Willenstenhelia thalia sp. nov.

(Figs. 48–54)

Type locality. South Korea, South Sea, Gwangyang Bay, sampling station 10, muddy sediments, 37.539231°N 126.343417°E .

Specimens examined. Female holotype dissected on one slide (collection number NIBRIV0000232706), male allotype dissected on one slide (collection number NIBRIV0000232707), two male paratypes and four female paratypes together on one SEM stub (collection number NIBRIV0000232708), three males and four females destroyed for DNA sequence (four females amplification successful, Codes 0245, 0444, 0146 & 0348), type locality, 18 November 2012, leg. K. Kim.

Two male paratypes and seven female paratypes together in ethanol (collection number NIBRIV0000232709), three females destroyed for DNA sequence (amplification successful, Codes 0143, 0241 & 0342), type locality, 30 July 2012, leg. K. Kim.

Thirty male paratypes and 30 female paratypes together in ethanol (collection number NIBRIV0000232710), one female destroyed for DNA sequence (amplification successful, Code 0547), South Korea, South Sea, Gwangyang Bay, sampling station 10, muddy sediments, 34.920944°N 127.785528°E, 18 November 2012, leg. K. Kim (Fig. 1) .

Three male paratypes and 12 female paratypes together in ethanol (collection number NIBRIV0000232711), one female destroyed for DNA sequence (amplification unsuccessful), South Korea, South Sea, Gwangyang Bay, sampling station 10, muddy sediments, 34.920944°N 127.785528°E, 18 February 2012, leg. K. Kim.

One female destroyed for DNA sequence (amplification unsuccessful), South Korea, South Sea, Gwangyang Bay, sampling station 15, muddy sediments, 34.797194°N 127.786444°E, 18 November 2012, leg. K. Kim (Fig. 1) .

Etymology. The species is named after Thalia (Ancient Greek: Θάλεια), one of nine Muses from Greek mythology, who was a patron of comedy. The species name is a noun in apposition (in the nominative case).

Description. Female (based on holotype and six paratypes). Body length from 373 to 441 µm (428 µm in holotype). Body segmentation, colour, nauplius eye, hyaline fringes, integument thickness as in Wellstenhelia calliope sp. nov. Surface appearance of prosomites extremely smooth and without minute pits even when examined on highest magnifications with scanning electron microscope; surface of urosomites mostly covered with numerous irregular rows of minute pits on dorsal and lateral sides, except for hyaline fringes which generally smooth and slightly frilled. Most somite ornamentation also similar to Wellstenhelia calliope, and presumed homologous pore and sensilla numbered with same Arabic numerals (see Figs. 48A, B, C, 49A, B, C, D) to allow easier comparison. Habitus (Figs. 48A, B, 53A) less robust than in Wellstenhelia calliope, with prosome/urosome length ratio about 1.1, body length/width ratio from 2.6 to 2.9, and cephalothorax from 1.6 to 1.7 times as wide as genital double-somite.

Rostrum (Figs. 48A, B, C, 53B) general shape and position of anterior pair of sensilla no. 1 as in Wellstenhelia calliope but without dorsal pore no. 2 and with two dorso-lateral pores centrally (no. α).

Cephalothorax (Figs. 48A, B, 49C, 53B, 54A, C) about 0.9 times as long as wide; comprising 30% of total body length. Surface of cephalothoracic shield with 28 unpaired and paired sensilla and pores, most of which probably homologous to those in Wellstenhelia calliope (indicated with Arabic numerals in illustrations), but 17 pores and sensilla missing (nos. 3, 6, 10, 14, 16, 17, 18, 19, 21, 23, 26, 27, 29, 33, 34, 37, 40); absolute and relative positions of some pores and sensilla differ; one lateral pair of sensilla probably homologous to that in Wellstenhelia euterpe (indicated with geometric shape in illustrations); one unpaired dorsal sensillum (no. II) probably homologous to that in Itostenhelia polyhymnia; three unique pairs of pores located in anterior half, one lateral (no. β) and two dorsal (nos. γ, δ).

Pleuron of second pedigerous somite (Figs. 48A, B, 49D) relatively narrow, with only four pairs of posterior sensilla, three probably homologous to Wellstenhelia calliope (nos. 46, 49, 50) and one probably homologous to Wellstenhelia euterpe (no. IV).

Pleuron of third pedigerous somite (Figs. 48A, B, 49D) similar to second pedigerous somite in shape and ornamentation, with only four pairs of posterior sensilla, all probably homologous to those in Wellstenhelia calliope (nos. 53, 54, 55, 56).

