Ilyodromus dikrus De Deckker, 1981

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 : 58-64

Ilyodromus dikrus De Deckker, 1981

Figures 6 to 9

1981 Ilyodromus dikrus n. Sp. — De Deckker: 61–63, FigS 11 & 12 .

nov. comb. 2011 Isocypris dikra ( De Deckker, 1981) —MartenS & SaVatenalinton: 50.

Type locality. Dam at Wasley Well, near Nallan, 21 km NNE of Cue, Western Australia, Australia ( De Deckker 1981). Approximate coordinates: S 27° 16’ 54”, E 118° 09’ 06” ( Figure 1 ).

Material investigated. Holotype male with soft parts dissected on a sealed slide (J1182a) and valves stored dry in a micropalaeontological slide (J1182b); two female paratypes with soft parts dissected on a sealed slide (J1184a, J1185a) and valves stored dry in a micropalaeontological slide (J1184b, J1185b); Three female paratypes with carapaces stored dry on micropalaeontological slides (J1189a, J1190a, J1191a). Two male paratypes with carapaces stored dry on micropalaeontological slides (J1186a, J1188a).

Measurements (in µm). ♀ RV: L = 1422–1470 (n = 2), H = 647–653 (n = 2). LV: L = 1425–1477 (n = 2), H = 657–660 (n = 2). Cp: L = 1228–1505 (n = 4), H = 547–718 (n = 3), W = 447 (n = 1).

♂ RV: L = 1373–1558 (n = 2), H = 615–714 (n = 2). LV: L = 1368–1537 (n = 2), H = 552 (n = 1). Cp: L = 1258–1338 (n = 2), H = 562–598 (n = 2).

Diagnosis. Adults up to 1500 µm in length. Carapace appears as an inclined parallelogram in lateral view, valve surfaces with deep, broad striations and finer striations nested within them. CIL of both valves broad anteriorly and posteriorly, extending inwardly by approximately 1/4 of valve length, and with a blunt anteroventral peg (remnants of reduced inner list) in LV. A1 second segment stout, with length to width ratio approximately 1:2. A1 third segment with dorsal seta longer than ventral seta, and RO 0.4 times length of second segment of A1. A2 second endopodal segment short, longest natatory setae 3.9 times length of second endopodal segment. Males with L5 Rpp and Lpp asymmetrical, Rpp with second segment subtriangular whereas Lpp with second segment hook shaped. Male Hp ls protruding further than ms, narrow with parallel margins and rounded distal margin; Hp ms broad proximally, narrowing distally to flattened distal margin.

Differential diagnosis (electronic supplementary file, Table S1). Ilyodromus dikrus can be differentiated from all other species of the genus by the lateral carapace outline appearing as an inclined parallelogram, with the apex of the anterior margin being well above mid-height and the apex of the posterior margin being well below midheight. Males of this species are also distinguished by the narrow ls of the Hp having parallel margins.

Description. Female carapace elongate ( Figure 6 A & B), with anterior and posterior edges pointed in ventral view at both ends ( Figure 6 K). Greatest width situated at mid-length. In lateral view, dorsal margin straight along dorsal hinge, with pronounced concavity in front of the anterodorsal hinge. Ventral margin slightly sinuous at midlength. Anterior margin broader than posterior one, apex of anterior margin well above mid-height and posterior margin well below mid-height. No obvious overlapping of either valve over the other around entire margin ( Figure 6 A–D). Greatest height situated at approximately 1/3 from anterior. All external valve surfaces with deep, broad striations ( Figure 6 A–D), with finer striations nested within them ( Figure 6 J).

In interior view, RV and LV with similar size and shape ( Figure 6 E & F). Both valves with CIL broad, and extending around entire valve, aside from dorsal hinge area; CIL extending inward, by approximately 1/4 of valve length anteriorly, and by approximately 1/5 of valve length posteriorly, much narrower ventrally at mid-length. Ventral margin straight, with only slight sinuation at mid-length. CIL in LV often with a blunt anteroventral peg (remnants of reduced inner list) ( Figure 6 G & I).

