Ilyodromus williamsi ( McKenzie, 1966 )

Martens, Koen, 2017, On the affinity of Isocypridinae and Herpetocypridinae, with redescriptions of four species of Ilyodromus Sars, 1894 (Crustacea, Ostracoda), Zootaxa 4318 (1), pp. 47-81 : 73-77

Ilyodromus williamsi ( McKenzie, 1966)

Figures 15 to 17

1966 Isocypris williamsi n. Sp. —McKenzie: 266–268, FigS 3 a–e. nov. comb. 1981 Ilyodromus williamsi ( McKenzie 1966) —De Deckker: 63.

Type locality. About 10 miles west of Inverway, Northern Territory, Australia ( De Deckker 1981). Approximate coordinates: S 17° 50’ 15”, E 129° 29’ 10” ( Figure 1 ). In McKenzie (1966), the type locality is given as above. However, the appendix lists more accurately that the Holotype originated from sample “583, a shallow pool, creek about 10 miles west of Inverway, N.T.; May 1965.”

Material investigated. Holotype female with soft parts dissected on a sealed slide (Museum Victoria: J18) and valves stored dry in a micropalaeontological slide (Museum Victoria: J17).

Measurements from McKenzie (1966) (in µm). ♀ Cp: L = 1080 (n = 1), H = 540 (n = 1), W = 430 (n = 1).

Diagnosis. Adults up to approximately 1000 µm in length, subreniform in lateral view. LV with CIL broad anteriorly but narrow posteriorly, and a pronounced inner list around entire margin. A1 second segment with large RO, third segment with dorsal and ventral setae subequal in length, fourth segment also stout, fifth segment with two ventral claw-like setae. A2 with natatory setae 3.7 times the length of the second protopodal segment. L6 with h2 2.5 times the length of the third endopodal segment, and h1 0.7 times the length of h3. L7 with first endopodal segment only marginally longer than the others. CR with Ga short, only 0.4 times the length of the ramus, and Sp bristle-like.

Differential diagnosis (electronic supplementary file, Table S1). This species can be distinguished from most other Ilyodromus species by the strongly sinuated LV ventral margin in lateral view. This character is also shared with I. varrovillius , but I. williamsi can be distinguished from this species based on the prominent list of the LV both anteriorly and posteriorly (absent in I. varrovillius ).

Description. Female carapace in lateral view subreniform ( Figure 15 A & B), dorsal margin rounded, ventral margin sinuous at mid-length, curve of anterior margin more elongate than posterior, apex of anterior margin below mid-height and apex of posterior margin at mid-height, greatest height at mid-length; in dorsal view pointed anteriorly but rounded posteriorly, greatest width at mid-length. External surfaces of some individuals with fine striations.

RV and LV ( Figure 15 A & B) with similar size and shape (RV in Figure 15 B possibly distorted due to damage, see remarks section below). LV with CIL broad anteriorly, narrow posteriorly and ventrally, and extending around entire valve, aside from dorsal hinge area; CIL extending inwardly, by approximately 1/5 of valve length anteriorly, under 1/10 of valve length posteriorly, and much narrower ventrally at mid-length. Ventral sinuous at mid-length. CIL in LV with a pronounced inner list beginning at postero-dorsal margin near the hinge boundary, extending around ventral and anterior margins, and terminating anterodorsally near the hinge boundary. Central muscle scars with five scars, one of these smaller than the others, and two larger scars positioned anteroventral to the main group.

A1 ( Figure 16 B) seven segmented (with first two fused segments counted as only one segment). The length of first segment approximately twice the width, with one short dorsal seta and two long ventral setae. Length to width ratio of second segment approximately 1:2, this segment with a long medio-dorsal seta and a RO ( Figure 16 A) of approximately 0.7 times the length of the segment; this RO ( Figure 16 A) two-segmented, with distal flagella. Third A1 segment elongate, with length approximately twice the width, and additionally with one ventral seta and one dorsal seta, both approximately twice the length of the segment. Fourth segment stout, with length approximately 0.8 times the width, carrying two ventral setae, the ventral-most approximately 2/3 the length of the other, and two dorsal natatory setae, both of these longer than the ventral setae. Fifth segment with length to width ratio approximately 1:1, with two dorsal natatory setae, and two shorter and claw-like ventral setae, the latter under half the length of the dorsal natatory setae, the ventral-most being the shortest. Sixth segment ( Figure 7 C) with length to width ratio approximately 1:1, with an apical group of four long natatory setae, dorsal to this group one very short seta. Seventh segment with length approximately 1.5 times its width, distally carrying two long setae, a broader ventral claw-like seta and a dorsal aesthetasc Ya.

