Ilyodromus armacutis, Koenders, 2017

Koenders, Annette, 2017, Patterns of genetic divergence in the Ilyodromus amplicolis lineage (Crustacea, Ostracoda), with descriptions of three new species, Zootaxa 4318 (1), pp. 1-46 : 9-17

Ilyodromus armacutis n. sp.

Figures 5 to 9

Diagnosis. Adults largest known of the genus, up to 2500 µm in length, valve surfaces with deep, broad striations and finer striations nested within them. Calcified inner lamella of both valves broad anteriorly and posteriorly, extending inwardly by approximately 1/3 of valve length, and with a blunt antero-ventral peg (remnants of reduced inner list) in LV. A1 third segment with dorsal seta longer than ventral seta, and RO 0.3 times the length of second segment of A1. A2 with longest natatory setae 0.7 times length of second endopodal segment. Males with L5 Rpp and Lpp almost symmetrical, Hp ls narrow proximally, pointed medially, and with broad and curved distal margin, proximally with lateral rounded protrusion elongated; ms with rounded edge, reaching as far as ls.

Differential diagnosis. This species can be distinguished from others of the Ilyodromus amplicolis lineage by the Hp ls having a lateral point on one side, but a straight margin on the other. In other species the Hp ls is either rounded (as in I. sensaddito n. sp. and I. hiatus n. sp.), or less pointed (as in I. amplicolis ). For females, the lengths of natatory setae on the A2 are unique among all species of this lineage. In I. armacutis n. sp. the two longest setae are flanked by three very reduced setae on one side, and a seta of intermediate length on the other; whereas in I. amplicolis , the longest seta is flanked by four short setae on one side and a seta of intermediate length on the other; in I. sensaddito n. sp. all five natatory setae are very reduced and flanked by one longer seta; in I. hiatus n. sp., one natatory seta is flanked by four reduced natatory setae on one side, and a longer seta on the other.

Etymology. The name of this species is derived from the latin words ’arma’ (meaning shield) and ’cutis’ (meaning pointed), after the male lateral shield of the hemipenis being sharply pointed medially. This character is the most obvious feature that distinguishes this species from Ilyodromus amplicolis .

Type locality. Main pool at north east of Drummond nature reserve (Drummond Pool), Western Australia, Australia. Coordinates: S 31° 18’ 46”, E 116° 24’ 37” ( Figure 2 ).

Type material. Holotype male with soft parts dissected on a sealed slide (WAM57838) and valves stored dry in a micropalaeontological slide (WAM57838); two male paratypes with soft parts dissected on sealed slides (WAM57823, WAM57824) and valves stored dry in micropalaeontological slides (WAM57823, WAM57824); three male paratypes with carapaces stored dry on micropalaeontological slides (WAM57825, WAM57826, WAM57827).

Allotype female with soft parts dissected on a sealed slide (WAM57837) and valves stored dry in a micropalaeontological slide (WAM57837); two female paratypes with soft parts dissected on sealed slides (WAM57822, WAM57840) and valves stored dry in micropalaeontological slides (WAM57822, WAM57840); three female paratypes with carapaces stored dry on micropalaeontological slides (WAM57828, WAM57829, WAM57830).

Four female paratypes with valves stored in micropalaeontological slides (WAM57831, WAM57832, WAM57834, WAM57835). One female paratype critical point dried with RV removed stored in a micropalaeontological slide (WAM57836).

Many in toto paratype specimens in EtOH recollected from type locality by David Cale on 19 September 2011 stored in a vial ( WAM 57841—Main pool at north east of Drummond nature reserve, Western Australia, Australia).

Measurements (in µm)

♀ RV: L = 2431–2489 (n = 3), H = 1117–1156 (n = 3). LV: L = 2442–2492 (n = 3), H = 1117–1144 (n = 3). Cp: L = 2550–2569 (n = 3), W = 922–939 (n = 2), H = 1172 (n = 1).

♂ RV: L = 1856–2085 (n = 2), H = 889–925 (n = 2). LV: L = 2003–2097 (n = 2), H = 889–928 (n = 2). Cp: L = 1947–2014 (n = 3), W = 683–688 (n = 2), H = 911 (n = 1).

Description. Female carapace up to 2500 µm and elongate ( Figure 5 A, C & E), with anterior and posterior edges pointed in dorsal and ventral view, but with anterior one more elongated than posterior one ( Figure 5 C & E). Greatest width situated along middle third. In lateral view ( Figure 5 A), dorsal margin almost straight along dorsal hinge for about half its length, sloping sharply in caudal direction, and with slight concavity in front of anterodorsal hinge. Ventral margin sinuous at mid length. Anterior margin broader than posterior, with apex of both anterior and posterior margins below mid-height. LV only slightly overlapping RV dorsally. Greatest height situated along middle third. All external valve surfaces with deep, broad striations, with finer striations nested within them ( Figure 5 L).

