Diacyclops parasuoensis, Karanovic, Tomislav, Grygier, Mark J. & Lee, Wonchoel, 2013

Karanovic, Tomislav, Grygier, Mark J. & Lee, Wonchoel, 2013, Endemism of subterranean Diacyclops in Korea and Japan, with descriptions of seven new species of the languidoides-group and redescriptions of D. brevifurcus Ishida, 2006 and D. suoensis Ito, 1954 (Crustacea, Copepoda, Cyclopoida), ZooKeys 267, pp. 1-76 : 18-27

Diacyclops parasuoensis sp. n. Figs 91326D, E27C

Type locality.

South Korea, Jeollanamdo, Gurye city, Yangcheon, Seomjin River, 35°12'04.7"N, 127°35'29.3"E, interstitial water from coarse sand and gravel.

Type material. Holotype female dissected on two slides (NIBRIV0000232650). Allotype male from type locality dissected on two slides (NIBRIV0000232651). Other paratypes from type locality: four males, 15 females and 19 copepodids together in ethanol (NIBRIV0000232652), two males and eigth females on one SEM stub (NIBRIV0000232653); two males and three females on one SEM stub (NIBRIV0000232648), two females dissected on one slide each (NIBRIV0000232670 & NIBRIV0000232670), and two males dissected on one slide each (NIBRIV0000232654 & NIBRIV0000232655); all collected 19 June 2010, leg. J.-L. Cho.

Additional paratypes: seven males, three females and five copepodids together in alcohol (NIBRIV0000232656), from South Korea, Jeollanamdo, Gurye city, Yangcheon, Seomjin River (different locality), 35°11'25.4"N, 127°23'00.7"E, interstitial water from sandy banks, 19 June 2010, leg. J.-L. Cho.

Additional paratypes: two males, two females, and one copepodid together in alcohol (NIBRIV0000232657), from South Korea, Gangwondo, Wonju city, Jijeong, Seom River, 37°23'30.16"N, 127°51'08.39"E, interstitial water from sandy banks, 24 June 2010, leg. J.-L. Cho.

Additional paratypes: one female and seven copepodids together in alcohol (NIBRIV0000232658), from South Korea, Gangwondo, Wonju city, Buron River, 37°14'01.23"N, 127°44'58.78"E, interstitial water from sandy banks, 24 June 2010, leg. J.-L. Cho.

Additional paratypes: two males, one female, and seven copepodids together in alcohol (NIBRIV0000232659), from South Korea, Gyungsangbukdo, Sangju city, Young River, 36°31'42.8"N, 128°14'02.7"E, interstitial water from sandy banks, 1 July 2010, leg. J.-L. Cho.

Additional paratypes: two males and seven females together in alcohol (NIBRIV0000232660), from South Korea, Gyungsangbukdo, Uljin city, Geunnam, Wangpi stream, 36°57'41.4"N, 129°22'46.4"E, interstitial water from sandy banks, 18 May 2010, leg. J.-L. Cho.

Etymology.

The species name is an adjective composed of the existing specific name suoensis and the Greek prefix para (= near, beside), and refers to the relatively close apparent relationship between these two congeners.

Description.

Female (based on holotype and five paratypes from type locality). Total body length, measured from tip of rostrum to posterior margin of caudal rami (excluding caudal setae), from 437 to 462 µm (444 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 consisting of 40 pairs and three unpaired (mid-dorsal) pores and sensilla (those probably homologous with those of Diacyclops ishidai sp. n.indicated with same Arabic numerals; presumably novel pairs indicated with Roman numbers and numbered consecutively from anterior to posterior end of body, and from dorsal to ventral side in Figs 9A, B, 10A, B, C); no spinules except on anal somite, caudal rami, and appendages. Habitus (Fig. 9A, B) relatively slender, not dorso-ventrally compressed, with prosome/urosome length ratio 1.5 and greatest width in dorsal view at first third of cephalothorax, body prominently arched backwards between prosome and urosome. Body length/width ratio about 3.5 (dorsal view); cephalothorax 1.86 times as wide as genital double-somite. Free pedigerous somites without lateral or dorsal expansions, all connected by well developed arthrodial membranes, and all with narrow and smooth hyaline fringes. Pleural areas of cephalothorax and free pedigerous somites very short, not covering insertions of cephalic appendages or praecoxae of swimming legs in lateral view.

