Rhinoecetes spinicaudus, Kodama, Masafumi, Ohtsuchi, Naoya & Kon, Koetsu, 2016

Kodama, Masafumi, Ohtsuchi, Naoya & Kon, Koetsu, 2016, A new species of the genus Rhinoecetes Just, 1983 (Crustacea: Amphipoda: Ischyroceridae) from Japan, Zootaxa 4169 (1), pp. 133-144 : 135-143

Rhinoecetes spinicaudus sp. nov.

[New Japanese name: Toge-o-tsunoaruki]

Material examined. All the specimens were collected at 34°39’57”N, 138°56’05”E, Nabeta Bay , Shimoda-shi , Shizuoka, on 19 February 2015, by M. Kodama using Ekman-Birge type bottom sampler.

Holotype: male, 6.9 mm (NSMT-Cr 24583), 2 m, sandy bottom.

Paratypes: 1 male, 5.5 mm (NSMT-Cr 24584), 1 female, 4.3 mm (NSMT-Cr 24585), 1 male, 4.5 mm (KMNH IvR 500883), 1 ovigerous female, 4.7 mm (KMNH IvR 500,884), 1 m, sandy bottom.—1 male, 5.0 mm (NSMT-Cr 24586), 1 male, 5.6 mm (NSMT-Cr 24587), 1 male, 5.3 mm (NSMT-Cr 24588), 1 male, 4.8 mm (NSMT-Cr 24589) (examined by SEM), 1 male, 4.9 mm (NSMT-Cr 24590) (examined by SEM), 1 ovigerous female, 4.0 mm (NSMT- Cr 24591) (illustrated), 1 ovigerous female, 5.1 mm (NSMT-Cr 24592), 1 ovigerous female, 4.4 mm (NSMT-Cr 24593), 1 ovigerous female, 4.8 mm (NSMT-Cr 24594), 1 female, 4.6 mm (NSMT-Cr 24595), 1 male, 6.5 mm (KMNH IvR 500885), 1 male, 6.0 mm (KMNH IvR 500886), 1 male, 7.7 mm (KMNH IvR 500887), 1 ovigerous female, 4.4 mm (KMNH IvR 500888), 1 ovigerous female, 5.5 mm (KMNH IvR 500889), 1 ovigerous female, 5.3 mm (KMNH IvR 500890), 2 m sandy bottom.—1 male, 5.0 mm (NSMT-Cr 24596), 1 ovigerous female, 4.8 mm (NSMT-Cr 24597), 1 male, 4.4 mm (KMNH IvR 500891), 1 ovigerous female, 4.9 mm (KMNH IvR 500892), 8 m sandy bottom.

Diagnosis. Head with shallow midanterior depression; rostrum narrow, projecting downwards, pointed apically. Antenna II, flagellar article I with 5–8 robust setae; article II with 4–5 robust setae. Gnathopod I propodus with 1–3 midposterior robust setae. Uropod I peduncle with distomesial robust seta and dorsolateral row of 4–7 robust setae; outer ramus broad, lateral margin finely spinulate; inner ramus broad, lateral margin with spinulation. Uropod II ramus, with dorsolateral row of short robust setae. Uropod III ramus with distomesial short robust seta and 3–5 distolateral serrulate setae.

Description. Based on holotype male, 6.9 mm.

Body ( Fig. 2 A) subcylindrical, smooth on surface.

HEAD ( Fig. 3 B, C) as long as pereonites I and II combined along dorsal midline; anterolateral frontal eaves slightly extended, weakly concealing antenna I; midanterior depression shallow, encompassing approximately 0.3 of head length; rostrum straight, narrowly triangular, pointed apically, reaching beyond apexes of eye lobes, producing downwards, upper margin basally concave in lateral view; eye lobes truncate, anteroventral angle with several short simple setae.

Antenna I ( Fig. 2 A) approximately as long as head and pereonites I–V combined, bearing dense simple, pappose or bifid setae; peduncle 3-segmented, with length ratio approximately 9: 10: 8; flagellum 8-segmented, approximately 0.6 length of peduncle, articles I–VI similar, article VII slightly shorter than articles I-VI, article VIII vestigial; accessory flagellum absent; calceoli absent.

Antenna II ( Figs. 2 A, 3A) approximately as long as head and pereonites I–VII combined; peduncle 5- segmented as long as head and pereonites I–VI combined, ventral projection on peduncular article II triangular with long simple and pappose setae apically, peduncular article III with 1 short robust seta on dorsally distomesial angle, articles III–V with length ratio 4: 7: 5, having dense rows of long and short simple setae; flagellum 4- segmented with length ratio 4.5: 2: 0.6: 0.2, armed with dense long and short simple setae, article I with 2 rows of robust setae, outer and inner rows including 3 and 5 robust setae respectively, article II with 2 rows of curved robust setae, each rows including 2 robust setae, article III with 2 unguiform robust setae, article IV cylindrical and vestigial, every robust setae on flagellum bearing single short plumose seta.

