Wellstenhelia erato sp. nov.

(Figs. 18–23)

Type locality. South Korea, South Sea, Gwangyang Bay, sampling station 12, muddy sediments, 34.951389°N 127.734361°E (Fig. 1) .

Specimens examined. Female holotype dissected on one slide (collection number NIBRIV0000232681), male allotype dissected on one slide (collection number NIBRIV0000232682), female paratype on SEM stub (collection number NIBRIV0000232683), male paratype in ethanol (collection number NIBRIV0000232684), type locality, 14 October 2012, leg. K. Kim.

Two females destroyed for DNA sequence (amplification unsuccessful), type locality, 18 November 2012, leg. K. Kim.

Etymology. The species is named after Erato (Ancient Greek: Ξρατώ), one of nine Muses from Greek mythology, who was a patron of erotic poetry and song. The species name is a noun in apposition (in the nominative case).

Description. Female (based on holotype and three paratypes). Body length from 820 to 885 µm (882 µm in holotype). Body segmentation, colour, nauplius eye, hyaline fringes, integument thickness and surface appearence as in Wellstenhelia calliope sp. nov., including minute sparse pits visible only on highest magnifications on scanning electron microscope. Most somite ornamentation also similar to Wellstenhelia calliope, and presumed homologous pore and sensilla also numbered with same Arabic numerals (see Figs. 18A, B, C, 19A, 20A, B, C, D) to allow easier comparison. Habitus (Figs. 18A, B) more robust, with prosome/urosome length ratio close to 1.4, body length/width ratio about 2.8, and cephalothorax 2.3 times as wide as genital double-somite.

Rostrum (Fig. 18C) with wide base and narrow anterior part in dorsal view (arrowed in Fig. 18C) but without any other difference in shape or ornamentation to that in Wellstenhelia calliope .

Cephalothorax (Figs. 18A, B, 20A, 23A) about 0.9 times as long as wide; represents 31% of total body length. Surface of cephalothoracic shield with 36 paired or unpaired sensilla and pores, most of which probably homologous to those in Wellstenhelia calliope (indicated with Arabic numerals in illustrations) and Wellstenhelia clio (indicated with currency symbols in illustrations), but seven pores and sensilla missing (nos. 6, 9, 10, 16, 19, 33, 35); absolute and relative positions of some pores and sensilla differ.

Pleuron of second pedigerous somite (Figs. 20 B, 23B) as in Wellstenhelia calliope, except lateral sensilla no. 48 much closer to sensilla no. 49, and anterior pair of pores no. 43 more widely spaced.

Pleuron of third pedigerous somite (Figs. 20C, 23B) as in Wellstenhelia calliope, except one additional lateral pore present (no. *) and anterior pair of pores no. 51 less widely spaced.

Pleuron of fourth pedigerous somite (Figs. 20D, 23B) as in Wellstenhelia calliope, except anterior lateral pore no. 57 missing.

First urosomite (Figs. 18A, B, 23B, C) as in Wellstenhelia calliope, except lateral pore no. 67 missing.

Genital double-somite (Figs. 18A, B, 19A, 23C) as in Wellstenhelia calliope, except no large dorsal or lateral spinules (arrowed in Figs. 18A, B, 19A), ventral pair of sensilla no. 73 more widely spaced, and two large seminal receptacles clearly visible inside.

Last threeurosomites (Figs. 18A, B, 19A, 23E, F) as in Wellstenhelia calliope, except lateral pore no. 80 missing.

Caudal rami (Figs. 18A, B, D, 19A, 23F) short and stout, similar in shape to those in Wellstenhelia clio, about as long as anal somite, cylindrical, 2.3 times as long as wide (ventral view), parallel, and with space between them less than one ramus width; ornamentation and armature as in Wellstenhelia calliope, except central part of inner margin without spinules and ventral pore no. 83 missing.

Antennula (Fig. 18E) as in Wellstenhelia calliope, except first segment without dorsal pore, with less sharp process, and with additional row of minute spinules basally (arrowed in Fig. 18E).

Antenna (Fig. 18F) as in Wellstenhelia calliope, except first exopodal segment inflated in distal half and third exopodal segment without central row of spinules (both arrowed in Fig. 18F).

Labrum, paragnaths, mandibula, maxillula, maxilla, and maxilliped as in Wellstenhelia calliope .

Swimming legs (Fig. 19B, C) segmentation, ornamentation, armature, and proportions of various armature elements as in Wellstenhelia calliope, except first endopodal segment of second leg without anterior pore.

Fifth leg (Figs. 18B, 19A, D, 23D) segmentation, general shape, number of armature elements, and even most ornamentation as in Wellstenhelia calliope, except innermost endopodal seta proportionately shorter (arrowed in Figs. 18B, 19A, D), second endopodal seta from inner side proportionately longer (arrowed in Figs. 18B, 19A, D), and anterior pore of exopod closer to inner margin (arrowed in Fig. 19D). Length ratio of endopodal setae, starting from inner side, 1: 4.2: 5.2: 3. Length ratio of exopodal setae, starting from inner side, 1: 0.5: 0.3: 0.55: 0.5: 0.5.

Sixth leg (Fig. 19E) simple narrow cuticular plate, unornamented, with single smooth and short outer seta flanking single minute inner spine; latter about same size as some larger spinules on genital double-somite.