Rostrum (Fig. 10A, B) well developed, membranous, not demarcated at base, broadly rounded and furnished with one pair of frontal sensilla (no. 1).

Cephalothorax (Figs 9A, B, 10A, B) relatively small, 1.2 times as long as its greatest width (dorsal view), narrower at posterior end in dorsal view, with widest part at first third, only slightly oval; representing 34% of total body length (together with rostrum). Surface of cephalic shield ornamented 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, latter being incorporated into cephalothorax.

Second pedigerous somite (Fig. 9A, B) relatively short, tapering anteriorly, ornamented with just one pair of dorsal sensilla (no. 61), serially homologous to pair no. 50 on first pedigerous somite.

Third pedigerous somite (Fig. 9A, B) slightly longer than second and significantly narrower in dorsal view, also widest at posterior margin in dorsal view and with slightly flared latero-posterior corners, ornamented with one unpaired dorsal pore (no. I) and four pairs of large sensilla (nos. 63, 64, 72, 74); recognition of serially homologous pairs not easy, but probably dorsolateral pair of sensilla no. 64 serially homologous to pair no. 61 on second pedigerous somite.

Fourth pedigerous somite (Fig. 9A, B) significantly shorter and narrower than third, with slightly flared latero-posterior corners, ornamented with only one unpaired dorsal pore (no. II) and two pairs of large sensilla (nos. 75, 77); recognition of serially homologous pairs slightly easier than for two previous prosomites (probably II=I, 75=63, 77=72).

Fifth pedigerous somite (Figs 9A, B, 10C) short, significantly narrower than fourth pedigerous somite or genital double-somite in dorsal view, ornamented with one unpaired dorsa pore (no. III) and two pairs of large dorsal sensilla (nos. 80, 81); recognising serially homologous pairs relatively easy, i.e. III=II, 80=75 and 81=77; hyaline fringe very narrow and smooth.

Genital double-somite (Figs 9A, B, 10C, 26E) large, with deep lateral recesses at level of sixth legs and only slightly swollen antero-ventrally, widest at first third of its length and gradually tapering posteriorly, only slightly longer than its greatest width (dorsal view); ornamented with two pairs of central dorsal sensilla (nos. 86, 88), one pair of posterior lateral sensilla (no. 96), and one pair of ventral posterior pores (no. 97); central dorsal sensilla probably serially homologous to those on fifth pedigerous somite (i.e. 86=80, 88=81), but posterior sensilla and pores without homologous pairs; three minute lateral pores posterior to sixth legs visible only under highest magnification of SEM (Fig. 26E). Copulatory pore small, ovoid, situated ventrally at about two-thirds of genital double-somite length; copulatory duct narrow, directed anteriorly, well sclerotised. Hyaline fringe wavy, not serrated. Seminal receptacle butterfly-shaped, with relatively short anterior expansion and long lateral arms, constricted at middle, and with even shorter and slightly narrower posterior expansion, together representing 30% of double-somite’s length; Ovipores situated dorso-laterally at first third of double-somite length, covered by reduced sixth legs.

Third urosomite (Figs 9A, B, 10C) relatively short, about 1.8 times as wide as long and 0.35 times as long as genital double-somite in dorsal view, also with wavy hyaline fringe, ornamented with one pair of lateral posterior sensilla (no. 100) and one pair of ventral posterior pores (no. 102); serially homologous pores and sensilla easy to recognize, i.e. 100=96 and 102=97.

Preanal urosomite (Figs 9A, B, 10C, 26D) slightly narrower and shorter than third, also with wavy hyaline fringe, unornamented.

Anal somite (Figs 9A, B, 10C, 26D) slightly narrower and significantly shorter than preanal somite, with short medial cleft; ornamented with one pair of large dorsal sensilla (no. 104), one pair of small ventral pores (no. 106), and continous posterior row of small spinules. Anal sinus smooth. Anal operculum wide, short, convex, not reaching posterior margin of anal somite, representing 57% of anal somite’s width.