In interior view ( Figure 5 G–J), RV and LV with very similar shape and size ( Figure 5 C & D). Both valves with calcified inner lamella broad, and extending around entire valve, aside from dorsal hinge area; calcified inner lamella extending inwardly, by approximately 1/3 of valve length anteriorly and posteriorly. LV calcified inner lamella with a blunt antero-ventral peg (remnants of reduced inner list) ( Figure 5 K). Central muscle scar consisting of a central group of five small scars with one additional small scar set apart antero-dorsally, and another large elongated scar set apart antero-ventrally from the central group.

A1 ( Figure 6 B) seven segmented (with first two fused segments counted as only one segment). Length of first segment approximately twice the width, with one dorsal seta and two ventral setae all of similar length. Length to width ratio of second segment approximately 1:1, this segment with a long medio-dorsal seta and a RO of approximately 0.3 times the length of the segment; this RO ( Figures 6 A & 7A) two segmented, with distal flagella. Third A1 segment elongate, with length approximately 2.5 times the width, additionally with one ventral seta, approximately 1.5 times the length of the segment, and a longer dorsal seta with length more than twice that of the segment. Fourth segment with length approximately 1.5 times the width, carrying two ventral setae, the ventralmost shorter, and two dorsal natatory setae, both of these longer than the ventral setae. Fifth segment with length approximately twice its width, with two dorsal natatory setae, and two shorter ventral setae, the latter approximately half the length of the dorsal natatory setae, the ventral-most being the shortest and broadest. Sixth segment with length approximately twice its width, apically with two reduced setae, dorsal to this group one very short seta, and ventral to this group two setae of approximately half the length of the two reduced setae. Seventh segment with length approximately twice its width, distally carrying one long seta, one short seta, a broader ventral claw-like seta and a dorsal aesthetasc Ya with length subequal to the other short seta.

A2 ( Figure 6 D) first protopodal segment 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 of similar length to 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; distally this segment with three reduced and two longer natatory setae, flanked by another seta of intermediate length ( Figure 7 B), the three reduced setae being less than 0.25 times the length of the anterior-most seta, and less than 0.1 times the length of the second endopodal segment; ventro-distally this segment with a large bristled seta, with length approximately 0.8 times that of the segment. Second endopodal segment with four medio-ventral tsetae, two medio-dorsal setae on the opposite segment margin, three subapical z-setae and three distal claws (G1– G3), G1 and G2 of similar length, almost twice the length of the second endopodal segment with G3 shorter, 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–G2, and almost twice the length of the shorter claw on this segment (Gm); one solitary g seta and one seta fused at the base with aesthetasc y3, all being similar length to claw Gm.

Md coxa ( Figure 6 E) 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 6 C) with length ratios of four palp segments 4:1:2:1. First palp segment the largest, with length approximately 1.5 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 of similar length and carrying rows of long setules; α seta short and spine-like distally, but with a broad base; total length of α seta approximately 0.3 times the length of the S setae. Second palp segment stout, its width about 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; last seta in this group originating more distally, about half the length of the S setae, and covered with rows of setules; this segment also with a group of three antero-dorsal setae, two of which smooth and slightly longer than 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, the three most distal ones longer than the first palp segment, the most proximal one slightly shorter; 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 of similar length; ventral margin of third segment with two subapical setae, one very short, the other long, approximately 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 shorter than the other two, and three setae approximately 2/3 the length of the longest claw.

Mx ( Figure 8 A) endopodite two segmented. First segment with six setae on the dorso-apical margin, three of these setae smooth and of similar length, two slightly 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 with three claws and three setae. Third endite with one proximal seta on anterior margin, distally with 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 one third of the length of the other setae. Chaetotaxy of endites I and II not elaborated. Respiratory plate with approximately 24 rays, six of these reflexed.

L5 ( Figure 8 B) protopodite 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 6 rays.

L6 ( Figure 8 D) a walking limb, with first two segments bearing d1 and d2 setae antero-distally, with d2 being approximately 1.5 x longer than d1. Endopod four-segmented. First endopodal segment with e seta; posterior margin hirsute and arranged into four groupings. Second endopodal segment with f seta antero-distally and approximately the same length as the third endopodal segment. Third endopodal segment with g seta anterodistally, plus another seta alongside (broken in illustrated specimen). Terminal segment with h2 developed into a long serrated claw, about 2.6 times the length of the third endopodal segment, seta h3 0.7 times the length of h1, h3 approximately twice the length of the terminal segment. Length ratios of first to fourth endopodal segments approximately 6:4:3:1.