Pleuron of fourth pedigerous somite (Fig. 48A, B) with only three pairs of posterior sensilla, all homologous to those in Wellstenhelia calliope (nos. 59, 60, 61).

First urosomite (Figs. 48A, B, 53C, F) as in Wellstenhelia calliope, except pair of anterior lateral pores no. 63 and pair of dorso-lateral sensilla no. 66 missing; dorsal surface additionally ornamented with several rows of minute spinules, while ventral surface smooth.

Genital double-somite (Figs. 48A, B, 49A, B, 53C, F) shape as in Wellstenhelia calliope, except anterior part much more inflated laterally, almost as wide as first urosomite, with lateral expansions produced into posteriorly directed, large spiniform processes (arrowed in Figs. 48B, 49A); paired copulatory pores small ovoid holes in anterior part, situated ventro-laterally; paired genital apertures situated laterally, covered by reduced sixth legs; only more sclerotized part of genital apparatus visible inside; ornamentation similar to that in Wellstenhelia calliope, except most large spinules missing (only three at base of ventral sensillum no. 73, arrowed in Fig49A, and two close to dorsal sensillum no. 71), lateral pair of sensilla no. 72 also missing, and dorsal anterior pairs of sensilla nos. 69 & 70 more widely spaced; dorsal surface additionally ornamented with many parallel rows of minute spinules, ventral surface smooth; no ventral suture between genital apertures.

Third urosomite (Figs. 48A, B, 49A) as in Wellstenhelia calliope, except lateral pair of sensilla no. 76 and most large spinules missing, two or three large spinules only at base of sensilla; dorsal surface additionally ornamented with four parallel rows of minute spinules, while ventral surface smooth.

Preanal somite (Figs. 48A, B, 49A, 53D) without sensilla as in Wellstenhelia calliope but with pair of unique lateral pores (no. ε), two large posterior spinules dorso-laterally on each side, and three parallel rows of minute dorsal spinules.

Anal somite (Figs. 48A, B, 49A, 53D, 54B) general shape and ornamentation as in Wellstenhelia calliope, except for fewer spinules along medial cleft and distal margin, lateral pores nos. 79 & 80 missing, and anal operculum well developed, broad and smooth, reaching beyond distal margin of anal somite.

Caudal rami (Figs. 48A, B, 49A, 53D, 54B) shape and armature similar to those in Wellstenhelia calliope, except more elongated, middle lateral seta much shorter and more slender, inner margin without spinules, anterior ventral pore no. 82 moved more towards outer margin (arrowed in Fig. 49A), and posterior ventral pore no. 83 missing; rami cylindrical, about 1.9 times as long as anal somite, 4.6 times as long as wide (ventral view), slightly divergent or parallel, and with space between them about one ramus width.

Antennula (Figs. 48D, 49E, F, 53B) similar to that in Wellstenhelia calliope but sixth and seventh segments almost completely fused, and eighth segment with only three lateral setae and without apical aesthetasc; only ornamentation short row of small spinules in proximal half of first segment and one central row of minute dorsal spinules, distal row of spinules and spiniform process missing; aesthetasc on fourth segment just reaching distal tip of appendage; setal formula 1.10.9.6+ae.3.(4.4).5.

Antenna (Fig. 48E) as in Wellstenhelia calliope, except allobasis and first exopodal segment proportionately shorter, endopod with single slender lateral seta, and inner apical fused seta much longer, spiniform, and geniculate.

Labrum (Fig. 53E) and paragnaths (Fig. 53E) as in Wellstenhelia calliope .

Mandibula (Figs. 50A, B, 53E) similar to that in Wellstenhelia calliope, except cutting edge without central seta and with shorter dorsalmost seta, basis with additional row of slender spinules, and exopod with three lateral setae inserted much closer to each other.

Maxillula (Figs. 48F, 49G, 53E) as in Wellstenhelia calliope, except cutting edge with stronger combs on two ventralmost spines (arrowed in Fig. 48F), basis almost completely fused with endopod and exopod, and endopod much wider distally (arrowed in Fig. 49G).

Maxilla (Figs. 50C, 53E, 41A) as in Wellstenhelia calliope, except dorsal endite on syncoxa with only two setae, syncoxa with only several outer spinules, and endopod with only four slender setae.

Maxilliped (Figs. 48G, 53E) segmentation, armature and most ornamentation as in Wellstenhelia calliope, except outer distal corner of coxa produced into pronounced and blunt process and second endopodal segment fused basally to first endopodal; also setae on first endopodal segment proportionately much longer.

First swimming leg (Fig. 50D) as in Itostenhelia polyhymnia, except coxa without proximal row of anterior spinules (arrowed in Fig. 50D) and distal inner seta on second endopodal segmented inserted closer to distal margin.

