Ilyodromus hiatus, 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 : 25-32

Ilyodromus hiatus n. sp.

Figures 15 to 19

Diagnosis. Adult females up to 2000 µm in length; both valves with apex of anterior and posterior margins below mid-height in lateral view, calcified inner lamella broad anteriorly and posteriorly, extending inwardly by approximately 1/5 of valve length anteriorly and 1/6 posteriorly. A1 third segment with dorsal seta longer than ventral seta. A2 with one natatory seta flanked by four reduced natatory setae on one side, and a longer seta on the other. Males with L5 Rpp and Lpp second segments glove-like and nearly symmetrical, Hp ls protruding further than ms, narrow proximally, widening to rounded latero-distal margin with apex pointing postero-laterally, proximally with lateral rounded protrusion; ms elongated with broad base narrowing to rounded distal margin with apex pointing postero-medially.

Differential diagnosis. This species can be identified among others of the Ilyodromus amplicolis lineage by the glove-like second segments of the L5 prehensile palps. In other species of this lineage, these segments are much narrower and hook-like. If only females are present, the lengths of natatory setae on the A2 are also unique for all species of this lineage. 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; 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. 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; in I. sensaddito n. sp. all five natatory setae are very reduced and flanked by one longer seta.

Etymology. This species has been named after the Latin word for ’gap’, for the notably large ventral overlap of the LV over RV, comparison to other species of this lineage.

Type locality. Granite rock pool on top of Wannara Rock, north of Wubin, Western Australia, Australia. Coordinates: S 29° 31’ 22.944”, E 116° 47’ 41.135” ( Figure 2 ).

Type material. Holotype male with soft parts dissected on a sealed slide (WAM57868) and valves stored dry in a micropalaeontological slide (WAM57868); two male paratypes with soft parts dissected on sealed slides (WAM57866, WAM57867) and valves stored dry in a micropalaeontological slide (WAM57866, WAM57867); three male paratypes with carapaces stored dry on micropalaeontological slides (WAM57869, WAM57870, WAM57871). One male paratype with valves stored in a micropalaeontological slide, and soft parts used for DNA extraction (RS620). One male paratype critical point dried with RV removed stored in a micropalaeontological slide (WAM57878).

Allotype female with soft parts dissected on a sealed slide (WAM57872) and valves stored dry in a micropalaeontological slide (WAM57872); one female paratype with soft parts dissected on sealed slides (WAM57876) and valves stored dry in a micropalaeontological slide (WAM57876); three female paratypes with carapaces stored dry on micropalaeontological slides (WAM57873, WAM57874, WAM57875). Six female paratypes with valves stored in micropalaeontological slides, and soft parts used for DNA extraction (WAM57860, WAM57861, WAM57862, WAM57863, WAM57864, WAM57865). One female paratype critical point dried with RV removed stored in a micropalaeontological slide (WAM57877).

Many in toto paratype specimens in EtOH recollected from the type locality by RS on 17 September 2013 stored in a vial ( WAM 57879 – Wannara rock north of Wubin, Western Australia, Australia).

Measurements (in µm). ♀ RV: L = 2066 (n = 1), H = 952 (n = 1). LV: L = 2034 (n = 1), H = 950 (n = 1). Cp: L = 1989–2068 (n = 3), W = 755–777 (n = 2), H = 921 (n = 1)

♂ RV: L = 1698–1702 (n = 2), H = 765–777 (n = 2). LV: L = 1685–1694 (n = 2), H = 777–788 (n = 2). Cp: L = 1665–1740 (n = 3), W = 627–644 (n = 2), H = 746 (n = 1).

Description. Carapace larger for females ( Figure 15 A, C & E) than males ( Figure 15 B, D & F), with length up to 2000 µm versus 1700 µm respectively, but otherwise similar in shape. Carapace elongate ( Figure 15 A–F), with anterior and posterior ends equally pointed in dorsal and ventral view. Greatest width situated at mid-length. In lateral view ( Figure 15 A & B), dorsal margin almost straight along dorsal hinge for about 2/3 its length, sloping at approximately equal angles in postero-ventral and antero-ventral directions. Ventral margin sinuated. Anterior margin with apex well below mid-height, posterior margin at mid-height. LV with strong overlap of RV ventrally, but negligible overlap around the remaining margins. Greatest height situated along middle third. All external valve surfaces with deep, broad striations, with finer striations nested within them ( Figure 15 L).

