Psychrodromus Danielopol & McKenzie 1977

Genus Psychrodromus Danielopol & McKenzie, 1977

Holotype: 1 male. No. K-42856 - Zoological Museum Hamburg

Paratypes: on slides: 1 male, 4 females. No. K-42857a-e – Zoological Museum Hamburg undissected in ethanol: 5 males, 55 females, and 31 larvae. No. K-42858 - Zoological Museum Hamburg

Type locality: near road P12 between Kairouan and Maktar, near Kef El Garia tunnel; effluent of a karstic spring, only few meters east of the pond below a waterfall (O- 108 in Fig. 1) (35°46’14.8” N, 09°26’44.9” E); very low discharge (Fig. 2).

Etymology: The new species is named after the country where the animals were found.

Material examined: 12 adult males, 76 adult females, and 38 juveniles collected on the 8th November 2010, leg. B. Scharf and C. Zaibi. Associate fauna: 7 adult and 2 juvenile Darwinula stevensoni.

Diagnosis: small Psychrodromus species, carapace elongated, anterior and posterior very broadly rounded, dorsal margin only slightly arched, LV of both sexes with a small inner list, male clasping organs of maxilliped with one, or two additional truncated setae, respectively.

Description (Figs. 3–6): Female: Surface of carapace smooth, ventrally white, dorsally green with a transition from white to green, surface ventrally hirsute, valves with conical inclusions (in transmitted light shown as small dark dots, figured on the anterior part of the valves in Fig. 3). Carapace in lateral view: elongated (ratio of height to total length: 0.48–0.52). LV overlaps anteriorly, posteriorly and ventrally RV (Figs. 6 F to J). A small list developed inside on the LV anteriorly, posteriorly, and ventrally, running in parallel to the shell edge, forming a groove between the list and the edge. Dorsal margin of carapace only slightly arched; greatest height of LV posterior, of RV anterior of the middle of the carapace; ventral margin moderate concave. Fused zone anteriorly and posteriorly narrowed, with a large number of slender radial canals, ventrally widened. Calcareous inner lamella of both valves anteriorly wider than posteriorly (ratio of maximal width of inner lamella to total length of carapace: anteriorly 0.17–0.19, posteriorly 0.07–0.09). Carapace in dorsal view: greatest width in the middle, both ends slightly rounded.

Dimensions (n=13): length: 1.16–1.23 mm, height: 0.57–0.62 mm.

Antennule, first two segments fused forming a large basis with Rome organ, aesthetasc y a at joint 5. Terminology of the joints according to Boxhall et al. (2010), length of the joint 4a in relation to the length of the 2 short setae of joint 3b: 20: 50: 36 (Fig. 4 A).

Antenna with very short natatory setae of nearly the same length (relation of the length of the penultimate joint to the length of the natatory setae: 45: 20), length of the claws: G1: G2: G3: GM: Gm = 100: 64: 81: 78: 32 (Fig. 4 B).

Mandibule with a masticatory process with strong teeth, a jointed setose palp and a well developed vibratory plate (Fig. 4 D).

Food rake with 7 teeth (Fig. 4 E).

Maxillule with 2 serrate teeth bristles (Zahnborsten) on third endite and with a slightly spatula-like terminal segment of the palp (Fig. 4 F).

Maxilliped L5 lacks ‘c’ bristle on the protopod (Figs. 4 G and H).

Walking leg L6 5-segmented with a well developed apical claw (Fig. 5 A).

Cleaning leg L7 with a pincer-organ and 1 long seta on penultimate segment at around mid-length (Fig. 4 K).

Uropod: Uropodal attachment without Triebel loop, but with basal triangular reinforcement characteristic for the subfamily Herpetocypridinae (Fig. 5 B). Uropodal rami slightly asymmetric, right uropodal ramus forming a simple curve while left uropodal ramus slightly S-shaped; well developed anterior seta, posterior claw of about 60 % of length of anterior claw; posterior seta a truncated spine with few setulae apically; posterior margin of rami with many irregularly set setulae (Figs. 5 B, C, F).

Male: Carapace similar to that of female, except in lateral view: here, greatest height of both valves posterior of the middle of the carapace (Figs. 3 A, B and Figs. 6 A to E).

Dimensions (n=9): length: 1.10–1.21 mm, height: 0.56–0.60 mm.

Antennule, mandible, walking leg L6, cleaning leg L7, and uropod of males are similar to those of the females.

Antenna, claw GM at the distal segment is strongly pectinated. Relation of this claw to aesthetasc y3 is 86: 100, i.e. y3 is longer than the claw (Fig. 4 C).

Maxilliped L5, right and left clasping organ asymmetric. Finger of right clasping organ longer and less curved than left finger. Left clasping organ with two bristles and two additional truncated setae (Fig. 4 I), right clasping organ with only 1 seta of this type (Fig. 4 J).

Zenker organ with number of rings of spines varying between 25 and 27 (n=5).

Copulatory organ with hemipenis with small rounded medial shield, very wide lateral shield, a lateral appendix, postlabyrinthal spermiduct with one additional circular whorl (Fig. 4 L).

Systematic position. The new species belongs to the genus Psychrodromus erected by Danielopol & McKenzie (1977), although the elongated shape of the carapace resembles carapaces of the genus Herpetocypris. However, the selvage of the LV of Herpetocypris is largely inwardly displaced, which is not the case in Psychrodromus. In some species of Psychrodromus a list exists, which runs close and in parallel to the edge on the left valve (e.g. P. betharrami (Fig. 2 in Baltanás et al. (1993) and P. t u n i s i c u s n. sp.), but this is not the selvage. Further arguments for the species designation within the genus Psychrodromus include straight radial pore canals, both valves with small selvages, A1 with Rome organ, natatory setae on A2 very short, uropod weakly asymmetric, posterior setae of uropod shaped as spines, hemipenis with lateral appendix and only one additional circular whorl.

Psychrodromus tunisicus n. sp. differs from the other Psychrodromus species by the size and the shape of the carapace, the claws and setae of the uropod, characteristic additional truncated setae on the clasping organs, and the male copulatory organ (see also the following determination key for the genus Psychrodromus).