in both the Australian and Japanese material but is present on leg 2 in the Japanese material only.

The male is described here for the first time. Only a single male was found and it was collected in amplexus with a female. Sexual dimorphism in H. pholas is noted in the shape of the dorsal cephalothoracic shield and its chitinous supporting frame, and in body shape. In addition, the caudal rami of the male are more elongate than those of the female and the principal caudal seta is markedly longer than the ramus in the male but shorter than the ramus in the female. The proximal segment of the antennule of the male is armed with only 7 setae (compared with 9 in the female) and has a flattened flange located distally on the posterior margin. The antenna carries a tapering process distally on the basis and the subchela is divided and armed with 2 setal elements in the male while the female lacks any process on the basis and the entire subchela forms an unarmed recurved claw. Legs 1 and 2 have a similar armature in both sexes, the main difference being the presence of 5 setae on the distal exopodal segment of leg 1 in the male compared to only 4 in the female. The major dimorphism regarding the legs is the presence of 2 pairs of posteriorly directed spinous processes on the interpodal bars of both legs in the male whereas the female lacks such processes.

Uyeno & Nagasawa (2013) noted that all their females were “armed with a chitinous lamella attached to the anterior border of urosome”. A similar chitinous structure was present on the females from Australia ( Fig. 13 View FIGURE 13 ). Careful removal revealed that it consisted of a complex chitinous plate incorporating a pair of spermatophores and their spermatophore tubules ( Fig. 14A View FIGURE 14 ). This structure is interpreted here as a coupler which is transferred together with the spermatophores onto the female during mating. It probably functions to help locate and attach the spermatophores to the female genital area in the correct position for them to discharge into the paired copulatory pores. Similar coupling structures are well known in certain calanoid copepods, such as the pontellid Labidocera aestiva Wheeler, 1900 ( Blades & Youngbluth, 1980), but this is the first report of such a structure in the Hatschekiidae .

The type host of H. pholas was the tetraodontid Arothron stellatus caught off Pearl Banks in Sri Lankan waters ( Wilson, 1906) and it was subsequently collected off Kerala, India on the same host ( Pillai, 1967). The subsequent host record, from Leiodon cutcutia (Hamilton, 1822) (as Tetraodon cutcutia ) caught in Indian waters ( Pillai, 1967, 1968) was considered doubtful by Uyeno & Nagasawa (2013) because L. cutcutia inhabits fresh and brackish waters but no Hatschekia species has ever been reported from such reduced salinity regimes. The present record from Australian waters is also from A. stellatus which remains the only confirmed host of H. pholas .