Daphnia (Ctenodaphnia) pusilla (Serventy, 1929)

(Figs 1–8)

Daphniopsis pusilla Serventy, 1929: 65–67, Plate IX: Figs A–D; Smirnov & Timms 1983: 106, Fig. 125; Sergeev & Williams 1983: 294–299, Figs 1–4; Sergeev & Williams 1985: 127–128, Fig. 6; Hann 1986: Fig. 5F; Sergeev 1990a: 6, Figs 5 C–D; Hebert & Wilson 2000: 806, Figs 9A–C.

Daphnia pusilla (Serventy) in Benzie 2005: 286–288, Figs 1185–1197.

Ephippial morphotype 1 in Kokkinn and Williams 1987: 68, 71, Figs 1a–c.

Taxonomic comment. Hyalodaphnia cristata var. pusilla Sars, 1890 was described before (Sars 1890), but never used since its first description.According to the International Commission on Zoological Nomenclature (ICZN 2000: Introduction, Article 11), “an author will be required (without a ruling by the Commission) not to displace a name which has been used as valid by at least 10 authors in 25 publications during the past 50 years, and encompassing a span of not less than ten years, by an earlier synonym or homonym which has not been used as valid since 1899”. Therefore we accept Daphnia pusilla (Serventy, 1929) as a valid species and do not need to displace one.

Type locality. “ Rifle-Range Swamp or a closely adjacent water-body on Rottnest Island, Western Australia ” (Sergeev and Williams 1983) .

Type material. Lost. Sergeev & Williams (1983) did not find the material of Serventy (1929) in any collections in Australia, i.e. in Western Australian Museum. They deposited “a representative collection of specimens” collected by I.A.E. Bayly from type locality to the Western Australian Museum (WAM 255-82). Such specimens have the status of topotypes, no neotype was selected. Here we also studied the sample from exact type locality from the same date as Sergeev & Williams (1983) and also collected by I.A.E. Bayly who separated collected material and sent a portion to B. V. Timms, who re-sent the sample to Prof. N.N. Smirnov.

Material studied here. South Australia. Many males, ephippial & parthenogenetic females from unnamed pond N of Eleanor River, Kangaroo Island, coll. on 22.01.1971 by B. V. Timms,AAK 2013-094. Few parthenogenetic females from unnamed pond SW of Kingscote, Kangaroo Island, coll. on 11.01.1976 by B. V. Timms, AAK 2013- 093. Two females from Kangaroo Island, NNS MGU 2961 and 2962. Many parthenogenetic females from Porter’s Lag., via Burra (33.83°S, E 38.87°E), coll. on 16.01.1976 by B. V. Timms, AAK 2013-095, NNS MGU 2963, NNS MGU 2968 . Western Australia. Many males, ephippial & parthenogenetic females from Lake Negri, Rottnest Island (31.99°S, 115.51°E), coll. on 12.11.1974 by I.A. Bayly, GLAG051. Many parthenogenetic females from Rifle Range Swamp, Rottnest Island, coll. on 09.07.1974 by I.A. Bayly, AAK 2013-096 (topotypes). Many ephippial & parthenogenetic females from a shallow ephemeral wetland W004, Buntine-Marchagee region (30.2°S, 116.33°E, coll. on 24.08.2005 by R. J. Shiel, AAK M-0154. Tasmania. Many males, ephippial & parthenogenetic females from a pond 6 in Cape Portland, NE Tasmania, coll. on 17.03.1992 by R. Walsh, AAK 2005-062.

Short diagnosis. Adult parthenogenetic female with body from almost transparent to remarkably pigmented in brown, postero-dorsal angle without a caudal spine. Head with ocular dome, post-ocular depression from shallow to relatively deep, a shallow depression in posterior portion of head. Posterior extremity of head shield as an angle, fornices rounded. Spinules are absent on dorsal and ventral margin, ventral margin in the middle with a group of long setae. Postabdomen elongated, tapering distally. Postabdomen with outer side with three successive pectens along the dorsal margin, the first and second pecten consisting of relatively long teeth, the third pecten consisting of numerous fine setules, not reaching tip of claw. Antenna I as a small projection with nine aesthetascs, tips of aesthetascs projecting beyond rostrum, antennular sensory seta small. Limb I with a relatively short anterior seta 4. Limb II with anterior seta 1 length about 0.36 of a closest posterior seta length. Limb III with seta 1’ relatively short. Limb V exopodite supplied with two distal setae.

Ephippium ovoid, lacking any projections, with a single large resting egg.

Adult male with rectangular-ovoid body, caudal spine fully absent. Head large, anterior most extremity lies ventrally to middle body axis, post-ocular depression present, a shallow incision in posterior head portion. Abdomen with a shallow mound on each segment. Postabdomen elongated, anal teeth small, present only on anal margin; gonopore opens subdistally. Antenna I long, antennular sensory seta located near flagellum, as long as aesthetascs. Limb I outer distal lobe large, with a small rudimentary setae; inner distal lobe with a single seta 1. Limb II with seta 1 longer than in female and supplied by somewhat longer setules. Limb V as in female.

