Ilyodromus candonites 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 : 65-70

Ilyodromus candonites De Deckker, 1981

Figures 10 to 14

1981 Ilydromus candonites n. Sp. —De Deckker: 56–61, FigS 9 f–j & 10.

Type locality. Small granite rock pool at the summit of Mt. Chudalup, near Northcliffe, Western Australia, Australia ( De Deckker 1981). Approximate coordinates: S 34° 45’ 51.3”, E 116° 05’ 13.24” ( Figure 1 ).

Material investigated. Holotype male with soft parts dissected on a sealed slide (J1175a) and valves stored dry in a micropalaeontological slide (J1175b); two female paratypes with soft parts dissected on a sealed slide (J1176a, J1177a) and valves stored dry in a micropalaeontological slide (J1176b, J1177b); two female paratypes with carapaces stored dry in micropalaeontological slides (J1180a, J1181a). Two male paratypes with carapaces stored dry in micropalaeontological slides (J1178a, J1179a).

Two female topotypes recollected from type locality with soft parts dissected on a sealed slide and valves stored dry in a micropalaeontological slide (WAM57216, WAM57218); one male topotype recollected from type locality with soft parts dissected on a sealed slide and valves stored dry in a micropalaeontological slide (WAM57217). Three female topotypes recollected from type locality with carapaces stored dry in micropalaeontological slides (WAM57219, WAM57220, WAM57221). Three male topotypes recollected from type locality with carapaces stored dry in micropalaeontological slides (WAM57222, WAM57223, WAM57224).

Many in toto topotype specimens in EtOH recollected from type locality by RS on the 23rd September 2013, stored in two vials ( WAM 57225—Small granite rock pool at the summit of Mt. Chudalup, near Northcliffe, Western Australia, Australia).

Measurements (in µm). ♀ RV: L = 1382–1435 (n = 3), H = 683–718 (n = 3). LV: L = 1447 (n = 2), H = 722– 747 (n = 2). Cp: L = 1480–1490 (n = 3), H = 727–777 (n = 3).

♂ RV: L = 1093–1144 (n = 3), H = 568 (n = 1). LV: L = 1099–1150 (n = 3), H = 568–592 (n = 3). Cp: L = 1149 (n = 1), H = 562 (n = 1).

Diagnosis (based also on description by De Deckker, 1981). Adults up to approximately 1400 µm in length, in lateral view subrectangular, anterior margin appearing more elongated than posterior, apex of anterior and posterior margins very low; valve surfaces with deep, broad striations and finer striations nested within them. Valves with CIL broad anteriorly but narrow posteriorly, extending inwardly by approximately 1/5 of valve length anteriorly but only 1/10 of valve length posteriorly, LV with one pointed anteroventral and one pointed posteroventral peg (remnants of reduced inner list) and faint inner list along posterior margin extending anteriorly and terminating near the apex of the anterior margin. RV with selvage at periphery of valve margin posteriorly. A1 third segment with dorsal seta longer than ventral seta, and RO 0.3 times length of second segment of A1. A2 second endopodal segment with natatory setae reduced. A1 terminal segment without broad claw-like ventral seta, as seen in other species. Md palp with S2 seta longer than S1. L6 with d1 longer than d2. CR with length ratios of claws Sa, Ga, Gp and Sp to ramus 0.1, 0.4, 0.4 and 0.2, respectively. Males with L5 Rpp and Lpp similar but not symmetrical, both with second segment hook shaped but angular, the shape resembling a drawer handle. Male Hp ls protruding further than ms, narrow, curved and widest distally; Hp with ms embracing ls proximally, and with an additional lobe embracing the proximal 1/2 of the ms.

Differential diagnosis (electronic supplementary file, Table S1). This species can be distinguished from all others of the genus, aside from Ilyodromus substriatus Sars, 1894 , by the pointed shape of the anteroventral and posteroventral pegs on the LV CIL. It can be further differentiated from I. substriatus by the RV having a faint selvage at the periphery of the valve margin (whereas the selvage is prominent posteriorly in the RV of I. substriatus ) and by the RO to A1 second segment length ratio being far lower (about 0.3 in I. candonites versus 1.0 in I. substriatus ).

Description. Carapaces dark green to dark blue in colour. Female sub-rectangular ( Figure 10 A & B), with anterior and posterior edges pointed in ventral view at both ends. Greatest width situated at mid-length. Ventral margin sinuous at mid-length. Anterior margin more elongate than posterior, apex of anterior margin low, below mid-height and posterior margin very low below mid-height. Strong overlap of LV over RV around entire margin, including dorsal hinge area. Greatest height spanning middle 1/2 of carapace. All external valve surfaces with deep, broad striations, with finer striations nested within them ( Figure 10 D).