A2 ( Figure 16 C) first protopodal segment with distal setae not visible in illustrated holotype. Second protopodal segment (fused with first) with one distal seta on internal side of the segment. Exopod a small rudimentary plate, with three setae, the anterior-most of similar length as the segment, the middle seta approximately 1/3 this length, and the posterior-most very short. First endopodal segment with aesthetasc Y elongate and two-segmented, with both segments equal in length; distally this segment with group of five long natatory setae, flanked by another short adjacent seta, the five posterior natatory setae being 3.7 times the length of the second endopodal segment; ventro-distally this segment with a large bristled seta, approximately the same length as the segment. Second endopodal segment with four medio-ventral t-setae, two medio-dorsal setae on the opposite segment margin, three subapical z-setae, a short aesthetasc (y2) and three distal claws (G1–G3); G1 and G2 approximately 1.7 times the length of the second endopodal segment, G3 slightly shorter. Terminal segment approximately 1.5 times as long as its basal width, distally with one seta fused at the base with aesthetasc y3, GM a claw reaching as far as G1–G3, and Gm reduced as a seta.

Md coxa ( Figure 16 D) distally with teeth accompanied by few setae, and more proximally from largest tooth an elongate seta covered in stiff setules, as typical of Cypridoidea.

Md palp ( Figure 16 E & F) with length ratios of four palp segments 8:2:5:2. First palp segment the largest, with length approximately 1.3 times the width; this segment with a group of four setae; the most proximal seta long and smooth, this followed by an S1 seta of similar length, an α seta, then most distally a broader S2 seta (broken in illustrated holotype); α seta short and spine-like distally, but with a broad base; total length of α seta approximately 0.4 times the length of the S1 seta. Second palp segment stout, its width over twice its length, and with six setae posteriorly; β seta short and stout, covered with long setules, and almost of the same length as the α seta; four subsequent setae all of similar length as the S1 seta; terminal seta in this group originating more distally, about 3/4 the length of the S1 seta, and covered with rows of long setules; this segment also with a group of three anterodorsal setae, two of which with similar length to the S setae, and the most distal one shorter. Third palp segment elongate with length approximately 1.4 times its width, antero-distally with four smooth setae, all of similar length; distal margin of third palp segment with γ seta anteriorly, and a row of three additional setae, γ seta elongate and with short setules covering the distal half, one neighbouring seta half the length of the γ, two others positioned posteriorly the same length as γ seta; ventral margin of third segment with two setae missing in illustrated holotype. Terminal palp segment short, with length approximately 1.5 times its width; distally with a group of three claws, one much longer than the other two, one seta of similar length and two setae with length half that of the largest claw.

Mx ( Figure 17 A) endopodite two-segmented. First segment with six setae on the dorso-apical margin, three of these setae smooth and of similar length, two shorter and hirsute, the most proximal one slightly longer; a seventh seta being the shortest, based more medially than the others, and pointed posteriorly. Second endopodal segment spatulate, distally bearing three claws and three setae. Third endite distally with small seta on the posterior margin, and at the distal margin, two strongly developed but smooth claws positioned between seven setae dorsally, and one seta ventrally, all setae on the distal margin of similar length, apart from one seta based slightly more proximal, this seta stout, bent, covered with long stiff setules and approximately half the length of the other setae (this seta positioned behind the others on illustrated holotype). Chaetotaxy of endites I and II not elaborated. Respiratory plate not visible in holotype specimen and not illustrated.

L5 not visible in holotype dissection slide.

L6 ( Figure 17 B) a walking limb, with first two segments bearing d1 and d2 setae antero-distally, these setae subequal. Endopod four-segmented. First endopodal segment with e-seta; posterior margin hirsute and arranged into five groupings. Second endopodal segment with f-seta antero-distally and slightly shorter than the third endopodal segment. Third endopodal segment with g-seta, plus one shorter seta antero-distally, both over half the length of the third endopodal segment. Terminal segment with h2 developed into a long claw, about 2.5 times the length of the third endopodal segment, seta h1 0.7 times the length of h3, h3 1.5 times the length of the terminal segment. Length ratios of first to fourth endopodal segments approximately 8:6:5:2.

L7 ( Figure 17 C) a cleaning limb, basal segment with setae d1 and d2 on anterior margin and dp posterodistally, d1 the shortest, d2 longest, and dp intermediate length. First endopodal segment only marginally longer than the others, and bearing a bristled antero-distal e-seta with length approximately the same as d2 seta. Second and third endopodal segments fused, with a bristled f-seta approximately in the middle of this fused segment, this seta approximately half the length of the e-seta. Third endopodal segment without g-seta. Terminal segment fused with third endopodal segment to form a pincer organ, and bearing three setae: seta h1 forming a comb-like seta, h2 short, approximately half the length of the f-seta, a reflexed seta h3 approximately 1.4 times the length of the f-seta.

CR ( Figure 17 E) symmetrical and elongated, each with two serrated claws, an apical Sa seta, and a pectinated claw-like Sp. Claw Ga approximately 0.4 times length of the ramus. Length ratios of claws Sa, Ga, Gp and Sp to ramus 0.1, 0.4, 0.4 and 0.2, respectively. Attachment of CR ( Figure 17 D) bearing an obvious branch at proximal end, and a distal bifurcation.