In inner view ( Figure 15 G–J), RV and LV with very similar shape and size. Both valves with calcified inner lamella broad, and extending around entire valve, aside from dorsal hinge area; calcified inner lamella extending inwardly, by approximately 1/5 of valve length anteriorly and 1/6 of valve length posteriorly. LV calcified inner lamella without antero-ventral peg visible (remnants of reduced inner list) ( Figure 15 K).

A1 ( Figure 16 A) 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 longer ventral setae. Length of second segment approximately 1.5 times the width, this segment medio-dorsally with a seta, medio-ventrally with a RO with length approximately 0.2 times the length of the segment; this RO ( Figure 16 B, Figure 17 A) two segmented, with short distal flagella. Third A1 segment elongate, with length approximately 2.5 times the width, with one ventral seta of similar length to that of the segment, and a dorsal seta of twice the length of the ventral seta. Fourth segment with length approximately 1.5 times the width, carrying two ventral setae, the ventral-most shorter, and two dorsal setae with length far greater than the longer ventral seta. Fifth segment with length approximately twice its width, with two dorsal natatory setae, and two shorter ventral setae, the latter with length under half that of the dorsal natatory setae, the ventral-most being the shortest and broadest. Sixth segment with length approximately twice its width, apically with four long natatory setae. Seventh segment with length approximately twice its width, distally carrying two long natatory setae, a broader ventral claw-like seta and a dorsal aesthetasc Ya with length approximately half the other short setae.

A2 ( Figure 16 C) 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 almost the length of the segment, the middle seta approximately 3/4 this length, and the posterior-most very short. First endopodal segment with aesthetasc Y elongate and two segmented; distally this segment with one natatory seta flanked by four reduced natatory setae on one side, and a longer seta on the other ( Figure 16 D, Figure 17 B), the four reduced setae being less than 0.3 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 0.6 times that of the segment. Second endopodal segment with four medioventral t-setae, an additional short seta proximal to the t-setae, two medio-dorsal setae on the opposite segment margin, three subapical z-setae, three distal claws (G1–G3) and a short aesthetasc (y2); G1 and G2 of similar length, both subequal to that of the second endopodal segment with G3 shorter. Terminal segment approximately twice as long as its basal width, distally with Gm a seta, and GM a claw, reaching as far as G3; one solitary g seta and one seta fused at the base with aesthetasc y3, y3 and g seta being almost equal to the length of GM.

Md coxa ( Figure 16 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 16 F) with length ratios of four palp segments 6:1:4:1. First palp segment the largest, with length approximately twice 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 and bent 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 0.25 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 to that of the S setae; last seta in this group originating more distally, about 2/3 the length of the S setae, and covered with rows of setules; this segment also with a group of three antero-dorsal setae, the most distal one shortest. Third palp segment elongate with length approximately twice its width, antero-distally with four smooth setae, three with similar length to the first palp segment, the more proximal one 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, but the two flanking the γ seta broader; ventral margin of third segment with the same length as its width; distally with a group of three claws, one longer than the other two, one setae of similar length to the longest claw, and two setae approximately half the length of the longest claw.

Mx ( Figure 18 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 longer and hirsute, the most proximal one slightly longest; 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 ventral 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 (not shown in illustrated specimen). Chaetotaxy of endites I and II not elaborated. Respiratory plate with approximately 24 rays, six of these reflexed.

L5 ( Figure 18 B) protopod with two a-setae of similar length based proximally on the anterior margin, one short 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 18 C) a walking limb, with first two segments bearing d1 and d2 setae antero-distally, with d2 being approximately 1.5 times the length of d1. Endopodite four-segmented. First endopodal segment with e seta; posterior margin hirsute and arranged into five groupings. Second endopodal segment with f seta antero-distally of approximately 0.7 times the length of the third endopodal segment. Third endopodal segment with g seta anterodistally, plus another shorter seta alongside. Terminal segment with h2 developed into a long serrated claw, about 2.5 times the length of the third endopodal segment, seta h3 approximately the same length as h1, h3 approximately 1.5 times the length of the terminal segment. Length ratios of first to fourth endopodal segments approximately 8:6:5:2.

L7 ( Figure 18 D) a cleaning limb, basal segment with setae d1 and d2 on anterior margin, and seta dp posterodistally, all with similar length. First endopodal segment the longest, and bearing a bristled antero-distal e seta with length slightly shorter than 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, and a reflexed seta h3 shorter than the f seta.