Caudal rami (Figs 9A, B, 10C, 26D) almost cylindrical, slightly divergent, inserted close to each other (space between them about half as wide as one ramus), with deep dorso-median anterior depression (as continuation of anal sinus and narrowed ventral part of base); approximately 2.9 times as long as wide (ventral view) and 2.3 times as long as anal somite, with long dorsal seta (arrowed in Fig. 10C) and longer rami than in Diacyclops ishidai or Diacyclops brevifurcus (arrowed in Fig. 10C); armed with six setae (one dorsal, one lateral, and four terminal); 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 1.3 times as long as ramus, inserted at 5/6 of ramus length, biarticulate at base (inserted on small pseudo-joint) and pinnate distally. Lateral seta inserted dorso-laterally at 2/3 of ramus length, about as long as ramus width, sparsely unipinnate laterally and uniarticulate at base. Outermost terminal seta stout, spiniform, 0.6 times as long as ramus, densely bipinnate. Innermost terminal seta small and slender, sparsely pinnate, half as long as outermost terminal seta. Principal terminal setae with breaking planes, bipinnate; inner principal terminal seta about 1.9 times as long as outer one and 4.7 times as long as caudal rami.

Antennula (Fig. 9C) 11-segmented, slightly curved along caudal margin, directed laterally, not reaching posterior margin of cephalothoracic shield, ornamented just with arched proximo-ventral row of spinules on first segment (no pits or other integumental structures), with armature formula as in Diacyclops ishidai . Only one terminal seta on ultimate segment biarticulate basally, and most larger setae sparsely pinnate at distal end; both aesthetascs very slender, that on eighth segment reaching posterior margin of tenth segment (arrowed in Fig. 9C). One seta on fourth and one on fifth segment spiniform and short; all other setae slender; one apical seta on eleventh segment fused basally to aesthetasc. Length ratio of antennular segments, from proximal end and along caudal margin, 1: 0.5: 0.9: 0.5: 0.4: 0.7: 1.3: 1.1: 0.7: 0.9: 1.1.

Antenna (Fig. 9D) five-segmented, strongly curved along caudal margin, comprising very short coxa, much longer basis, and three-segmented endopod. Coxa without armature or ornamentation, about half as long as wide. Basis cylindrical, 2.1 times as long as wide, ornamented with four short, diagonal rows of spinules (two on ventral side, two on dorsal surface) close to caudal margin, armed with two subequal smooth setae on distal inner corner (exopodal seta absent). First endopodal segment narrowed basally but generally cylindrical, 1.6 times as long as wide and 0.7 times as long as basis, with smooth inner seta at 2/3 length and patch of large spinules on caudal margin. Second endopodal segment also with narrowed basal part, 2.3 times as long as wide, about 1.1 times as long as first endopodal segment, bearing only six smooth setae along inner margin (arrowed in Fig. 9D), ornamented with one row of spinules along caudal margin. Third endopodal segment cylindrical, 2.2 times as long as wide and slightly shorter than second endopodal segment, ornamented with two rows of slender spinules along caudal margin, armed with seven smooth apical setae (four of them strong and geniculate).

Labrum (Fig. 11A) relatively large trapezoidal plate, with muscular base and strongly sclerotised distal margin (cutting edge), ornamented with two diagonal rows of 12 long and slender spinules each on anterior surface, and central transverse row of minute spinules between them. Cutting edge almost straight, with 14 sharp teeth between produced and rounded lateral corners.

Mandibula (Fig. 11B, C) composed of coxa and small palp. Coxal gnathobase cutting edge with five slender spinules on anterior surface, seven apical teeth, and dorsalmost unipinnate seta; ventralmost tooth strongest and quadricuspidate (Fig. 11C), second and fourth teeth from ventral side bicuspidate, all other teeth unicuspidate; two dorsalmost simple teeth partly fused basally. Palp twice as wide as long, unornamented, armed with three apical setae: two long and bipinnate and one short and smooth; pinnate setae subequal in length, directed posteriorly, not reaching posterior margin of cephalic shield (see also Fig. 9B).

Maxillula (Fig. 11D, E) composed of praecoxa and two-segmented palp, unornamented. Praecoxal arthrite bearing four very strong distal spines (three of them smooth, blunt, and fused at base; one distinct at base, sharp, and with two proximal spinules) and six medial elements (proximalmost one longest and plumose, two distalmost ones large and strong, three in between small and slender). Palp composed of coxobasis and one-segmented endopod. Coxobasis with slender, bipinnate proximal seta (probably representing exopod) and three medial setae (two slender and smooth, one strong and pinnate). Endopod with three slender, bipinnate setae.

