Archaeodiacyclops Alekseev & Chaban, 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 : 30-31

Genus Archaeodiacyclops Alekseev & Chaban gen. nov.

lsid: zoobank.org: act: 616EF90A-8D63-4ED7-879B-A7B188D52F0E

Diagnosis. Medium-sized species (833–1410 μm) with slender, cyclopiform habitus. Genital double-somite about as long as broad, widest at first third. Receptaculum seminis with bulky anterior part and almost unexpanded posterior part in salisae -group or with thin anterior part and sac-like posterior part in uruguayensis -group. Anal somite with denticles along most of the lower margin; anal operculum weakly developed, almost straight in salisae -group or rather strongly developed in uruguayensis -group. Caudal branches cylindrical, parallel, from 3.3 to 5.1 times as long as wide; inner surface bears rows of hairs-setules and denticles (one proximal row of hairs in salisae -group and several rows of hairs in uruguayensis -group); outer surface may bear transverse row of denticles in proximal third of branch ( uruguayensis -group); lateral seta longer than width of caudal branch, attached at level of lower quarter/third of length of outer margin, plumed with long hairs, with or without denticles at point of attachment; innermost seta is longer than outer one (1.2–1.6: 1) and shorter or subequal to length of caudal rami (0.6–1.1: 1), uniformly plumed with long hairs; outermost seta from 0.5 to 0.8 times as long as caudal ramus, uniformly plumed with long hairs, has denticles at point of attachment; dorsal seta shorter than outermost seta, subequal or longer (0.75–1.2: 1) and from 0.4 to 0.8 times as long as caudal ramus, usually with few long hairs distally.

Antennule 17-segmented, short, does not extend beyond middle of first free thoracic somite. Antenna with long exopodite seta; endopodite segment 2 with 9 setae. Labrum with rounded lateral projections and 10-12 blunt teeth on cutting edge; frontal surface with 2 rows of long hairs. Mandible with several teeth and plumose seta on cutting edge; palp with two long plumose and one short naked seta. Maxillule of typical cyclopid structure; palp surface without ornamentation. Maxilla 5-segmented, with first two segments partly fused, without ornamentation on surface. Maxilliped 4-segmented; segment setation: 3/2/1/3 or 3/1/1/3; second segment ornamented with two or three lateral rows of spinules and one medial group of spinules; third segment bearing one medial group of spinules.

Swimming legs two-branched, three-segmented. Exp1 and Exp2 with 1 inner seta and 1 outer spine. Exp3 spine formula 2/3/3/3 or 3/4/4/4; setal formula 4.4.4.4. Enp1 with 1 inner seta; Enp2 with 2 inner setae. Enp3 spine formula 1/1/1/2; setal formula 5.5.5.3. All swimming legs with finely plumose setae, except lateral-most seta of P1 Exp3, which has tiny spinules along its lateral side. Intercoxal plate has simplified ornamentation; in a number of species it is bare on both sides; in some there are hairs on frontal side of some swimming legs; in some species, there are denticles on caudal side of P4 intercoxal plate. Inner spine of P4 Enp3 may be longer or shorter than outer spine, or they may be subequal. Distal setae of P4 Enp3 may extend beyond tips of distal spines in some species. P1 Bas with inner spine, which may be homogeneously covered with setules of similar length or heterogeneously covered with long setules proximally and short setules distally. P1 Bas with or without distal arc of spinules between Enp and Exp frontally.

Fifth pair of legs two-segmented; first (proximal) segment about as wide as long or slightly wider, with 1 outer seta; second (distal) segment cylindrical, longer than wide, armed with 1 apical seta and 1 inner spine. Inner spine is almost as long as distal segment or longer (0.9–2.0:1), with several denticles at insertion site.

Sixth leg represented by small plate armed with two short smooth spines and plumose longer outer seta.

Males are noticeably shorter and more slender than females. Structure of appendages similar to females, although length ratios slightly differ. Antennule geniculate. P6 armed with strong inner spine and two plumose setae.

New genus currently includes five species.

Type species. Archaeodiacyclops okhensis sp. nov.

Other species. Archaeodiacyclops uruguayensis ( Kiefer, 1935) comb. nov.; A. salisae (Reid, 1992) comb. nov.; A. ecabensis ( Fiers & Ghenne, 2000) comb. nov.; A. pilosus ( Fiers & Ghenne, 2000) comb. nov.

Etymology. New genus is named using prefix Archae - in reference to a number of “archaic” characters in its structure, namely 17-segmented A1 (the largest number known in the subfamily), caudal branches with hairs on inner side, as well as unreduced segmentation of swimming legs.