Archaeodiacyclops okhensis Chaban & Alekseev, 2024

Alekseev, Victor & Chaban, Olga, 2024, Archaeodiacyclops, new genus with “ archaic ” features (Crustacea, Copepoda, Cyclopoida), with description of new species from water bodies of northern Sakhalin Island, Zootaxa 5514 (1), pp. 28-40 : 32-37

Archaeodiacyclops okhensis Chaban & Alekseev sp. nov.

( Figs. 3-6 )

lsid: zoobank.org: act: 28ED8DB9-CD5C-4B38-9909-CED2EB41F765

Type material. Holotype female, mounted on permanent slide, #55606; paratype male, mounted on permanent slide, #55607; preserved paratypes: 3 females undissected, #55608; deposited in Federal Collection of Zoological Institute of Russian Academy of Sciences.

Type locality. Russia, North of Sakhalin Island, Sakhalinka river on outskirts of Okha town , under bridge (53°37’03.4”N 142°56’20.6”E) .

Etymology. The species is named after the town where it was found.

Description of female holotype. Female body length without caudal setae 1029–1233 μm. Habitus slender, with largest width near posterior margin of cephalothorax ( Fig. 2A ). Cephalothorax 1.1 times as long as wide, with maximum width close to posterior margin. Two last thoracic somites laterally slightly produced and curved. Genital double-somite slightly longer than wide, tapering posteriorly, with lateral sharp angles anteriorly ( Fig. 2D ); receptaculum seminis with large round anterior part and almost not extended posteriorly; pore-canal short, straight. Hyaline frills of abdominal somites smooth. Anal somite with small thin spinules along posterior margin; anal operculum weakly developed, almost straight ( Fig. 3B ).

Caudal rami 4.5–5.1 times longer than wide, almost parallel ( Fig. 2C ); lateral surface smooth except tiny spines at insertion of outermost caudal setae; medial surface with one proximal row of hair-setules and two more posterior rows of tiny spines; dorsal and ventral surfaces are covered with groups of tiny spines poorly visible under low magnification (more abundant on ventral surface) ( Fig. 3A, B ). Lateral seta inserted at about posterior 1/4 of lateral margin of ramus, without spines at insertion place. Innermost seta 1.3–1.4 times as long as outermost seta, and 0.6 times as long as ramus length. Dorsal seta 0.8–0.9 times as long as outermost seta. All caudal setae plumose (dorsal setae with only few hairs). Distal setae length ratio beginning from outermost one: 1.0/ 5.5–5.9/ 8.0–8.3/1.3–1.4.

Antennule (A1) 17-segmented, short, not reaching posterior edge of cephalothorax. Setation of antennular segments, beginning from first: 8/4/2/6/4/2/2/1/1/0/1/1+ae/0/1/2/2+ae/7+ae ( Fig. 3C ). First segment with one row of spinules. Aesthetasc on 12 th segment reaching 1/3 of 15 th segment or distal border of 14 th segment. Aesthetasc on 16 th segment reaching 3/4 to 9/10 of 17 th segment.

Antenna (A2) composed of basipodite with two medial setae and one long exopodal seta, and three endopodal segments; endopodite setation: 1, 9, 7 ( Fig. 5A ). Basipodite ornamentation composed of few sparse proximal groups of spines, rather variable ( Fig. 5B–D ); medial setae smooth, exopodal seta covered with short spinules.

Mouthparts. Labrum ( Fig. 3D ) composed of large plate ornamented with two groups of long setules; distal margin with 10 large blunt teeth between rounded lateral projections. Mandible ( Fig. 3E ) composed of coxa and palp bearing three setae (two long plumose and one short naked) as typical for the family; coxal gnathobase with group of long spinules, plumose seta, adjacent shorter seta with two denticles and blunt teeth of different size. Maxillule composed of praecoxa and palp of typical cyclopid structure; setation and ornamentation according to figure 4B; palp naked. Maxilliped ( Fig. 4C ) 4-segmented as typical for the family; segment setation: 3/2/1/3; second segment ornamented with two lateral rows of spinules and one medial group of spinules; third segment bearing one medial group of spinules. Maxilla ( Fig. 4A ) 5-segmented as typical for the family, with first two segments partly fused, without ornamentation.