Maxilla (Fig. 11F, G) 5-segmented but praecoxa partly fused to coxa on anterior surface, unornamented. Proximal endite of praecoxa robust, armed with two sparsely bipinnate setae; distal endite slightly smaller than proximal and unarmed. Proximal endite of coxa with one bipinnate seta; distal endite highly mobile, elongated and armed apically with two pinnate setae, proximal one considerably longer and stronger. Basis armed with two setae, expanded into robust claw. Claw shorter than larger seta and furnished with longitudinal row of six spinules at midlength along concave (dorsal) margin (arrowed in Fig. 11F); larger seta with convex ventral margin, robust and spiniform; smaller seta smooth and slender, inserted on posterior surface. Endopod two-segmented with segmentation easily discernable; proximal segment armed with two robust, unipinnate setae; distal segment with one robust, unipinnate apical seta and two slender and much shorter subapical setae. Longest seta on distal endopodal segment 0.8 times as long as longer seta on proximal endopodal segment. All strong setae on basis and endopod, as well as basal claw, unguiculate.

Maxilliped (Fig. 11H) four-segmented, composed of syncoxa, basis, and two-segmented endopod; second endopodal segment minute, armed with only two setae (arrowed in Fig. 11H). Ornamentation consisting of two rows of long, slender spinules on basis (one on posterior surface, other on anterior surface), as well as two spinules on anterior surface of first endopodal segment. Armature formula: 2.2.1.2. All inner setae pinnate, very strong, and those on basis and endopod also unguiculate.

All swimming legs (Figs 11I, J, K, 12A) relatively small, composed of minute, triangular praecoxa, large, rectangular coxa, short basis, and slender exopod and endopod. Exopods and endopods approximately equally long on all legs, their segmentation formula (exopod/endopod): 2/2.3/2.3/3.3/3. Ultimate exopodal segment spine formula 3.3.3.3 and setal formula 5.4.4.4. All setae on endopods and exopods slender and plumose, except apical seta on exopod of first leg pinnate along outer margin and plumose along inner (Fig. 11I); no modified setae observed. All spines strong and bipinnate. Intercoxal sclerite of all swimming legs with slightly concave distal margin and without any surface ornamentation except on anterior surface of fourth leg.

First swimming leg (Fig. 11I) shorter than other swimming legs; praecoxa unarmed, ornamented with distal row of large spinules on anterior surface; coxa 2.3 times as wide as long, ornamented with distal row of spinules on anterior surface (outer half of row composed of large spinules, inner half of minute spinules), and small pore on anterior surface close to inner margin, armed with slender and sparsely plumose seta on inner-distal corner; basis almost pentagonal, 0.7 times as long as coxa, armed with long and slender outer seta and strong and bipinnate inner-distal element (latter reaching to 1/3 length of second endopodal segment, i.e. much shorter than in Diacyclops ishidai ; arrowed in Fig. 11I), ornamented with row of extremely slender spinules along inner margin, two posterior rows of minute spinules on anterior surface (one at base of inner seta, other at base of endopod), and one cuticular pore on anterior surface close to outer margin; exopod with single outer spine and single inner seta on first segment, with three outer spines and five setae (three inner, two apical) on second segment, ornamented with distal rows of spinules on anterior surface of first segment, row of slender inner spinules on both segments, and extremely minute spinules at base of almost all setae and spines on anterior surface; endopod armed only with inner seta on first segment, second segment with only three inner setae (arrowed in Fig. 11I), one apical spine, and one outer seta, ornamented with slender spinules along inner margins of both segments, with shorter and stronger spinules along distal margin of first segment on anterior surface, and with minute spinules at base of most setae (those at base of apical spine larger) on anterior surface; apical spine on second endopodal segment slightly outwardly unguiculate, and only slightly longer than segment or inner setae; second endopodal segment about 1.5 times as long as wide and also 1.7 times as long as first endopodal segment, lacking inner notch (arrowed in Fig. 11I).

Second swimming leg (Fig. 11J) similar to that of Diacyclops ishidai , but second endopodal segment without inner notch showing ancestral segmentation (arrowed in Fig. 11J) and with proximal two setae shorter than others (arrowed in Fig. 1J); apical spine on second endopodal segment proportionally longer than in Diacyclops ishidai , 0.7 times as long as segment or distal inner seta; second endopodal segment about 1.9 times as long as wide and 1.8 times as long as first endopodal segment.