A1 ( Figure 7 B) seven segmented (with first two fused segments counted as one segment). Length of first segment approximately two times 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 7 A) of approximately half the length of the segment; this RO ( Figure 7 A) two-segmented, with distal flagella not visible. Third A1 segment elongate, with length approximately twice the width, additionally with one ventral seta, approximately twice the length of the segment and a longer dorsal seta, with length over twice that of the segment. Fourth segment with length to width ratio approximately 1:1, carrying two ventral setae, the ventral-most being approximately half the length of the other, and two dorsal natatory setae, both of these much longer than the ventral setae. Fifth segment with length to width ratio approximately 1:1, with two dorsal natatory setae, and two shorter ventral setae, the latter about 1/3 the length of the dorsal natatory setae, the ventral-most being the shortest. Sixth segment ( Figure 7 C) with length approximately 1.5 times its width, with an apical group of four natatory setae, dorsal to this group one very short seta. Seventh segment with length approximately twice its width, distally carrying two long setae, a broader ventral claw-like seta and a dorsal aesthetasc Ya.

First protopodal segment of A2 ( Figure 7 C) with two latero-distal setae, as typical of Cypridoidea. Second protopodal segment (fused with first) with one distal seta on inner side of the segment. Exopod a small rudimentary plate, with three setae, the anterior-most short and half the length of the segment, the middle seta approximately 1/ 3 this length, and the posterior-most very short. First endopodal segment with aesthetasc Y elongate and twosegmented, with distal segment approximately 1.5 times the length of the proximal one; distally this segment with group of five natatory setae, flanked by another short adjacent seta, the five posterior natatory setae being four 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 and three distal claws (G1–G3), the latter all of similar length, approximately 1.5 times the length of the second endopodal segment, and a short aesthetasc (y2). Terminal segment approximately twice as long as its basal width, distally with two claws, the larger one (GM) reaching as far as G1–G3, and about 1.5 times the length of the shorter claw on this segment (Gm) and one seta fused at the base with aesthetasc y3.

Md coxa ( Figure 7 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 7 F) with length ratios of four palp segments 6:1:3:1. First palp segment the largest, with length approximately 1.6 times the width; this segment with a group of four setae; the most proximal seta long and smooth, followed by an S1 seta of similar length, an α seta, then most distally a broader S2 seta; both S setae carrying rows of long setules, the more proximal S1 longer and straight, the distal S2 shorter and bent; 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 S setae. 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 S setae; terminal seta in this group originating more distally, about half the length of the S setae, and covered with rows of long setules; this segment also with a group of three antero-dorsal setae, two of which smooth, with similar length to the S setae, and the most distal one shorter, with long setules over the distal half of its length. Third palp segment elongate with length approximately 1.5 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, three neighbouring setae three times the length of the γ; ventral margin of third segment with two setae, one very short, the other long, almost the same length as the S setae. 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, and two setae with length half that of the shorter claws.

Rake-like organ ( Figure 7 E) with elongate proximal arm, broadening abruptly to a 7–9 toothed rake structure.

Mx ( Figure 8 A) endopodite two-segmented. First segment with six setae on the antero-distal margin, three of these setae smooth and of similar length, two shorter and hirsute, the most proximally based one slightly longer, and 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 six setae anteriorly, and two seta posteriorly, 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. Chaetotaxy of endites I and II not elaborated. Respiratory plate not visible in type material and not illustrated.

L5 ( Figure 8 B) protopod with two a-setae of similar length, based proximally on the anterior margin, one long and hirsute b-seta on ventral margin, and a long, hirsute d-seta based anteriorly. Endite with 14 setae lining the antero-distal margin of varying length and shape. Endopodite with three hirsute distal setae, one longer than the other two.

L6 ( Figure 8 E) a walking limb, with first two segments bearing d1 and d2 setae antero-distally, with d2 being approximately two times longer than d1. 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 antero-distally plus another seta of the same length, both less than half the length of the third endopodal segment. Terminal segment with h2 developed into a long serrated claw, with length about 2.8 times the length of the third endopodal segment, seta h1 0.3 times the length of h3, h3 approximately six times the length of the terminal segment. Length ratios of first to fourth endopodal segments approximately 8:6:5:2.

L7 ( Figure 8 F) a cleaning limb, basal segment with setae d1 and d2 on anterior margin, and seta dp posterodistally, all of similar length. First endopodal segment the longest, 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 1/3 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 nearly twice the length of the f-seta.

CR ( Figure 8 C) symmetrical and elongated, each with two serrated claws, an apical Sa seta, and a claw-like Sp. Claw Ga half the length of the ramus. Length ratios of claws Sa, Ga, Gp and Sp to ramus 0.2, 0.5, 0.3 and 0.3, respectively. Attachment of CR ( Figure 8 D) bearing an obvious branch at proximal end, and a distal bifurcation.