Remarks. Isocypris williamsi was described in 1966 from 10 miles west of Inverway, Northern Territory, Australia ( McKenzie 1966). In this description, several appendages were partly illustrated with only the left valve. Later, De Deckker (1981) designated the species to the genus Ilyodromus based on the presence of the A1 RO, the similar morphology of the α, β, and γ setae of the Md palp, the spatulate distal segment of the Mx palp, the thick and pectinate Sp seta of the CR, and the CR attachment bearing an obvious single branch at the proximal end. These genus level characters were still considered valid after revising a number of species, including the one above and in Shearn et al. (2014), so the transfer of this species to Ilyodromus appears to have been justified. However, the illustration of these characters was limited in McKenzie (1966) and no illustrations were provided in De Deckker (1981). Detailed illustrations of all appendages will enable confirmation of the transfer of this species to Ilyodromus , and will provide a larger set of taxonomic characters with which to diagnose this species from others in the genus. Here, the only existing type specimen is used to illustrate all appendages and redescribe the species. Unfortunately due to restrictions in the way type material can be examined, SEM micrographs of the valves could not be taken. Instead, light microscope micrographs were taken of the left valve (internal view), and right valve (external view). This has highlighted some major differences of internal valve structure to some other species in the genus. However, to determine character states such as the inner pegs of the left valve that have been documented in other species, SEM micrographs are essential. Additionally, the right valve of the type specimen ( Figure 15 B) is damaged, with extensive cracks and possibly distortion of the valve outline. Therefore, although the redescription here provides a much larger set of taxonomic characters, the collection of topotypes to have pristine valves imaged using scanning electron microscopy would be of great benefit. Additionally, inspection of the holotype appendages in the glass slide revealed that this specimen had been stained, most likely with methylene blue (perhaps to make it easier to pick from a sample). For this reason it is likely that the pigment of the valves has also been altered, so it was not possible to know for sure what the original colour was.

Distribution and habitat. This species was documented from creek side pools, waterholes and temporary pools in north Western Australia and the Northern Territory at the time of its description (sample details from McKenzie (1966) listed below), but has not been documented since. The detailed redescriptions here reveal long natatory setae on the A1 and A2, suggesting that this species is adapted to more pelagic habitats than many other species in the genus that exhibit a more burrowing lifestyle (Sars 1894).

Samples investigated in McKenzie (1966):

Sample No. 583, 3 adults, 7 juveniles, May 1965; a shallow pool, creek about 10 miles west of Inverway , N.T.

Sample No. 584, 1 juvenile, May 1965. Small artificial dam, about 200 yd from 583, Northern Territory; May

1965.

Sample No. 590, 1 juvenile, June 1965; Large shallow waterhole, near junction of Broome-Derby-Fitzroy

Crossing roads, Western Australia; June 1965.

Sample No. 593, 1 juvenile, June 1965; Waterhole , Leopold Ranges, 25–30 miles west of Mt. House on Derby-

Mt. House road, Western Australia; June 1965.

Figures

FIGURE 1. DiStribUtion of Ilyodromus redeScribed in the preSent StUdy, ShoWing the locality of each SpecieS repreSented by a correSponding Symbol.

FIGURE 3. Ilyodromus amplicolis. ♀ (Holotype NMV J 1171 b) from granite rock pool on top of Boyagin Rock, betWeen Brookton and Pingelly, WeStern AUStralia, AUStralia A – G. A. A 2. B. A 1, firSt foUr SegmentS. C. A 1, SegmentS fiVe to SeVen. D. A 1, detail of RO. E. A 2, detail of natatory Setae. F. Md palp. G. Md coxa. ScaleS: A – C = 200 µm, D = 30 µm, E = 15 µm, F – G = 150 µm.

FIGURE 7. Ilyodromus dikrus ♀ (Paratype NMV J 1184 b) from dam at WaSley Well, near Nallan, 21 km NNE of CUe, WeStern AUStralia, AUStralia A – F. A. A 1, detail of RO. B. A 1. C. A 2. D. Md coxa. E. Rake-like organ. F. Md palp. ScaleS: A = 30 µm, B = 150 µm, C = 153 µm, D, F = 102 µm, E = 60 µm.

FIGURE 15. Ilyodromus williamsi ♀ (Holotype NMV J 18) from NT, 10 mileS WeSt of InVerWay, Northern Territory, AUStralia A – B. A. LVi. B. RVe. ScaleS: A – B = 500 µm.

FIGURE 16. Ilyodromus williamsi ♀ (Holotype NMV J 18) from NT, 10 mileS WeSt of InVerWay, Northern Territory, AUStralia A – F. A. A 1, detail of RO. B. A 1. C. A 2. D. Md coxa. E. Md palp, detail of alpha, beta and gamma Setae. F. Md palp, detail of Setae (note Some are miSSing, preSUmed damaged). ScaleS: A = 10 µm, B = 135 µm, C = 91 µm, D = 108 µm, E = 68 µm, F = 73 µm.

FIGURE 17. Ilyodromus williamsi ♀ (Holotype NMV J 18) from NT, 10 mileS WeSt of InVerWay, Northern Territory, AUStralia A – E. A. Mx palp and enditeS, WithoUt detail on Second and third enditeS. B. L 6. C. L 7. D. CR attachment. E. CR. ScaleS: A = 68 µm, B = 108 µm, C = 90 µm, D = 135 µm, E = 84 µm.