Third swimming leg (Fig. 11K) similar to that of Diacyclops ishidai , but third endopodal segment proportionally shorter, and third exopodal segment with pore on anterior surface; apical spine on third endopodal segment as long as segment and half as long as apical seta; third endopodal segment about 1.2 times as long as wide and 1.4 times as long as second endopodal segment.

Fourth swimming leg (Fig. 12A) generally similar to that of Diacyclops ishidai , but coxa with fewer spinules on posterior surface (arrowed in Fig. 12A), intercoxal sclerite without spinules on posterior surface but instead with six large spinules on anterior surface (arrowed in Fig. 12A), inner process of basis much smaller (arrowed in Fig. 12A), and proximal inner seta on third endopodal segment much shorter (Fig. 12A); third endopodal segment without pore on anterior surface, 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 1.5 times as long as outer apical spine, slightly shorter than segment, and less than half as long as distal inner seta; apical spines diverging at about 20° angle.

Fifth leg (Figs 10C, 12B) inserted ventrally, relatively small, two-segmented, with same armature as in Diacyclops ishidai and Diacyclops brevifurcus , but with very different shape and proportions. Protopod very small and narrow (arrowed in Fig. 12B), almost trapezoidal, about as long as greatest width, unornamented, armed with single slender outer seta inserted on short setophore and unipinnate distally. Exopod slightly narrower than in these congeners, almost cylindrical but with narrower proximal part, twice as long as protopod and 3.2 times as long as wide, unornamented, armed with long apical seta and subapical inner spine; apical seta bipinnate distally, 1.7 times as long as basal seta, 2.9 times as long as exopod, and 4.7 times as long as subapical spine, reaching to 2/3 length of genital double-somite; subapical exopodal spine small but strong, bipinnate, 0.63 times as long as exopod and twice as long as exopod’s greatest width.

Sixth leg (Figs 9A, B, 26E) small, short, and broad semicircular cuticular plate with single pore on anterior surface, two short and smooth spines, and one longer and distally unipinnate outermost seta; inner spine fused to plate, outer one articulated basally; outermost seta directed postero-dorsally.

Male (based on allotype and four paratypes from type locality). Total body length 402-437 µm (406 µm in allotype). Urosome with free genital somite. Habitus (Fig. 13A) even more slender than in female, with prosome/urosome length ratio about 1.5 and greatest width in dorsal view at second pedigerous somite. Body length/width ratio 3.6; cephalothorax about 1.7 times as wide as genital somite. Cephalothorax 1.2 times as long as wide and nearly cylindrical in dorsal view, representing 33% of total body length. Ornamentation of cephalothorax (Fig. 12D), free prosomites (Fig. 13A), and first and last two urosomites (Figs 12C, 13A, B) with same number and distribution of sensilla and pores as in female.

Genital somite (Figs 12C, 13A, B) 1.3 times as wide as long in dorsal view, with wavy hyaline fringe dorsally, ornamented with one unpaired dorsal pore (no. 85; N.B., this pore absent in female) and two pairs of dorsal and lateral sensilla (nos. 86, 88); two small circular spermatophores visible inside. Third urosomite (Figs 12C, 13A, B) homologous to posterior part of female genital double-somite, also ornamented with ventral pair of posterior pores (no. 97), but without lateral pair of sensilla (no. 96). Fourth urosomite (Figs 12C, 13A, B) also lacking lateral pair of sensilla present in female (no. 100), only ornamented with ventral posterior pair of pores (no. 102).

Caudal rami (Figs 12C, 13A, B) slightly shorter than in female and less divergent, but nonetheless with long dorsal seta (arrowed in Fig. 12C) and longer rami than in Diacyclops ishidai or Diacyclops brevifurcus (arrowed in Fig. 12C), with proportionally shorter outermost terminal seta than in female, but with very similar ornamentation and armature.

Antennula (Fig. 13C) strongly prehensile and digeniculate, 16-segmented (with ancestral sixteenth and seventeenth segments completely fused), ornamented with spinules only on first segment (as in female), with anvil-shaped cuticular ridges on anterior margin of fourteenth and fifteenth segments (distal geniculation), with much shorter fifteenth and sixteenth segments than in Diacyclops ishidai (both arrowed in Fig. 13C), and also fifteenth segment without aesthetasc and sixteenth segment with one additional minute seta. Armature formula as follows: 8+3ae.4.2.2+ae.2.2.2.2.2+ae.2.2.2.2 + ae.2.1.12+ae. All aesthetascs linguiform and most relatively slender, apical one on sixteenth segment fused basally to one seta; distribution of small setae as in Diacyclops ishidai .

