Diacyclops suoensis Ito Tak 1954

Female (based on eight specimens from Daido River). Total body length, measured from tip of rostrum to posterior margin of caudal rami (excluding caudal setae), from 391 to 473 µm (391 µm in holotype). Preserved specimens colourless; no live specimens observed. Integument relatively weakly sclerotised, smooth, without cuticular pits or cuticular windows. Surface ornamentation of somites very similar to that of Diacyclops parasuoensis, consisting of 38 pairs of pores and sensilla (those probably homologous with those of Diacyclops ishidai indicated with same Arabic numerals in Fig. 15A, B, C); no spinules except on anal somite, caudal rami, and appendages. Habitus (Fig. 14A) relatively robust, not dorso-ventrally compressed, with prosome/urosome ratio 1.3 and greatest width in dorsal view at posterior end of cephalothorax, only slightly arched backwards between prosome and urosome. Body length/width ratio about three (dorsal view); cephalothorax 2.1 times as wide as genital double-somite. Free pedigerous somites without lateral or dorsal expansions, all connected with well developed arthrodial membranes, and with narrow and smooth hyaline fringes, but arthrodial membranes not as exposed as in Diacyclops parasuoensis. Pleural areas of cephalothorax and free pedigerous somites short, not covering insertions of cephalic appendages or praecoxae of swimming legs in lateral view. Rostrum, cephalothorax, and three free pedigerous somites ornamented as in Diacyclops parasuoensis, except dorsal unpaired pores (nos. I, II) absent. Cephalothorax (Fig. 14A) relatively large (arrowed in Fig. 14A), 1.1 times as long as its greatest width (dorsal view), widest at posterior end in dorsal view and tapering towards anteriorly, oval; representing 35% of total body length (together with rostrum). Surface of cephalic shield ornamented as in Diacyclops parasuoensis with 20 pairs of long sensilla (nos. 2-4, 6, 7, 11, 12, 14, 15, 17, 21, 23, 24, 31, 38, 39, 42, 45, 48, 50); no pores visible; sensilla 39-50 belonging to first pedigerous somite, incorporated into cephalothorax. Second pedigerous somite (Figs 14A) relatively short, tapering posteriorly, ornamented with just one pair of dorsal sensilla (no. 61) as in Diacyclops parasuoensis. Third pedigerous somite (Fig. 14A) slightly longer than second and significantly narrower in dorsal view, also tapering posteriorly, ornamented with four pairs of large sensilla (nos. 63, 64, 72, 74); unpaired dorsal pore (no. I) absent. Fourth pedigerous somite (Figs 14A) significantly shorter and narrower than third, tapering posteriorly, ornamented with only two pairs of large sensilla (nos. 75, 77); unpaired dorsal pore (no. II) absent. Fifth pedigerous somite (Fig. 15A, B, C) short, significantly narrower than fourth pedigerous somite or genital double-somite in dorsal view, ornamented with two pairs of large dorsal sensilla (nos. 80, 81); unpaired dorsal pore (no. III) absent. Genital double-somite (Fig. 15A, B, C) large, slightly more slender than in Diacyclops parasuoensis, with deep lateral recesses at level of sixth legs and swollen antero-ventrally, widest at first third of its length and gradually tapering posteriorly, as long as its greatest width (dorsal view); ornamented with one pair of central dorsal sensilla (no. 86), one pair of posterior lateral sensilla (no. 96), and one pair of ventral posterior pores (no. 97); central dorsal pair of sensilla probably serially homologous to those on fifth pedigerous somite (i.e. 86=80), but posterior sensilla and pores without homologous pairs; dorsal pair of central sensilla no. 88 absent. Copulatory pore small, oval, situated ventrally at about midlenth of double-somite; copulatory duct narrow, siphon-shaped, well sclerotised. Hyaline fringe wavy, not serrated. Seminal receptacle butterfly-shaped, but with much thicker and shorter lateral arms than in Diacyclops parasuoensis (arrowed in Fig. 15C); representing 43% of double-somite’s length; oviducts broad and well sclerotised. Ovipores situated dorso-laterally at 2/5 of double-somite length, covered by reduced sixth legs. Third urosomite (Fig. 15A, B, C) similar in size and shape to that of Diacyclops parasuoensis, but lateral pair of sensilla (no. 100) absent, only ornamentation ventral posterior pair of pores (no. 102); hyaline fringe wavy. Preanal urosomite (Fig. 15A, B, C) as in Diacyclops parasuoensis, unornamented and with wavy hyaline fridge. Anal somite (Fig. 15A, B, C) also similar to that of Diacyclops parasuoensis, but with posterior row of spinules limited to ventral surface and composed of much smaller spinules. Anal sinus ornamented with two diagonal rows of minute spinules. Anal operculum wide, short, only slightly convex, not reaching posterior margin of anal somite, representing 54% of anal somite’s width. Caudal rami (Fig. 14A, 15A, B, C) proportionately longer than in Diacyclops parasuoensis (arrowed in Fig. 15C), and with shorter outermost terminal seta (arrowed in Fig. 15A), longer innermost terminal seta (arrowed in Fig. 15A), and much longer dorsal seta (arrowed in Figs 14A, 15A), approximately 3.4 times as long as wide (ventral view) and 2.5 times as long as anal somite; ornamented with one pore on tip of small protuberance on distal margin ventrally between two principal terminal setae (no. 108), and rows of small spinules at base of lateral setae. Dorsal seta slender, about 2.2 times as long as ramus and almost as long as outer principal terminal seta, inserted at 5/6 of ramus length, biarticulate at base (inserted on small pseudo-joint) and