Onychocamptus bengalensis (Sewell, 1934)

Boonyanusith, Chaichat, Saetang, Thanida, Wongkamheng, Koraon & Supiyanit Maiphae,, 2018, Onychocamptus Daday, 1903 from Thailand, with descriptions of two new species and two new records (Crustacea, Copepoda, Harpacticoida, Laophontidae), ZooKeys 810, pp. 45-89 : 60-61

Onychocamptus bengalensis (Sewell, 1934) Figs 16, 17, 18, 22B, 23C

Laophonte bengalensis Sewell, 1934: 98, fig 10a−k.

Onychocamptus bengalensis : Lang 1948: 1409, abb. 571.9, 1420, abb. 578.2; Hamond 1973: 406, figs 42−65; Song and Chang 1995: 72, 75, fig 6; Lee and Chang 2005: 40, fig. 5.

Material examined.

Two females and two males from Khao Thanan cave, Satun Province, southern Thailand, 07°03'43.2"N, 99°41'42.7"E; coll. C Boonyanusith and K Wongkamhaeng; 12 December 2014. One female from Samer-rach peat swamp, Trat Province, eastern Thailand, 12°28'04.0"N 102°21'20.6"E.; coll. S Maiphae and T Saetang; 25 May 2017.

Differential diagnosis.

Laophontidae . Caudal rami more than four times as long as wide in female and approximately three times as long as wide in male. Female P5 exopod and baseoendopod fused, endopodal lobe and exopod with three setae each. P4 exp-3 with three outer spines.

Redescription of adult female.

Female (Fig. 16A). Total body length measured from tip of rostrum to posterior margin of caudal rami, 530-550 µm (mean 536 µm, n = 3); preserved specimen colourless. Body covered with setules, cylindrical, gradually tapering posteriorly, with maximum width at posterior part of cephalothorax. Prosome 1.6 times as long as urosome. Rostrum small, completely fused to cephalothorax, with pair of apical sensilla. Cephalothorax as long as wide, approximately 0.5 times the length of prosome (Fig. 16A). Cephalothorax and all free thoracic somites with less developed posterior sensillum-bearing tubercles (Fig. 16A, B). Second and third urosomite fused ventrally forming genital double-somite, with dorsal and lateral remnant of original division (Fig. 16C). Posterior half of genital double-somite and subsequent somites with lateral sensillum-bearing tubercles. Other characters on urosomite as in O. tratensis sp. n.

Caudal rami (Fig. 16C, D). Cylindrical, parallel, 4.3 times as long as wide, with seven setae of different lengths. Anterolateral accessory seta (I) minute, inserted close to anterolateral seta (II) at distal third of rami. Posterolateral seta (III) inserted on minute protuberance. Outer terminal seta (IV) slender, fused at base to inner terminal seta (V), the latter longest, without fracture plane, approximately 0.6 times as long as body length. Inner accessory seta (VI) slender. Dorsal seta (VII) tria-rticulate, inserted at quarter of ramus. Length ratio of caudal setae to ramus length from seta I to seta VII: 0.2: 0.5: 0.8: 1.1: 9.7: 0.3: 0.8.

Antennule and mouthparts as in O. satunensis sp. n. and O. tratensis sp. n., except for allobasis of antenna with one bipinnate abexopodal seta (Figs 17A, 22B).

P1 (Fig. 17B), P2, P3 and P4 (Fig. 17C) as in O. satunensis sp. n. and O. tratensis sp. n., except for P4 exp-3 with three outer spines. Armature formula of P1-P4 as in Table 2.

P5 (Figs 17E, 23C). Baseoendopod and exopod fused; exopod rectangular. Baseoendopod with basal seta, endopodal lobe with three pinnate setae and with row of spinules at base of each seta. Exopod with three bipinnate setae, innermost longest and with spinules at its base.

P6 (Fig. 16C). Reduced to minute prominence at outer distal corner of genital field, with short slender seta.

Redescription of adult male.

Body (Fig. 18A). Total body length, measured from tip of rostrum to posterior margin of caudal rami, 450-470 µm (n = 2); habitus smaller than female; preserved specimen colourless. Prosome approximately 1.1 times as long as urosome. Cephalothorax as long as wide, 0.5 times as long as prosome. Other characters as in O. satunensis sp. n. and O. tratensis sp. n.

Caudal rami (Fig. 18A). slightly divergent, three times as long as wide; length ratio of caudal setae to ramus length from seta I to seta VII: 0.3: 0.7: 1.2: 1.5: 7.6: 0.4: 1.2.

Antennule, antenna, mouthparts, P1, P2, P3 as in O. satunensis sp. n. and O. tratensis sp. n., except for P4 exp-3 with three robust outer spines (Fig. 18D).

Armature formula of P1-P4 as in Table 2.

P5 (Fig. 18C). Baseoendopod absent. Exopod with two setae, inner approximately 1.4 times as long as outer.

P6 (Fig. 18B). Reduced to minute, rectangular protuberance with two setae apically; inner seta approximately 1.5 times as long as outer one.

Variability.

In male, one additional seta on left P1 enp-2 (Fig. 17F). In female, left P4 with 2-segmented, two apical elements of exp-2 fused (Fig. 17D).

Onychocamptus bengalensis is characterised by the relatively long caudal ramus and fusion of exopod and baseoendopod of P5 in the female. The length:width ratio of caudal ramus in female is variable (three times as long as wide in Lang (1948) and Song and Chang (1995), 3.8-4.0 times as long as wide in Lee and Chang (2005), and six times as long as wide in Hamond (1973)). The length:width ratio of the caudal ramus shown in Hamond (1973), is however, approximately 4.5 ( Song and Chang 1995). The length:width ratio of the caudal ramus of the Thai specimens is 4.3. Other characters observed from the Thai specimens match well Hamond’s (1973) redescription, except for the basis of P3 and P4 with one row of setules along inner margin, and posterior margin of anal operculum with one row of spinules in the Thai material. The ornamentation of anal operculum is similar to that of the Korean specimens from the estuary of Ssangcheon Stream ( Lee and Chang 2005), but it differs from the Australian specimens by lacking ornamentation at posterior margin ( Hamond 1973).

Distribution.

This species has been recorded from Calcutta (India) ( Sewell 1934), brackish lagoons in northern coastal suburbs of Sydney (Australia) ( Hamond 1973), crab burrows in a mud flat a little apart from shore line in Chindo Island (Korea) ( Song and Chang 1995), and from Ssangcheon Stream (Korea) ( Lee and Chang 2005).

In this study, we found the species in i) Samer-rach peat swamp, Trat Province, eastern Thailand; water temperature, 27.65 °C; pH, 5.38; salinity, 6.06 ppt; conductivity, 11285.2 µS cm-1; total dissolved solids, 6982 mg L-1; dissolved oxygen, 3.86 mg L-1, ii) Khao Thanan cave, Satun Province, southern Thailand; this is a limestone cave which normally dries out during the dry season (April); water pH, 8.85; dissolved oxygen, 2.7 mg L-1; conductivity, 2.1 µS cm-1; salinity varies seasonally because of the effect from the sea nearby.