In interior view, RV and LV with similar shape, but LV larger ( Figure 10 E & F). Both valves with CIL broad anteriorly but not posteriorly, and extending around entire valve, aside from dorsal hinge area; CIL extending inward, by approximately 1/4 of valve length anteriorly, and by approximately 1/10 of valve length posteriorly, much narrower ventrally at mid-length. Selvage at periphery of valve margin in RV ( Figure 10 F). Ventral margin sinuous at mid-length in both valves. LV CIL with pointed posteroventral ( Figure 10 I) and anteroventral ( Figure 10 J) pegs (remnants of reduced inner list) and with a weakly developed inner list obvious posteriorly, continuing around the ventral margin and terminating near the apex of anterior margin ( Figure 10 E).

A1 ( Figure 11 A) seven segmented (with first two fused segments counted as only one segment). Length of first segment approximately 1.5 times the width, with one short dorsal seta and two longer ventral setae. Length to width ratio of second segment approximately 1:1, this segment with a short medio-dorsal seta and a RO ( Figure 11 A & B) of approximately 1/3 the length of the segment, this RO two-segmented, with distal flagella not visible. Third A1 segment elongate, with length nearly three times the width, additionally with one ventral seta, approximately 1.5 times the length of the segment, and a longer dorsal seta, with length over twice the segment length. Fourth segment with length to width ratio approximately 1:1, carrying two ventral setae, the ventral-most one 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 approximately 1.5 times the width, 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, broadest, and claw-like. Sixth segment with length approximately 1.5 times its width, with an apical group of four natatory setae. Seventh segment with length approximately twice its width, distally carrying two long setae, a shorter ventral seta and a dorsal aesthetasc Ya.

First protopodal segment of A2 ( Figure 11 C–E) with two latero-distal setae, as typical of Cypridoidea (missing in damaged illustrated paratype ( Figure 11 C), but clearly visible in topotype ( Figure 11 D)). Second protopodal segment (fused with first) with one distal seta on inner side of the segment (again missing in damaged illustrated paratype ( Figure 11 C), but clearly visible in topotype ( Figure 11 D)). Exopod a small rudimentary plate, with three setae of variable length, the anterior-most between 1/2 to 2/3 the length of the segment, the middle seta approximately 1/2 this length, and the posterior-most very short. First endopodal segment with aesthetasc Y elongate and two-segmented, with proximal segment approximately 1.5 times the length of the distal one; distally with group of five reduced natatory setae ( Figure 11 E), flanked by another longer adjacent seta, the five posterior natatory setae being 0.2 times the length of the second endopodal segment; ventro-distally this segment with a large bristled seta, approximately 0.8 times the length of the segment. Second endopodal segment with four medioventral t-setae, two medio-dorsal setae on the opposite segment margin, three subapical z-setae and three distal claws (G1–G3), with G3 shorter than G1 and G2, and a short aesthetasc (y2). Terminal segment approximately twice as long as its basal width, distally with GM a claw and reaching as far as G1 and G2, Gm a seta alongside a g-seta and one other seta fused at the base with aesthetasc y3.

Md coxa ( Figure 12 A) 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 12 B) with length ratios of terminal three palp segments approximately 1:3:1. First palp segment the largest, 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; S2 seta longer than S1, both carrying rows of long setules; α seta short and spine-like distally, but with broad base; total length of α seta approximately 0.3 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 S1; terminal seta in this group originating more distally, under half the length of S1, and covered with rows of setules; this segment also with a group of three antero-dorsal setae, two of which smooth, with similar length to S1, and the most distal one half the length of the others, with long setules over most of its length. Third palp segment elongate with length approximately 1.5 times its width, antero-distally with four smooth setae, the most distal one shorter than the others; 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 1.2 times the length of the γ; ventral margin of third segment with two subapical setae, one very short, the other long, almost the same length as S1. Terminal palp segment short, with length approximately 1.2 times its width; distally with a group of three claws of variable length, and one seta of similar length to the claws.

Rake-like organ (not illustrated) with elongate proximal arm, broadening abruptly to a 7–9 toothed rake structure.

Mx ( Figure 12 C) 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 under half the length of the other setae. Chaetotaxy of endites I and II not elaborated. Respiratory plate (not illustrated) with approximately 26 rays, six of these reflexed.

L5 ( Figure 12 D) 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. Exopodite a respiratory plate with six setae.

L6 ( Figure 13 A) a walking limb, with first two segments bearing d1 and d2 setae antero-distally, with d1 being approximately 1.5 times longer than d2. Endopod four-segmented. First endopodal segment with e-seta reaching to approximately mid-length of third segment; posterior margin hirsute and arranged into four 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 one slightly longer seta, both less than half the length of the third endopodal segment. Terminal segment with h2 developed into a long serrated claw, with length approximately 2.5 times the length of the endopodal segment, seta h1 0.7 times the length of h3, and approximately twice the length of the terminal segment. Length ratios of first to fourth endopodal segments approximately 9:6:5:2.