Mouthparts. Upper lip ( Fig. 4 A) rounded, distal part finely setulose; epistome unfused. Mandible ( Fig. 4 B, C); palp 2-segmented, article I elongate with long simple and serrulate setae, article II vestigial, tipped with long serrulate seta; left and right incisors 6 and 5-dentate respectively; medial projection present on proximal region of palp; left and right laciniae mobiles with 4 and 2 teeth respectively; 4 left and 3 right accessory blades present; molar with molar blade. Lower lip ( Fig. 4 D) normal, inner plate and mandibular process present, outer and inner plate finely setulose in distal and mesial parts. Maxilla I ( Fig. 4 E); palp, article I wider than long without setae, article II gently inward curved and elongate beyond outer plate bearing 7 distal robust setae and 5 ventrofacial pappose setae; outer plate truncate and inward curved, apical margin with 2 simple, 4 bifid and 1 trifid robust setae; inner plate absent. Maxilla II ( Fig. 4 F); outer plate extending beyond inner plate with dense marginal simple setae; inner plate with dense marginal simple and plumose setae. Maxilliped ( Fig. 4 G); palp, article I triangular without setae, article II elongate beyond outer plate with dense pappose setae mesially and distolaterally, article III half length of article II with dense mesial setae and scattered laterofacial setae, terminal article stubby with long spine; outer plate with mesiofacial row of pappose setae and distal row of simple setae, bearing 10 mesial robust setae; inner plate with facial and distal rows of pappose setae, bearing 4 apical stubby setae.

PEREON. Gnathopod I ( Fig. 5 A), slender than Gnathopod II; basis, anterior and posterior margin with sparse setae; ischium subequal length to merus with anterior and posterior groups of setae; merus, posterior margin with dense rows of setae; carpus as long as propodus, posterodistal margin with long robust seta, anterior margin with sparse rows of setae, posterior margin with dense rows of setae; propodus slender with 3 midposterior robust setae and posterodistal short robust seta, posterior and mesial surface with rows of setae; dactylus, posterior margin having 7 teeth with several short simple setae.

Gnathopod II ( Fig. 5 B); basis, anterior margin with sparse short setae and fine spinules, posterior margin with long simple setae; merus with 4 posterior and 1 mesial groups of setae; carpus triangular, anterodistal margin with group of long setae, posterior area quadrately lobate and spinulate bearing short robust seta and some long setae; propodus, anterior margin bearing 4 groups of long simple or pappose setae, palm oblique and concave armed with strong tufts of simple or pappose setae and 2 robust setae, distoposterior margin with short robust seta; dactylus, posterior margin 8-teethed with several short simple setae.

Pereopods III and IV ( Fig. 5 C, D) similar; basis expanded, anterior margin with sparse setae, posterior margin with scattered setae; ischium longer than wide with several setae anteriorly and posteriorly; merus, anterior and posterior margins with several setae, anterodistal and posterodistal margins strongly convex bearing groups of long setae, anterior lobe reaching propodus beyond carpus, posterior lobe reaching middle of carpus; carpus degraded, posterior margin with long setae, anterior margin without setae; propodus slightly expanded bearing anterodistal and posterior setae; dactylus acutely conical, subequal length of carpus and propodus combined, anterior margin with single short simple seta.

Pereopods V and VI ( Fig. 6 A, B) similar; basis approximately as long as ischium and merus combined, weakly expanded, both anterior and posterior margins sparsely setose, with anterodistal group of long setae; ischium with several anterodistal short setae; merus sparsely setose, with anterodistal and posterodistal groups of setae; carpus facially spinulate, posterodistal margin with 1 long and 1 short robust setae, each robust setae bearing short plumose seta; propodus weakly curved, subequal length to carpus, bearing simple setae posteriorly and anterodistally; dactylus with several short simple setae.

Pereopod VII ( Fig. 6 C) longest among pereopods; basis weakly expanded, anterior and posterior margin with dense plumose setae; ischium with several anterodistal short simple setae; merus twice length of ischium anterior and posterior margin with several groups of bifid or serrulate setae on distal half; carpus subequal length to merus, anterior and posterior margin with some groups of bifid or serrulate setae, distomesial margin finely setulose; propodus weakly curved, longer and slender than carpus, anterior margin with sparse simple setae, posterior margin with several groups of long simple and plumose setae; dactylus with several simple setae.