CR ( Figure 18 F) symmetrical and elongated, each with two serrated claws, an apical Sa seta, and with Sp clawlike but with serration on all sides, and bent at 2/3 from the base ( Figure 17 C). Claw Ga approximately 0.5 times the length of the ramus. Length ratios of claws Sa, Ga, Gp and Sp to ramus 0.15, 0.5, 0.4 and 0.35, respectively. Attachment of CR ( Figure 18 E) with slightly bent main stem, bearing an obvious branch at proximal end, and a distal bifurcation.

Male Cp smaller than female ( Figure 15 D–F) and valves with inner margin of calcified inner lamella pointed proximally, and calcified inner lamella extending inward less than in females ( Figure 15 I & J). Aside from these features, male valves of similar appearance to females.

Male L5 prehensile palps near symmetrical ( Figures 19 B & C, Figure 17 D) with first segment length up to twice the central width; subapically with two large sensory organs; second segment narrow and glove-shaped with distal sensory organ. Rpp second segment with larger distal sensory organ than Lpp, and curved distally in ventral direction, whereas Lpp more two dimensional.

Male Hp ( Figures 19 A, Figure 17 E) Hp ls protruding further than ms, narrow proximally, widening to rounded latero-distal margin with apex pointing postero-laterally, proximally with lateral rounded protrusion; ms elongated with broad base narrowing to rounded distal margin with apex pointing postero-medially.

Distribution and habitat. This species has only been recorded at the type locality: Granite rock pool on top of Wannara Rock, north of Wubin, Western Australia, Australia, and two other nearby pools on the same granite outcrop. All pools were slightly acidic and without any macrophytes at the time of sampling (Supplementary file, Table S3). During sampling this species was easily observed crawling at the sediment surface, occasionally darting briefly above the sediment surface.

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 15. Ilyodromus hiatus n. sp. (RTJ 2) from granite rock pool on top of Wannara Rock, north of Wubin, Western Australia, Australia A – L. A. CpRl ♀ (WAM 57875). B. CpRl ♂ (WAM 57870). C. CpD ♀ (WAM 57873). D. CpD ♂ (WAM 57871). E. CpV ♀ (WAM 57874). F. CpV ♂ (WAM 57869). G. LVi ♀ (WAM 57872). H. RVi ♀ (WAM 57872). I. LVi ♂ (WAM 57866). J. RVi ♂ (WAM 57866). K. LVi ♀, detail of anterior peg (WAM 57872). L. CpRl ♀ detail of valve striation pattern (WAM 57873). Scales: A – J = 1000 µm, K = 400 µm, L = 10 µm

FIGURE 16. Ilyodromus hiatus n. sp. ♀ (WAM 57872, RTJ 2) from granite rock pool on top of Wannara Rock, north of Wubin, Western Australia, Australia A – G. A. A 1. B. A 1, detail of RO. C. A 2. D. A 2, detail of natatory setae. E. Md coxa. F. Md palp. Scales: A, C = 200 µm, B, D = 40 µm, E – F = 150 µm

FIGURE 17. Ilyodromus hiatus. n. sp. from granite rock pool on top of Wannara Rock, north of Wubin, Western Australia, Australia A – E. A. ♀ (WAM 57877), A 1, detail of RO (red). B. ♀ (WAM 57877), A 2, detail of natatory setae (red). C. ♀ (WAM 57877), CR, detail of Sp (red). D. ♂ (WAM 57878), Left and right L 5, detail of prehensile palp first (blue) and second (yellow) segments. E. ♂ (WAM 57878), Hp, detail of ls (blue) and ms (yellow). Colour only visible in electronic version. Scales: A = 20 µm, B = 50 µm, C – E = 200 µm

FIGURE 18. Ilyodromus hiatus n. sp. ♀ (WAM 57872, RTJ 2) from granite rock pool on top of Wannara Rock, north of Wubin, Western Australia, Australia A – F. A. Mx palp and endites, without detail on second and third endites. B. L 5. C. L 6. D. L 7. E. CR attachment. F. CR. Scales: A = 102 µm, B – F = 150 µm

FIGURE 19. Ilyodromus hiatus n. sp. ♂ (WAM 57868) from granite rock pool on top of Wannara Rock, north of Wubin, Western Australia, Australia A – C. A. Prehensile palp of left L 5. B. Hp. C. Prehensile palp of right L 5. Scales: A – C = 77 µm.