Male Cp ( Figure 6 B & C) and valves ( Figure 6 G & H) smaller than females, but otherwise of similar appearance.

Male L5 Lpp ( Figure 9 B) with first segment length approximately twice its central width; subapically with two sensory organs of similar length; second segment narrow and hook shaped with distal sensory organ; Rpp ( Figure 9 C) with first segment length over 2.5 times its width, second segment broader than Lpp second segment and subtriangular in shape with long distal sensory organ.

Male Hp ( Figure 9 A) elongated; ls narrow with parallel margins and rounded distally, protruding notably further than ms; ms with broad base narrowing to a flattened distal margin.

Remarks. Ilyodromus dikrus was described in 1981 from a dam at Wasley Well, near Nallan, 21 km NNE of Cue, Western Australia, Australia ( De Deckker 1981). In these descriptions, several important taxonomic features of the valves and appendages for this genus, first described in Shearn et al. (2014), were not visible in existing illustrations and were not described. Here, type material for this species is used in a redescription that includes these features.

Distribution and habitat. To date, there appear to be only two records of the species, and thus it has a limited known distribution within inland waters of Western Australia. This species was described from a dam, in fresh turbid water ( De Deckker 1981), but has also been recorded from turbid, fresh claypans at Coolcalalaya station, which supports a Gascoyne/Murchison distribution (Halse et al. 2000). Given that the natatory setae of the A1 and A2 are not heavily reduced, it may be that this species does not completely lack swimming power, and may have the ability to swim through the water column as well as traversing the sediment surface.

Figures

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

FIGURE 6. Ilyodromus dikrus from dam at WaSley Well, near Nallan, 21 km NNE of CUe, WeStern AUStralia, AUStralia A – J. A. CpRl ♀ (Paratype NMV J 1189 a). B. CpLl ♀ (Paratype NMV J 1190 a). C. CpRl ♂ (Paratype NMV J 1188 a). D. CpLl ♂ (Paratype NMV J 1186 a). E. LVi ♀ (Paratype NMV J 1184 a). F. RVi ♀ (Paratype NMV J 1184 a). G. LVi ♂ (Holotype NMV J 1182 a) indicating anterior peg. H. RVi ♂ (Holotype NMV J 1182 a). I. LVi ♀, detail of peg (Paratype NMV J 1185 a) indicating anterior peg. J. CpRl ♀, detail of A (Paratype NMV J 1189 a). K. CpV ♀ (Paratype NMV J 1191 a). ScaleS: A – H, K = 500 µm, I = 300 µm, J = 50 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 8. 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. Mx palp and enditeS, WithoUt detail on Second and third enditeS. B. L 5. C. CR. D. CR attachment. E. L 6. F. L 7. ScaleS: A, C = 77 µm, B, F = 102 µm, D, E = 150 µm.

FIGURE 9. Ilyodromus dikrus ♂ (Holotype NMV J 1182 b) from dam at WaSley Well, near Nallan, 21 km NNE of CUe, WeStern AUStralia, AUStralia A – C. A. HemipeniS. B. PrehenSile palp of left L 5. C. PrehenSile palp of right L 5. ScaleS: A = 68 µm, B – C = 108 µm.

FIGURE 11. Ilyodromus candonites ♀ S from Small granite rock pool at SUmmit of Mt. ChUdalUp, near Northcliffe, WeStern AUStralia, AUStralia A – E. A. A 1 (Paratype NMV J 1176 b). B. A 1, detail of RO (Paratype NMV J 1176 b). C. A 2, damaged With a nUmber of miSSing and broken Setae (Paratype NMV J 1176 b). D. A 2, ShoWing all Setae (Topotype WAM 57216). E. A 2, detail of natatory Setae (Topotype WAM 57216). ScaleS: A = 150 µm, B = 30 µm, C – D = 118 µm, E = 40 µm.

FIGURE 12. Ilyodromus candonites ♀ (Paratype NMV J 1176 b) from Small granite rock pool at SUmmit of Mt. ChUdalUp, near Northcliffe, WeStern AUStralia, AUStralia A – D. A. Md coxa. B. Md palp. C. Mx palp and enditeS, WithoUt detail of Second and third enditeS D. L 5. ScaleS: A – D = 102 µm.