Antenna, labrum, mandibula, maxillula, maxilla, swimming legs, and fifth leg as in female.

Sixth leg (Fig. 13B, D) large cuticular plate with single minute pore on anterior surface, armed with small inner spine and two bipinnate setae on outer distal corner; outermost seta 2.3 times as long as inner bipinnate seta, 4.3 times as long as innermost spine.

Remarks.

Diacyclops parasuoensis sp. n. can be easily distinguished from the Japanese Diacyclops suoensis and Diacyclops pseudosuoensis sp. n. by the size of the dorsal caudal seta, as well as by the proportions of many armature elements and the ornamentation of most appendages. Most of these differences are arrowed in Figs 14-17. However, very few differences in the pattern of pores and sensilla and the similar armature of the antenna in Diacyclops parasuoensis and Diacyclops suoensis probably indicate that these two species are not distantly related, and may form a monophyletic group together with Diacyclops pseudosuoensis and Diacyclops hisuta sp. n. They are all only remotely related to Diacyclops ishidai sp. n., Diacyclops brevifurcus , Diacyclops leeae sp. n., Diacyclops hanguk sp. n., and Diacyclops parahanguk sp. n.

It is quite clear that Diacyclops parasuoensis forms a sibling species pair with the Japanese Diacyclops hisuta (see below), and they can only be distinguished at this stage by the habitus shape (much more slender in Diacyclops parasuoensis ). We described Diacyclops hisuta for a population reported and partly described by Ueda et al. (1996) under the name Diacyclops suoensis (see synonymy below). Unfortunately, the description and illustrations they provide do not show details of any mouth appendages or the armature of the first three swimming legs, so many characters cannot be compared. If we assume these features are very similar to those of Diacyclops suoensis as illustrated by Ito (1957), who provided drawings of all swimming legs, Diacyclops hisuta would differ from Diacyclops parasuoensis additionally by the number of inner setae on the second endopodal segment of the first leg. This, however, remains speculative until Diacyclops hisuta is properly redescribed.

A great number of species (and subspecies) in the languidoides-group have been described based on a very limited set of characters, giving us no opportunity to compare fine details of the somite sensilla and pores pattern or the appendages ornamentation. This species-group was for a long time considered a single widely-distributed species, although some authors noticed a high level of morphological variability even in a small geographic area ( Ito 1954; Kiefer 1968). Even today, some authors consider as valid no fewer than 14 subspecies (see Dussart and Defaye 2006), many of them with overlapping ranges. It was Petkovski (1984) who first realized that most of these subspecies must be distinct biological species, after he found three of them occurring sympatrically in a single interstitial sample from Slovenia. He illustrated their major differences in caudal rami shape and ornamentation, shape of the genital double-somite and its seminal receptacle, and proportions of the third endopodal segment of the fourth leg and its apical spines. Since then these particular characters have normally been illustrated for new, presumably closely related taxa, while other features have not been studied in detail. Thus, most comparisons between different taxa in this species-group have to be inferred today from these three characters, especially the shape and armature of the caudal rami. Somewhat similar caudal rami to those of Diacyclops parasuoensis are found in the following European species: Diacyclops clandestinus (Kiefer, 1926), Diacyclops cristinae Pesce & Galassi, 1987, Diacyclops eriophori (Gurney, 1927), Diacyclops hypnicola (Gurney, 1927), Diacyclops insularis Monchenko, 1982, and Diacyclops paolae Pesce & Galassi, 1987 (see Gurney 1927; Herbst 1951; Kiefer 1968; Pesce and Maggi 1981; Monchenko 1982; Petkovski 1984; Pesce and Galassi 1987a, b; Galassi 1991). In all of these species, some small differences from Diacyclops parasuoensis can, however, be discerned in the proportions and armature of the caudal rami. Also, none of them has such elongated lateral arms of the anterior part of seminal receptacle, and most also differ from Diacyclops parasuoensis in some other significant morphological characters. For example, Diacyclops insularis has eight setae on the second endopodal segment of the antenna, and Diacyclops cristinae and Diacyclops hypnicola have an exopodal seta on the antenna.