L7 ( Figure 13 B & C) 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 eseta, slightly longer than d2 seta. Second and third endopodal segments fused, with a bristled f-seta approximately in the middle of this fused segment, this seta of approximately 1/2 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 1/3 the length of the f-seta, a reflexed seta h3 1.5 times the length of the f-seta.

CR ( Figure 13 E) symmetrical and elongated and narrow distally, each with two serrated claws, an apical Sa seta, and a claw-like Sp. 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 13 D) bearing an obvious branch at proximal end, and a distal bifurcation.

Male Cp ( Figure 10 C) and valves ( Figure 10 G & H) smaller than females, but otherwise of similar appearance.

Male L5 Lpp ( Figure 14 B) first segment stout, with length approximately 1.5 times its central width; apically with two sensory organs, one double the length of the other; second segment hook shaped but angular, the shape resembling a drawer handle, with distal sensory organ; Rpp ( Figure 14 C) with first segment length over twice its width and with only one apical sensory organ, second segment with similar shape to Lpp second segment but broader distally, and with pronounced outward bend proximally.

Male Hp ( Figure 14 A) ls narrow, curved, protruding notably further than ms, with parallel margins aside from distal 1/4, where it is broadest and terminating with a flat distal margin; ms with broad base narrowing to a rounded distal margin, with two lobes embracing ls proximally, and with a rounded lateral protrusion; an additional lobe embracing the proximal 1/2 of the ms.

Remarks. Ilyodromus candonites was described in 1981 from a small granite rock pool at the summit of Mt. Chudalup, near Northcliffe, Western Australia, Australia ( De Deckker 1981). In these descriptions, several important taxonomic features of the valves and appendages for this genus (later discovered for other species in Shearn et al. 2014) were not visible in illustrations and were not described. Unfortunately, the condition of type material was poor, and many slides had dried with appendages damaged and having missing segments and setae. It was thus necessary to resample Ilyodromus candonites at the type locality and designate topotypes to be used alongside the damaged holotype and paratypes to redescribe the species with the additional taxonomic characters.

Distribution and habitat. This species is adapted to temporary rock pools of Western Australia. The species was described from a small granite rock pool at the summit of Mt. Chudalup, near Northcliffe, Western Australia, Australia, but was also recorded nearby at Muirillup Rock ( De Deckker 1981) and quite some distance away in a pool at Pintha Rock ( Pinder et al. 2000), in the mid west of Western Australia. Like many other species in the genus, the natatory setae of the A1 and A2 are very reduced, suggesting this species lacks swimming ability and is a benthic form, as was observed in Ilyodromus amplicolis , and several other species by Sars (1894).

Figures

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

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 10. Ilyodromus candonites from Small granite rock pool at SUmmit of Mt. ChUdalUp, near Northcliffe, WeStern AUStralia, AUStralia A – J. A. CpRl ♀ (Paratype NMV J 1179 a). B. CpLl ♀ (Paratype NMV J 1180 a). C. CpRl ♂ (Paratype NMV J 1178 a). D. CpRl ♀, detail of A (Paratype NMV J 1179 a). E. LVi ♀ (Paratype NMV J 1176 a) indicating anterior and poSterior pegS. F. RVi ♀ (Paratype NMV J 1176 a). G. LVi ♂ (Holotype NMV J 1175 a) indicating anterior and poSterior pegS. H. RVi ♂ (Holotype NMV J 1175 a). I. LVi ♀, detail of poSterior peg (Paratype NMV J 1176 a), J. LVi ♀, detail of anterior peg (Paratype NMV J 1176 a). ScaleS: A – H = 500 µm, I – J = 50 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.

FIGURE 13. Ilyodromus candonites ♀ from Small granite rock pool at SUmmit of Mt. ChUdalUp, near Northcliffe, WeStern AUStralia, AUStralia A – E. A. L 6 (Paratype NMV J 1176 b). B. L 7, With miSSing Seta on Second Segment (Paratype NMV J 1176 b). C. L 7, ShoWing all Setae (Topotype WAM 57218) D. CR attachment (Paratype NMV J 1176 b). E. CR (Paratype NMV J 1176 b). ScaleS: A – C, E = 102 µm, D = 200 µm.

FIGURE 14. Ilyodromus candonites ♂ (Holotype NMV J 1175 b) from Small granite rock pool at SUmmit of Mt. ChUdalUp, near Northcliffe, WeStern AUStralia, AUStralia A – C. A. HemipeniS. B. PrehenSile palp of left L 5. C. PrehenSile palp of right L 5. ScaleS: A – C = 54 µm.