All swimming legs with 3-segmented endo- and exopodites; distal exopodite spine formula 3/4/4/4; setal formula according to genus description. In one of specimens, P4 asymmetrical with 3 spines on one of the Exp3 and 4 spines on another. All setae of swimming legs not modified, slender and plumose, except distal seta of P1 Exp3 which is covered with denticles along outer side and hairs along inner side. P1 Bas with inner spine, which heterogeneously covered with long setules proximally and short setules distally; this spine reaching distal margin of Enp2 ( Fig. 4E ). P1 Bas without distal arc of spinules between Enp and Exp frontally. P1–P3 intercoxal plates frontal surface with long hair-setules, caudal surfaces naked. P1–P4 Bas inner outgrowths covered with dense hairs. P4 Coxa caudal surface ornamented with abundant lateral groups of hair-setules/spinules, and two longitudinal rows of spinules proximally and distally. P4 intercoxal plate caudal surface with central row of rather long spinules ( Fig. 2F ); frontal surface naked. P4 inner coxal seta homogeneously covered with long hairs; this seta extending beyond inner outgrowth. P4 Enp3 1.9–2.3 times as long as wide; inner/outer distal spines length ratio 1.0–1.1: 1; inner spine 0.7 times as long as segment; distal setae reaching 1/2 to 3/4 of adjacent spines. P4 Exp3 distal spine 0.8–0.9 times as long as segment.

P5 two-segmented; proximal segment armed with outer seta; distal segment cylindrical, longer than wide, armed with apical seta and inner spine ( Fig. 2E ). Inner spine is almost as long as distal segment (0.9–1.2: 1), with several denticles at insertion site. P5 setae length ratio beginning from inner spine: 1.0/ 2.9–3.8/ 2.8–3.6.

P6 consists of small plate armed with two short smooth spines and plumose longer outer seta.

Male. Habitus shorter and slenderer than in female ( Fig. 2B ). Body length 798 μm (paratype). Cephalothorax 1.2 times as long as wide, with maximum width close to posterior margin. Two last thoracic somites without lateral curved extensions. Hyaline frills of abdominal somites coarsely dentate dorsally and ventrally. Caudal rami 4.1 times longer than wide, almost parallel; lateral surface with spines at insertion of outermost caudal setae and tiny denticles at insertion of lateral setae and at proximal third of branch; medial surface with one proximal row of hair-setules and three more posterior rows of tiny spines; dorsal and ventral surfaces are covered with groups and rows of tiny spines poorly visible under low magnification (more sparse than in female). Innermost seta longer than in female, 1.6 times as long as outermost seta, and 0.7 times as long as ramus length. Distal setae length ratio beginning from outermost one: 1.0/ 6.0/ 9.4/1.6. Antennule 14-segmented, with long slender aesthetascs. P6 with inner spine and two longer plumose setae ( Fig. 4D ). P6 appendages length ratio beginning from spine: 1/ 1.1/ 1.9.

Differential diagnosis. Archaeodiacyclops okhensis sp. nov. differs from A. uruguayensis comb. nov., A. ecabensis comb. nov. and A. pilosus comb. nov. in the presence of only one group of long hairs on the inner side of caudal rami, shape of receptaculum seminis, exopodite formula of swimming legs, ratio of distal spines of P4 Enp3. A. okhensis sp. nov. is most closely related to A. salisae comb. nov., having a similar habitus, shape of receptaculum seminis, ratio of distal spines of P4 Enp3, exopodite formula of swimming legs, the presence of only one group of hairs on the inner side and the absence of a proximal group of spinules on the outer side of caudal rami. A. okhensis sp. nov. differs from A. salisae comb. nov. in the presence of proximal angular projections on genital double-somite, relatively shorter caudal innermost setae, relatively shorter setae of P4 Enp3 not reaching tips of adjacent spines, and the presence of hairs on frontal side of P1 intercoxal plate.

For distinguishing species of the new genus, an identification key is provided below.

Figures

FIGURE 2. Sampling location of Archaeodiacyclops okhensis sp. nov.: A—a map of the area, black triangles indicate sampling points; B—sampling station 58; C—sampling station 59.

FIGURE 3. Archaeodiacyclops okhensis Chaban & Alekseev sp. nov.: female, A, C–F; male, B. A, B—habitus; C—caudal rami, ventral; D—genital double-somite, ventral; E—P5; F—P4, caudal. Scale bars: A, B—200; C–F—50 μm.

FIGURE 4. Archaeodiacyclops okhensis Chaban & Alekseev sp. nov.: female. A, B—caudal rami, ventral and dorsal ornamentation; C—antennule; D—labrum; E—mandible; F—maxilliped. Scale bars: A–C, F—50; D, E—20 μm.

FIGURE 5. Archaeodiacyclops okhensis Chaban & Alekseev sp. nov.: female.A—maxilla; B—P1, frontal; C—P2 coxopodite and intercoxal plate, frontal; D—P3 coxopodite and intercoxal plate, frontal; E—maxillule. Scale bars 50 μm.

FIGURE 6. Archaeodiacyclops okhensis Chaban & Alekseev sp. nov.: female, A, B; male, C, D. A—antenna, caudal; B— antenna basipodite, frontal; C—antennule; D—P6. Scale bars 50 μm.