Second swimming leg (Fig. 50E, F) as in Itostenhelia polyhymnia, except first exopodal segment without inner seta, third exopodal segment without distal inner seta (arrowed in Fig. 50F) and second endopodal segment with shorter proximal seta (arrowed in Fig. 50E).

Third swimming leg (Fig. 50G) as in Itostenhelia polyhymnia, except intercoxal sclerite with shorter distal projections, coxa without proximal row of spinules (arrowed in Fig. 50G), first exopodal segment without inner seta (arrowed in Fig. 50G), third exopodal segment without distal exopodal setae (arrowed in Fig. 50G) and with three outer spines, and third endopodal segment without proximal or distal inner setae (both positions arrowed in Fig. 50G).

Fourth swimming leg (Fig. 51A) as in Wellstenhelia calliope, except intercoxal sclerite with sharp distal projections, basis without inner spinules, first exopodal segment without inner setae (arrowed in Fig. 51A), third exopodal segment with only two outer spines and without proximal or distal inner setae (both arrowed in Fig. 51A), first endopodal segment smaller and with longer and stronger inner seta (arrowed in Fig. 51A), second endopodal segment without inner seta (arrowed in Fig. 51A), and third endopodal segment with only one inner seta (arrowed in Fig. 51A).

Fifth leg (Figs. 49A, 51B, 53F) segmentation as in Wellstenhelia calliope but general shape, armature, and ornamentation very different. Endopodal lobes completely fused, each with three elements, with enormous gap (arrowed in Fig. 51B) between two inner spiniform ones; outer element minute, smooth, and slender (arrowed in Fig. 51B); middle element strongest, unipinnate along outer margin, twice as long as inner element, and 3.5 times as long as outer slender seta. Exopod almost rectangular (arrowed in Fig. 51B), 1.5 times as long as wide, with longitudinal row of outer spinules in distal part, transverse row of inner spinules, one anterior pore and one posterior pore, with five setae, innermost one larger than some spinules (arrowed in Fig. 51B). Length ratio of exopodal setae, starting from inner side, 1: 5.5: 1.5: 1.8: 3.1.

Sixth leg (Fig. 49B) simple small cuticular plate covering genital aperture, inserted at midlength of genital double-somite laterally, with single smooth seta flanking short spiniform process.

Male (based on allotype and 16 paratypes). Body length from 375 to 403 µm (385 µm in allotype). Habitus (Fig. 52A), colour, rostrum (Fig. 52D), shape and ornamentation of cephalothorax (Figs. 52A, 54A, C), second pedigerous somite (Figs. 52A), third pedigerous somite (Fig. 52A), fourth pedigerous somite (Fig. 52A), ornamentation of first urosomite (Figs. 52A, B, 54E), anal somite (Fig. 52A, B), caudal rami (Figs. 52A, B, 54F), antenna, labrum, paragnaths, mandibula, maxillula, maxilla, maxilliped, first swimming leg, and third swimming leg as in female.

Genital somite (Figs. 52A, B, 54E) shorter and wider than in Wellstenhelia calliope and without large spinules; lateral pore no. 68 missing; additionally ornamented with several rows of minute spinules dorso-laterally.

Third urosomite (Fig. 52A, B) as in Wellstenhelia calliope, except lateral pair of sensilla no. 72 missing, ventral row of spinules not interrupted between sensilla pair no. 73, and these sensilla also more widely spaced; additionally ornamented with several rows of minute spinules dorso-laterally.

Fourth urosomite (Fig. 52A, B) pore and sensilla pattern as in female (i.e. lateral sensilla no. 76 missing), except for continuous ventral posterior row of large spinules.

Fifth urosomite (Fig. 52A, B) with lateral pore (no. ε) as in female, but with posterior ventral row of spinules of various sizes.

Caudal rami (Figs. 52A, B, 54F) long and slender (arrowed in Fig. 52A), about 4.5 times as long as wide in ventral view, with middle lateral seta hardly longer than spinules that flank it.

Antennula (Figs. 51D, 54D) as in Itostenhelia terpisichore, except aesthetascs on third and fourth segment shorter, sixth segment with more spinules, and seventh segment with one more seta; setal formula 1.11.5+ae.9+ae.1.2.3.4.5.

Second swimming leg (Fig. 52C) with coxa, basis, and exopod as in female. Endopod with second and third segment fused, with only three elements on ancestral segment, and with inner apical element (arrowed in Fig. 52C) shorter than outer.