Redescription. Adult parthenogenetic female. General. Body from almost transparent to remarkably pigmented in brown. In lateral view, body in general subovoid, relatively deep (depth/length ratio 0.48–0.66), maximum height in middle of valves (Figs 1A, 2A). Postero-dorsal angle without a caudal spine, ventral margin regularly convex. In anterior view body oval, with a low dorsal keel, brood chamber wider than head (Figs 1B–C, 2B).

Head relatively large, with a short, rounded rostrum (Figs 1D–E, 2C–D), ventral margin slightly concave, ocular dome expressed, post-ocular depression from shallow to relatively deep, then dorsal head margin regularly convex, a shallow depression in posterior portion of head. Posterior extremity of head shield as an angle, fornices rounded (Fig. 1C), therefore the head shield of Daphnia (Daphnia) type in contrast to the majority of Daphnia (Ctenodaphnia) . Eye capsule located ventrally to body middle axis. Compound eye relatively large, ocellus small.

Carapace ovoid, with a well-expressed rhomboid sculpture (Fig. 2E). Spinules are absent on dorsal margin while the postero-ventral and ventral margins are finely serrated (Fig. 2F–G). Ventral margin with long setae medially; short setae on postero-ventral and posterior portion of valve, with series of small setules between them (Figs 1G–H, 2F–G).

Abdomen relatively long, consisting of four segments, basal most with a straight process, second, third and fourth segments with low process covered by short setules (Fig. 1I–J).

Postabdomen elongated, tapering distally (Figs. 1I, 2H, 3A–C), ventral margin almost straight. Preanal margin straight, with series of minute setules (Fig. 3D), smoothed preanal angle, smooth postanal angle, a small tubercle on postanal margin (Fig. 1K). Anal portion with about 10-13 anal teeth. Postabdominal seta shorter than preanal margin. Postabdominal claw relatively long, curved, with a pointed tip (Figs 1 L-M, 3A–C), its ventral (convex) margin with a series of spines (Fig. 3C), dorsal (concave) margin with three successive pectens: the first and second pecten consisting of somewhat strong teeth; the third pecten consisting of numerous fine setules, not reaching tip of claw.

Antenna I (Figs. 1E, 2C–D) as a small projection (its length = 0.5–0.9 distance from its base to rostrum tip) with nine aesthetascs; tips of aesthetascs project beyond the rostrum; antennular sensory seta small.

Antenna II (Figs 1A, 3E–F) as in other ctenodaphnias: its body relatively short; branches approximately as long as basal segment; apical swimming seta somewhat longer than branches; lateral swimming setae approximately as long as apical ones.

Mandibles with well-developed masticatory surfaces (Fig. 3G).

Limb I without accessory seta; outer distal lobe (Fig. 4A: odl) cylindrical, with a long (its length/length of anterior seta 4 = about 3) seta distally armed with a short setules, and short second bisegmented setae; inner distal lobe (idl) with a long seta (anterior seta 1) armed distally by short setules. Endite 4 with a long anterior seta 2 and two posterior setae (a and b). Endite 3 with a long anterior seta 3 and two posterior setae (c and d). Endite 2 with a relatively short anterior seta 4 and four posterior setae (e–h). Two ejectors hooks of different length (Fig. 4A: ejh).

Limb II exopodite (Fig. 4B: ext) elongated, with two long setulated setae. Four endites with five setae (a–d and 1); anterior seta 1 with length about 0.35 of a soft seta length, covered distally by short setules; a minute spine at the base of seta 1. Gnathobase with two rows of setae: four anterior setae (Fig. 4C: 1’–4’); seta 2’ and 3’ very long; and numerous posterior setae of gnathobasic filter plate (a–f, only most distal setae are represented in this figure).

Limb III with a large, setulated pre-epipodite (Fig. 4D: pep) and ovoid epipodite (epp). Exopodite (Fig. 4D: ext) flat, bearing four distal setae (1–4) and two lateral setae (5–6); seta 2 covered by short setules distally (Fig. 4E); seta 3 longer than seta 4. Inner-distal portion with four endites (Fig. 4F). Endite 5 with a long anterior seta 1, armed distally with short setules, and a single posterior seta a. Endite 4 with a single anterior seta 2 and posterior seta b. Endite 3 with a large anterior seta 3 and two posterior setae (c and d). Endite 2 with an anterior seta 4 and four posterior setae (e-h). The rest of the inner-distal portion (modified gnathobase) as a singular large lobe bearing numerous posterior soft setae and three short anterior setae (Fig. 4F: 1’–3’): seta 1’ relatively long; setae 2’ and 3’ short, located somewhat distally to middle of gnathobase margin.

Limb IV with large setulated pre-epipodite (Fig. 4G: pep) and ovoid epipodite (epp). Exopodite flate, wide, bearing four distal (1–4) and two lateral (5–6) setae. Inner-distal portion of this limb with completely fused endites, with two setae of unclear homology (Fig. 4H: 1–2). Gnathobase filter plate bearing a numerous posterior setae.