L7 ( Figure 8 C) a cleaning limb, basal segment with setae d1 and d2 on anterior margin, and seta dp posterodistally; seta dp slightly longer than and d1 and d2. 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 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, under half the length of the f seta, a reflexed seta h3 approximately the same length as the f seta.

CR ( Figures 7 C & 8F) symmetrical and elongated, each with three serrated claws and apical Sa seta. Claw Ga nearly half the length of the ramus. Length ratios of claws Sa, Ga, Gp and Sp to ramus 0.15, 0.5, 0.4 and 0.2, respectively. Attachment of CR ( Figure 8 E) with slightly bent main stem, bearing an obvious branch at proximal end, and a distal bifurcation.

Male Cp smaller than that of female ( Figures 5 B, D & F) and with calcified inner lamella of both valves extending inward less than in females ( Figures 5 I & J), but otherwise of similar appearance.

Male L5 prehensile palps ( Figures 9 B & C) with first segment approximately twice their central width; subapically with two sensory organs; second segment narrow and hook shaped with distal sensory organ; first segment with one sensory organ shorter than the other in Lpp, of similar length in Rpp, but otherwise left and right prehensile palps symmetrical.

Male Hp ( Figure 9 A) ls narrow proximally, pointed medially, and with broad and curved distal margin, proximally with lateral rounded protrusion elongated; ms with broad base narrowing to rounded distal margin, approximately 1.5 times the width of ls at mid-length, and reaching as far as ls.

Distribution and habitat. This species has been recorded only at the type locality: Main pool at north east of Drummond nature reserve, Western Australia, Australia. The reduced natatory setae on the A1 and A2 ( Figure 7 B) suggest that this species may be a benthic form, as was observed for several other Ilyodromus species by Sars (1894). This is the largest known species of the genus. This may be explained by reduced predation, as it occurs in an ephemeral and turbid claypan with dominant vegetation of Melaleuca laterita A. Dietr. shrubland where fish are unlikely to occur.

Figures

FIGURE 2. Geographic origin of material of the Ilyodromus amplicolis lineage examined from Western Australia, showing the locality of each species represented by a corresponding symbol colour (colour only visible in electronic version). Symbols placed on circles originate from the centre of that circle, and represent pools that were positioned on the same rock outcrop.

FIGURE 5. Ilyodromus armacutis. n. sp. (DJC 17 - A) from Drummond Pool, Western Australia, Australia A – L. A. CpRl ♀ (WAM 57828). B. CpRl ♂ (WAM 57825). C. CpD ♀ (WAM 57829). D. CpD ♂ (WAM 57826). E. CpV ♀ (WAM 57830). F. CpV ♂ (WAM 57827). G. LVi ♀ (WAM 57822) indicating anterior peg. H. RVi ♀ (WAM 57822). I. LVi ♂ (WAM 57838) indicating anterior peg. J. RVi ♂ (WAM 57838). K. LVi ♀ detail of anterior peg (WAM 57840). L. CpRl ♂, detail of valve striation pattern (WAM 57825). Scales: A – J = 1000 µm, K = 50 µm, L = 25 µm.

FIGURE 6. Ilyodromus armacutis. n. sp. ♀ (WAM 57837, DJC 17 - A) from Drummond Pool, Western Australia, Australia A – F. A. A 1, detail of RO. B. A 1. C. Md palp. D. A 2. E. Md coxa. Scales: A = 30 µm, B, D = 300 µm, C, E = 150 µm

FIGURE 7. Ilyodromus armacutis. n. sp. ♀ (WAM 57836, DJC 17 - A) from Drummond Pool, Western Australia, Australia A – C. A. A 1, detail of RO (red). B. A 2, detail of natatory setae (red). C. CR, detail of Sp (red). Colour version of figure available online. Scales: A, C = 50 µm, B = 15 µm

FIGURE 8. Ilyodromus armacutis. n. sp. ♀ (WAM 57837, DJC 17 - A) from Drummond Pool, Western Australia, Australia A – F. A. Mx palp and endites, without detail on second and third endites. B. L 5. C. L 7. D. L 6. E. CR attachment. F. CR. Scales: A – C, F = 200 µm, D = 300 µm, E = 400 µm

FIGURE 9. Ilyodromus armacutis. n. sp. ♂ (WAM 57838, DJC 17 - A) from Drummond Pool, Western Australia, Australia A – C. A. Hp B. Prehensile palp of right L 5. C. Prehensile palp of left L 5. Scales: A – C = 77 µm.