Fourth swimming leg (Fig. 52D) as in female, except outer spine on second exopodal segment strongly curved and sclerotized, highly characteristic and visible even on undissected specimens (arrowed in Fig. 52D).

Fifth leg (Figs. 51C, 52A, 54E) general shape, segmentation, and most armature and ornamentation as in Itostenhelia polyhymnia, except endopodal lobes slightly more pronounced, exopod slightly larger, and number of exopodal and endopodal armature elements often asymmetrical. Endopodal lobe mostly with two elements, inner one twice as long as outer one; third element always innermost and smallest. Exopod 1.4 times as long as wide, with outer margin smooth or with several strong spinules, inner margin always with several small spinules, with four or five armature elements; innermost armature element as in female minute and slender seta, next one longest and slender seta (arrowed in Fig. 52A), third short and slender seta, followed by one or two strong and equally long apical spines. Length ratio of exopodal armature, from inner side, 1: 6.8: 2: 3.5.

Sixth legs (Figs. 52B, 54E) as in Itostenhelia polyhymnia, except both legs distinct at base.

Variability. Fusion between sixth and seventh antennular segment in female is variable, but is never complete (Fig. 49E, F). Male fifth leg is often asymmetrical (Fig. 51C), with more or less spinules on the endopodal lobe, with or without spinules on the outer margin of exopod, with two or three endopodal armature elements, and with four or five exopodal armature elements. Most morphological features are, however, extremely conservative, including the sensilla and pores pattern of somites, and length ratio of different armature on appendages.

Morphological affinities. Willenstenhelia thalia sp. nov. differs from all congeners by the minute inner seta on the female fifth leg exopod (Fig. 51B), minute outermost seta on the female fifth leg endopod (Fig. 51B), and robust and spiniform second seta from outer side on the female fifth leg endopod (Fig. 51B), which can all be considered as autapomorphic character states. It also has a more elongated female fifth leg exopod than any other congener, but this may be a plesiomophy in a larger group of stenheliins. Posteriorly pointed, large spiniform processes on the anterior part of the genital double-somite (Figs. 48B, 49A, B) also may be an autapomorphy, but this feature was not described or illustrated in at least two congeners. All these characters would suggest a somewhat isolated position of Willenstenhelia thalia in the genus Willenstenhelia gen. nov., which is not surprising given that this is the only Pacific representative. Other congeners include Willenstenhelia unisetosa (Wells, 1967) comb. nov. from Mozambique (Indian Ocean), Willenstenhelia urania sp. nov. from the Mediterranean Coast of Israel, Willenstenhelia terpsichore sp. nov. from the Slovenian Coast of the Adriatic Sea (northern Mediterranean), and Willenstenhelia minuta T. Scott, 1902 from the Indian Ocean and the Suez Canal (see below and Scott 1902; Gurney 1927; Lang 1948; Por 1964; Wells 1967; Marinov & Apostolov 1981). The males are still unknown for Willenstenhelia minuta and Willenstenhelia urania, and those for Willenstenhelia terpsichore are described quite incompletely. However, the latter species apparently has both exopodal and endopodal spines (or spiniform setae?) on the male fifth leg fused basally, forming strong spiniform processes, which is very different from the male fifth legs of Willenstenhelia thalia and Willenstenhelia unisetosa . Similarities in the male fifth leg between the latter two species, in addition to an identical armature formula of both male and female swimming legs, and a very similar anal operculum, may suggest that they are more closely related to each other than to other congeners, while the other three species seem to form a relatively compact group, at least as one can judge from a limited set of morphological characters given in their original reports.

Willenstenhelia thalia differs from Willenstenhelia unisetosa additionally by much longer caudal rami, more slender habitus, shorter and more spiniform innermost seta on the female fifth leg endopod, larger and more deeply concave space between two inner setae on the female fifth leg endopod, much shorter outer endopodal element on the male fifth leg endopod, and much longer second seta from inner side on the male fifth leg exopod. Wells (1967) also reported only three endopodal setae on the maxilliped and only one setae on the mandibular basis, but this may be because some setae may have been broken off on his specimen, as all other well-studied stenheliids have four endopodal setae on the maxilliped and three basal setae on the mandibula.

Willenstenhelia thalia differs from Willenstenhelia minuta, Willenstenhelia urania, and Willenstenhelia terpsichore additionally by the spiniform nature of the innermost element on the female fifth leg endopod, shorter second seta from outer side on the female fifth leg exopod, and presence of long spinules on the female fifth leg exopod. Its caudal rami are also significantly longer than those in Willenstenhelia minuta and Willenstenhelia terpsichore, but comparable in size to those in Willenstenhelia urania .