Daphnia hrbaceki Juračka, Kořínek & Petrusek, 2010, sp. nov.

Daphnia hrbaceki sp. nov. (Figs 1–8) Etymology. The new species is dedicated to the eminent Czech hydrobiologist Jaroslav Hrbáček (1921–2010), who initiated complex ecological studies of Daphnia populations in the former Czechoslovakia. The name in the Czech language also reflects the hunched body shape of some individuals. Type locality. A small fishless, recently (2004) excavated pool in the valley Český příkop (protected landscape area Kokořínsko, Czech Republic); N 50°28'54", E 14°41'10", alt. 289 m above sea level. The pool is 7 m long and 3 m wide with maximal depth ca. 2 m, situated in a deep, shaded valley with a cold microclimate. The type series was collected on 5 November 2007 by P. J. Juračka. Holotype. Adult parthenogenetic female (total body length 1.7 mm) mounted in Canada balsam and stained with a mixture of lignin pink and chlorazol black E; Natural History Museum, London (NHM 2010.39). Allotype. Adult male (body length without shell spine 1.0 mm) mounted and stained as above (NHM 2010.40). Paratypes. Males and females (45 specimens), preserved in 96% ethanol and a small amount of glycerol (NHM 2010.53-62). Additional specimens from the type series are deposited in the collection of the National Museum, Prague (P6E3005). Ephippial female (total body length 1.5 mm) stained and mounted as above (NHM 2010.41). Dissected parthenogenetic female treated with hot 10% potassium hydroxide and mounted as above (NHM 2010.42). Females and males (13 specimens) stained and mounted as above (NHM 2010.43-52). Diagnosis. Parthenogenetic female with median keel on head shield, some populations with induced necktooth on its posterior margin. Similar neckteeth may be present in juveniles and males. Antennule completely reduced, median mound strongly vaulted with reticulated apex. Ocellus pigmented. Shallow cervical depression. Shell spine short or absent. Gnathobase of second thoracic limb extended distally into angular projection. Postabdominal claw with second (middle) pecten of spinules or teeth of variable size and shape: either spinules slightly longer than those in proximal pecten, or large teeth longer than width of claw. Ephippium saddle-shaped, dorsal ridge smooth (without spinules), only reticulated; posterior carapace margin included into ephippium. Ephippial surface ultrastructure with many minute pits surrounded by fine lamellae. Male with medium-sized rostrum hardly covering antennular socket. Antennule short, two to three times longer than wide. One of the three terminal setae on 2nd endopodite bent and heavily setulated. Pre-anal margin of postabdomen weakly depressed, anal margin convex. Size. Total body length (without shell spine): parthenogenetic female 1.0– 1.7 mm; ephippial female 1.2–2.0 mm; male 0.9–1.2 mm. Description. Parthenogenetic female. Head: high, strongly vaulted apical part with median keel increasing in width dorsally. Keel extremely developed in some individuals; forming hump-shaped structure (Figs. 1C, E; 2B, F). Neckteeth rarely present in adult females (Figs. 1E; 2B, E). Dorsal margin with shallow cervical depression (Fig. 1B). Frontal contour of head concave above rostrum. Rostrum not prominent, its tip bent ventrally in some specimens. Tip of rostrum obtusely rounded and split into two lobes by suture or line between head shield and ventral side of head in lateral aspect (Fig. 4 C, E). Mid-antennular mound well developed, markedly reticulated on apex. Optic vesicle contiguous with frontal part of head. Ocellus pigmented. Fornix rounded at base of second antenna. Antennule: not protruding, its body reduced, seen as lateral areole on median mound with 9 sensory setae; single lateral seta anterior to areole (Fig. 4 C, E). Antenna: Setal formula of natatory setae: 0-0-1-3/1-1-3. Presumably sensorial setae and spinules: two setae on concertina-like basal joint, one apical spine-like on its outer side, one seta on inner side between both branches, one apical spinule on dorsal margin of second segment 4-segmented branch. Dark rings at base of distal part of swimming setae may be present in some individuals or populations. Surface of all segments covered with transversal groups of small teeth. First maxilla: carrying three robust, curved and heavily setulated setae and one short stump-like distal seta. Carapace: approximately sub-ovoid, length of posterior spine variable, forming up to 15% of body length (without shell spine) or completely reduced. Spinules on ventral margin cover 1/3 to 2/3 of its length, spinulation on dorsal margin developed only in posterior 1/4 of margin or only near posterior spine. Spinulation of dorsal margin completely missing in some individuals. Fringe of sub-marginal setae absent. Thoracic limbs: agree with the re-description of Daphnia curvirostris in Ishida et al. (2006) with the exception of 2nd limb gnathobase, which extends in front of longest clearing seta into noticeable rectangular corner or small lobe (Fig. 5E, F). Postabdomen: elongated, tapering distally, pre-anal face even, covered with scattered groups of fine spinules, anal margin slightly convex, fringed with up to 15 strong teeth that increase in length distally. Distal portion of postabdominal setae slightly shorter than proximal one. Abdominal processes gradually diminishing distally, first twice as long as second, third reduced to 1/3 up to 1/2 of second one in specimens preserved in formalin. Terminal claw long, with three groups (pectens) of teeth and spinules. Proximal one of 13–19 minute spinules, middle pecten variable in size: either 8–9 large teeth markedly longer than width of claw or 11–13 spinules that only slightly exceed in length those of other two pectens. Distal row of about 60 fine spinules, not reaching tip of claw. Differences in size and length of claw spinules were observed among samples collected in different times of season, and between individuals from the wild and those cultured in laboratory (Fig. 3 A–D). FIGURE 1. Daphnia hrbaceki. A. Adult male (K). B. Adult parthenogenetic female (K). C. Adult ephippial female (K). D. Adult male (RB) with necktooth indicated by arrow. E. Adult parthenogenetic female (RB) with morphology presumably induced by invertebrate predators; arrow indicates a hump-shaped dorsal outline of the carapace. F. Adult male (RB), hook-like apical seta (2nd limb) indicated by arrow. G. Adult male (RB), postabdomen (contrast increased at gonopore area); arrows indicate gonopore (g) and middle pecten on postabdominal claw (p). FIGURE 5. Daphnia hrbaceki and Daphnia curvirostris. A. D. hrbaceki, adult male (RB); antennules (A1) and rostrum (r), arrow (fr) indicates valves fringed with row of long, sub-marginal feathered setae. B. D. curvirostris, adult male, arrows as in Fig. 5 A (LL). C. D. hrbaceki, adult male (K); antennule (indicated by arrow). D. D. curvirostris, adult male (KP); antennule (indicated by arrow). E, F. D. hrbaceki, adult females (RB); 2nd thoracic limb, gnathobase, arrows indicate gnathobase extending distally into angular projection. Ephippial female. Dorso-posterior part of head shield swollen, forming bulge over dorsal suture between carapace and head shield (Figs. 1C, 2G). Ephippial surface covered with sclerotized pneumatic cells reaching up to postero-dorsal angle of shell without any gap. Two resting eggs perpendicular to dorsal margin, egg chambers well separated from each other. Free post-molting ephippium (Fig. 6 A, B) asymmetrically saddle-shaped, with maximal width between centre and proximal third of its length. Dorsal ridge without any spinescence, only reticulated (Fig. 7 A, B). Postero-dorsal corner includes part of vaulted posterior margin and remnants of short shell spine, which is lost in older, freely floating ephippia. Surface ultrastructure with many minute pits surrounded by fine lamellae (Fig. 8 A). Male. Head: rounded in frontal part around optic vesicle, apical contour only feebly convex, gradually descending dorsally to level of attachment of posterior antennal muscle or to necktooth (if present) (Figs. 1A, 2A, 4A). Compound eye large, filling half of frontal portion of head shield, ocellus pigmented. Obtuse rostrum short, covering only antennular socket. Antennular (ventral) part of head extends ventrally forming posterior wall of antennular sockets (Fig. 4 A, B). Antennule: in adult males directed towards compound eye, its segment short, two to three times longer than wide, reaching hardly to pigmented part of compound eye. Flagellum inserted on conical butt elevated over shallow socket for sensory papillae. Dorsal seta inserted distally at about four fifths of antennular length (Figs. 4 B, 5C). Antenna: surface sculpture of all segments weaker than in female. Carapace: ventral aspect: wide anterior gap between valves fringed with row of long, sub-marginal feathered setae. Setae most densely spaced along anterior fold of valves, gradually shortened to mid carapace margin (Fig. 5A). No gap or sub-marginal setae at distal part of ventral margin, only small marginal spines and groups of submarginal setules present. Dorsal margin feebly convex. Thoracic limbs conform with the description of Daphnia curvirostris male in Ishida et al. (2006). Hook-like seta of 2nd limb is shown in Fig. 1F. Postabdomen: all abdominal processes reduced, proximal one very small, others mostly missing. Pre-anal part with shallow depression, anal margin convex, fringed with up to 12 lateral spines (Fig. 1D). Gonopores open ventrally of last three largest marginal spines (Fig. 1G). Distal part of postabdominal setae slightly shorter than their proximal part. Middle pecten on terminal claw with either 6–7 spines or 10–12 spinules (Fig. 3 E). Differential diagnosis. The new species has to be differentiated from several other taxa present in the region of its occurrence: Daphnia curvirostris, members of the Daphnia pulex group, and Daphnia longispina (O. F. Müller, 1776), as well as related taxa in Asia and two taxa showing so

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