bipinnate distally. Lateral seta minute, inserted dorso-laterally at 2/3 of ramus length, about half as long as ramus width, unipinnate laterally and uniarticulate at base. Outermost terminal seta stout, spiniform, 0.4 times as long as ramus, densely bipinnate. Innermost terminal seta more slender and 0.8 as long as outermost terminal one, also densely bipinnate. Principal terminal setae with breaking planes, bipinnate; inner principal terminal seta about 1.8 times as long as outer one and 4.2 times as long as caudal rami. Antennula (Fig. 14B) unornamented, segmentation and armature as in Diacyclops parasuoensis and Diacyclops ishidai, except eighth segment proportionatelly longer (arrowed in Fig. 14B) and aesthetasc on eighth segment proportionatelly shorter (reaching posterior margin of ninth segment; arrowed in Fig. 14B). Length ratio of antennular segments, from proximal end and along caudal margin, 1 : 0.5 : 0.9 : 0.5 : 0.4 : 0.7 : 1.2 : 1 : 0.6 : 0.8 : 1. Antenna (Fig. 14C) with fewer spinules on basis than in Diacyclops parasuoensis, but all other ornamentation, as well as segmentation and armature without any difference. Labrum (Figs 16A, 26F) relatively large trapezoidal plate with mascular base and strongly sclerotised distal margin (cutting edge), ornamented with two diagonal rows of 11 to 13 long and slender spinules on anterior surface; cutting edge almost straight, with 16 to 18 sharp teeth between produced and rounded lateral corners. Mandibula (Fig. 16B, C, D) very similar to that of Diacyclops parasuoensis, but with additional small unicuspidate tooth on cutting edge (part of dorsalmost, partly fused group of three theeth). Maxillula (Figs 16E, F, 26F) segmentation, armature, and ornamentation as in Diacyclops parasuoensis, but free distal spine on praecoxal arthrite more pinnate and endopodal setae less pinnate. Maxilla (Fig. 16G) segmentation, armature, and ornamentation as in Diacyclops parasuoensis, except basal claw slightly longer (arrowed in Fig. 16G), and with only three small spinules on dorsal margin. Maxilliped (Fig. 16H) with more spinules in anterior row on basis, but other ornamentation, as well as segmentation and armature, as in Diacyclops parasuoensis. All swimming legs (Fig. 17A, B, C, D) generally similar to those in Diacyclops parasuoensis; but second endopodal segment of first leg with one additional seta (arrowed in Fig. 17A) and all legs with small differences in ornamentation of some segments and proportions of some setae. Segmentation formula, as well as ultimate exopodal segment spine and setal formulae, as in Diacyclops parasuoensis. First swimming leg (Fig. 17A) without large spinules on coxa and with inner notch on second endopodal segment (arrowed in Fig. 17A), with additional inner seta on second endopodal segment (arrowed in Fig. 17A); apical spine on second endopodal segment slightly outwardly unguiculate, 0.9 times as long as segment, and 0.7 times as long as distal inner seta; second endopodal segment about 1.7 times as long as wide and 1.6 times as long as first endopodal segment. Second swimming leg (Fig. 17B) without large spinules on posterior surface of coxa and with inner notch on second endopodal segment (arrowed in Fig. 17B); apical spine on second endopodal segment 0.7 times as long as segment or distal inner seta; second endopodal segment about 1.8 times as long as wide and 1.6 times as long as first endopodal segment. Third swimming leg (Fig. 17C) also without large spinules on posterior surface of coxa (arrowed in Fig. 17C), with two short rows of large spinules on anterior surface of intercoxal sclerite (arrowed in Fig. 17C), and with normally developed proximal seta on third endopodal segment (arrowed in Fig. 17C); apical spine on third endopodal segment 0.7 times as long as segment and less than half as long as apical seta; third endopodal segment with pore on anterior surface, about 1.5 times as long as wide and 1.6 times as long as second endopodal segment. Fourth swimming leg (Fig. 17D) without large spinules on posterior margin of coxa (arrowed in Fig. 17D), and with proximal inner seta on third endopodal segment longer than distal inner seta (arrowed in Fig. 17D); third endopodal segment about 1.2 times as long as wide, and 1.2 times as long as second endopodal segment; inner apical spine on third endopodal segment 1.2 times as long as outer apical spine, 0.8 times as long as segment, and less than 0.4 times as long as distal inner seta; apical spines diverging at about 20° angle. Fifth leg (Fig. 17E) very similar to that of Diacyclops parasuoensis, but basal seta and exopod proportionally shorter; protopod small and narrow, rhomboidal in shape and about as long as greatest width, unornamented, armed with single outer slender, short seta, this being inserted on very short setophore and unipinnate distally; exopod slightly narrower than protopod, almost cylindrical but with narrowed proximal part, 1.8 times as long as protopod and 2.2 times as long as wide, unornamented, armed with long apical seta and subapical inner spine; apical seta bipinnate distally, 3.3 times as long as basal seta, 4.8 times as long as exopod, and five times as long as subapical spine, reaching to 2/3 length of genital double-somite; subapical exopodal spine only slightly shorter than exopod and twice as long as exopod’s greatest width. Sixth leg (Fig. 15A, B) as in Diacyclops parasuoensis, but with slightly longer outer seta. Male. Not collected.

Japan, Yamaguchi prefecture, Tsunoshima island, Tsuno city, approximately 34°21'N, 130°52'E, well with a pump.