Limb V with a rounded epipodite (Fig. 4I: epp), triangular exopodite supplied with two distal setae (Fig. 4I–J: 1 and 2) and one large, curved lateral seta (3). Inner portion of limb with a setulated margin and one large seta.

Juvenile female. Body elongated, subovoid; dorsal margin straight, postero-dorsal angle without a caudal spine (Fig. 5A, 6A–C). Head rounded, its anterior extremity lies in middle body axis, post-ocular depression not developed; compound eye relatively small. In first instar, dorsal organ in posterior head portion on the type of Daphnia s. str. (Kotov and Boikova 2001).

Ephippial female. Body with strongly modified dorsal carapace margin forming ephippium with a single resting egg (Figs 5B–C, 7A–D). Ephippium ovoid, its dorsal margin inflated, with a special pattern of minute depressions and projections (Fig. 7E); central portion of ephippium with minute depressions (Fig. 7F). The ephippium belong to the Morphotype 1 of Kokkinn and Williams (1987) in contrast to opinion of Benzie (2005).

Adult male. General. Body rectangular-ovoid, elongated, dorsal margin of valves straight, slightly elevated above head, postero-dorsal angle rounded, caudal spine fully absent (Figs 5D, 8A–B).

Head large (Fig. 5E–F), without rostrum; its ventral margin convex; anterior most extremity lies ventrally to middle body axis due to inflating of eye capsule (Fig. 5E); post-ocular depression present; a shallow incision in posterior head portion. Compound eye very large, ocellus minute.

Carapace with anterior margin almost straight; antero-ventral angle of valves projected, covered by long setae; ventral margin with a depression in its anterior portion, supplied by especially long setae (Fig. 5G); postero-ventral portion of valves covered by short setae on inner portion (Fig. 5H).

Abdomen with a shallow mound on each segment.

Postabdomen elongated, slightly tapering distally (Figs. 5I, 8C–D); preanal margins slightly convex; anal teeth small, present only on anal margin; postanal margin with a low tubercle. Gonopore opens subdistally of postadomen. On outer portion of postabdominal claw, first pecten with a relatively strong spines, second pecten with shorter spines, third pecten with a numerous fine setules not reaching the tip of claw (Fig. 5J).

Antenna I long, with series of minute setules on its body (Figs. 5E, F, K). Nine aesthetascs short; antennular sensory seta located near flagellum, as long as aesthetascs. Flagellum very long, length longer than antenna I body. Second segment of flagellum covered by short setules (Figs. 5K–L).

Antenna II (Figs 5M, 8E) remarkably longer than in female; basal segment distally with posterior seta and anterior spine longer than in female; exopodite with a spine on proximal segment longer than in female (Fig. 5M).

Limb I outer distal lobe (Fig. 9A–B: odl) large, with a small rudimentary setae (Fig. 9B: arrow) and very large seta covered by small setules in its distal part (Fig. 9C). Inner distal lobe (Fig. 9A–B: idl) with a bent copulatory hook and a single seta 1, seta 1’ not found. Endite 4 with two anterior (2-2’) (Fig. 9D) and two posterior (a–b) setae. Seta 4 longer than in female.

Limb II (Fig. 9E) with seta 1 longer than in female and supplied by somewhat longer setules (Fig. 9F).

Limb V (Fig. 9G) as in female.

Size. Adult parthenogenetic females 1.2–3.1 mm in length. Adult males 1.1–1.5 mm in length.

Differential diagnosis. All Australian species of “ Daphniopsis ” (species with robust, thick body lacking a caudal spine and a spinulation on dorsal margin) form a monophyletic D. pusilla group proposed based on the genetic characters (see Discussion). Its morphological diagnosis is dubious to date, i.e. because the limbs of all its members need to be redescribed according to recent standards, but, most probably, no other “ Daphniopsis ” species groups will be found in Australia. Only two taxa from this group, D. pusilla and D. truncata (Hebert & Wilson, 2000), have a single resting egg in the ovoid ephippium (this is a very rare character for whole genus Daphnia) lacking any anterior and posterior projections, but the former species has a relatively large antenna I body (length of antenna I = 0.5–0.9 distance from its base to tip of rostrum), while it is strongly reduced in the latter.

Distribution. Salty lakes (from 3 to 71‰) in different regions of southern portion of Australia: south-western Western Australia including Rottnest Island (Serventy 1929; Sergeev and Williams 1983; Sergeev and Williams 1985), Victoria, South Australia and Tasmania (Sergeev and Williams 1983; Smirnov and Timms 1983; Shiel and Dickson 1995; Smirnov 1995). Due to the fact that some previous records (i.e. Bayly and Edwards 1969) could deal with future D. truncata (Hebert and Wilson 2000), Benzie (2005) doubted the presence of D. pusilla in South Australia, but on checking the samples from South Australia and Tasmania, we confirm its presence there. Previous records should be reviewed carefully and discussed, as they could belong to other “ Daphniopsis ” taxa.