Male (based on allotype). Body length 760 µm. Habitus, colour, rostrum (Fig. 22A), shape and ornamentation of cephalothorax, second pedigerous somite (Figs. 21C), and third pedigerous somite, shape and most ornamentation of fourth pedigerous somite (Fig. 21D), ornamentation of first urosomite (Figs. 20E, 21A, B), ornamentation of last threeurosomites (Figs. 20E, 21A, B), armature and ornamentation of caudal rami (Figs. 20E, 21A, B), antenna (Fig. 21E), labrum, paragnaths, mandibula, maxillula, maxilla, maxilliped, first swimming leg, second swimming leg (Fig. 22B), and coxae, bases and exopods of third and fourth swimming legs as in female. Prosome/urosome ratio 1.45, greatest width at posterior end of cephalothorax, body length/width ratio about 3.1; cephalothorax 2.4 times as wide as genital somite in dorsal view. Genital somite and third urosomite not fused.

Pleuron of second pedigerous somite (Fig. 21C) with sensilla no. 48 not so close to sensilla no. 49 as in female but everything else same.

Pleuron of fourth pedigerous somite (Fig. 21D) with anterior lateral pore no. 57 present, all other ornamentation as in female.

First urosomite (Figs. 20E, 21A, B) narrower than in female but also with three pairs of dorsal sensilla (nos. 64, 65, 66) and one pair of lateral pores (no. 63).

Genital somite (Figs. 20E, 21A, B) somewhat wider than in Wellstenhelia calliope, but with all ornamentation same, except pore no. 68 situated more ventrally (arrowed in Fig. 21A), i.e. much closer to pore no. #, and lateral row of spinules less broken near sensilla no. 70 (arrowed in Fig. 21A); large spermatophore visible inside genital somite, longitudinally placed on right side, about as long as that in Wellstenhelia calliope .

Third urosomite (Figs. 20E, 21A, B) as in Wellstenhelia clio, i.e. ventral row of spinules uninterrupted between sensilla no. 73, except few more large spinules present dorsally from sensilla no. 71.

Anal somite (Figs. 20E, 21A, B) as in female, except lateral pore no. 80 present.

Caudal rami (Figs. 20E, 21A, B) shorter than in female (arrowed in Fig. 21A, B), with length/width ratio in ventral view of just below 1.5, but armature and ornamentation as in female, including missing ventral pore no. 83.

Antennula (Fig. 22A) shape, segmentation, ornamentation, and most armature as in Wellstenhelia calliope, except second segment proportionately shorter (arrowed in Fig. 22A), aesthetascs proportionately longer, and second, third and fourth segment with one, one, and two additional setae respectively (latter arrowed in Fig. 22A); setal fomula thus 1.12.7+ae.9+ae.1.2.1.4.6+ae.

Third swimming leg endopod (Fig. 22C) without anterior pore and with distal inner seta on third segment slender.

Fourth swimming leg endopod (Fig. 22D) without anterior pore on first segment and with inner seta on first segment and distal seta on third segment slender and shorter than those in female.

Fifth leg (Fig. 21A, B) shape, armature, and ornamentation as in Wellstenhelia calliope, except exopod proportionately larger and with much strong inner armature element.

Sixth leg (Fig. 21A, B) shape, ornamentation, and number of armature elements as in Wellstenhelia calliope, except inner and middle elements strong spines (both arrowed in Fig. 21A) and middle one exceptionally long, reaching beyond midlength of fourth urosomite; length ratio of armature elements, starting from inner side, 1: 1.8: 1.

Variability. All morphological features are extremely conservative among the four female specimens examined, while only one male was available for morphological examination.

Morphological affinities. Wellstenhelia erato sp. nov. differs from all congeners by the minute innermost endopodal seta on the female fifth leg (Fig. 19D), which can be considered as a clear autapomorphy of this species. Other possible autapomorhies include an inflated first exopodal segment of the antenna (Fig. 18F) and very strong elements on the male sixth leg (Fig. 21A, B). However, the former character was not described for Wellstenhelia hanstromi (Lang, 1948) comb. nov. and Wellstenhelia melpomene sp. nov., while males (or male characters) are still unknown for Wellstenhelia hanstromi, Wellstenhelia melpomene, Wellstenhelia euterpe sp. nov., and Wellstenhelia bocqueti (Soyer, 1971) comb. nov. Wellstenhelia calliope sp. nov. and Wellstenhelia bocqueti also have the innermost endopodal seta on the female fifth leg relatively short, although not as short as in Wellstenhelia erato, but the former differs by its much longer caudal rami, while the latter has a deep notch on the female fifth leg endopod, as well as the principal caudal setae confluent at base.

Other congeners can be easily distinguished from Wellstenhelia erato by many characters. Wellestenhelia hanstromi and Wellstenhelia qingdaoensis (Ma & Li, 2011) comb. nov. have much longer caudal rami; Wellstenhelia euterpe sp. nov. has much shorter caudal rami and only three setae on the female fifth leg endopod; while Wellstenhelia clio sp. nov. and Wellstenhelia malpomene sp. nov. have a much longer innermost seta on the female fifth leg endopod. Each species can additionally be distinguished from Wellstenhelia erato by some other feature in the proportion of armature elements or ornamentation of somites. Numerous differences between this species and Wellstenhelia calliope and Wellstenhelia clio are indicated by arrowheads in Figs. 18A, B, C, E, F, 19A, D, 20E, 21A, B, E. These include absence of large dorsal spinules on the genital double-somite, absence of many sensilla and pores on prosomite, but also some novel features, such as lateral pore on the third pedigerous somite (no. *). Wellstenhelia erato is also the largest of all Korean sympatric congeners, and probably the largest species of Wellstenhelia gen. nov.