Coxal plates I–VII separated; plates I–IV produced anteroventrally, bearing long and short sparse setae on anterior and posterior margin; plates V and VI bilobate with marginal simple setae, anterior lobe rounded, posterior lobe slender; plate VII semicircular with marginal simple setae. Coxal gills ( Fig. 6 D); gill on pereopod III slender; gills on pereopods IV and V longer than wide, broader than gill on pereopod III; gill on pereopod VI slender, produced anteriorly.

PLEON. Pleopods I–III ( Fig. 7 B) almost same; peduncle expanded mesially, proxolateral margin with several plumose setae, distolateral margin with sparse simple setae, distomesial margin with coupling spines; Outer ramus 9-segmented with 11 lateral and 9 mesial plumose setae; Inner ramus 9-segmented, slightly longer than outer ramus with 10 lateral and 9 mesial plumose setae.

UROSOME ( Fig. 7 A). Uropod I ( Figs. 7 C); peduncle with distomesial robust seta and 5 dorsolateral branched robust setae and distolateral group of serrulate setae, lateral margin finely spinulate, distoventral margin finely setose; outer ramus broad, approximately 0.5 length of peduncle, with dorsolateral row of branched robust setae, several robust setae present on apex, fringed with dense spinules laterally; inner ramus broad, approximately 0.8 length of outer ramus, fringed with long spines in distal 0.8 laterally, spines weakly curved toward flexor surface, mesial margin finely spinulate, long and short robust setae present on apex. Uropod II ( Figs. 7 D); peduncle with several mesial simple and distolateral serrulate setae, distoventral margin dentate; ramus approximately 0.5 length of peduncle, with dorsolateral row of short robust setae, apex with long and short robust setae. Uropod III ( Fig. 7 E); peduncle distomesially expanded, with several simple setae on distomesial margin; ramus rounded and reduced, posterior margin with single short robust seta and 4 serrulate setae. Telson ( Fig. 7 A) entire, 40 percent wider than long, distolateral corner rounded, dorsal surface with 2 pairs of pappose setae on both left and right side.

Female characters. Generally similar to males ( Fig. 1 B), but antenna I flagellum with smaller number of articles, antennae I and II shorter and less setose, gnathopod sI and II dactyli with smaller number of teeth, compared with males. Pereopods II–V coxal plates bear slender oostegites.

Variations. The midanterior depressions on the head are quite shallow and barely recognizable in some specimens. Eye lobes sometimes miss setation. Robust setae on antenna II flagellums and uropods, as well as plumose setae on pleopods, vary in numbers individually. Gnathopod I propodi bear usually 2 and sometimes 1 or 3 robust setae of which numbers are occasionally different between left and right.

Coloration in life. Body generally whitish yellow; head, pereonites I and II mottled dark brown. Antennae I and II generally colorless, peduncular articles with terminal whitish bands.

Distribution. Known only from type locality, Nabeta Bay, Izu Peninsula, Japan.

Habitat. Found among sandy bottoms mixed with gravels and shell debris, at 1–8 m deep.

Ecological notes. This species was always found carrying a nest made of vacant gastropod shells, polychaete tubes, or sand grains. They jump backward by using antenna II. Some males were found within the nest compounded with 1 female’s nest (abode complex). A male in the abode complex shows the agonistic behavior against neighboring males.

Etymology. Species name spinicaudus means “thorny tail” in Latin, derived from the uropod I rami with spinulate lateral margins.

Remarks. This species can be distinguished from all the other congeners by the spinulate lateral margins on both inner and outer rami of uropod I ( Figs. 7 C, 8B), and the presence of dorsolateral row of small robust setae on uropod II ramus ( Figs. 7 D, 8C) and single small robust seta on uropod III ramus ( Fig. 7 E).

Among congeners, Rhinoecetes spinicaudus sp. nov. is closely similar to R. albomaculosus in having the narrow rostrum, the shallow midanterior depression on the head, and the single distomesial robust seta on uropod I peduncle. However, in addition to the three characters mentioned in the preceding paragraph, the following characters can distinguish the present new species from R. albomaculosus : (1) upper margins of rostrums are basally concave in lateral view, while those of R. albomaculosus are straight ( Figs. 3 B, 8A versus Just 2012a: fig. 18); (2) antenna II peduncular articles IV do not have any white spots, while those of R. albomaculosus have 1–3 large medial white spots; (3) robust setae on antenna II flagellums are thicker and shorter rather than those of R. albomaculosus ( Fig. 3 A versus Just 2012a: fig. 18); (4) uropod I outer and inner rami are broader (proportional length against basal width: less than 3.5 and less than 3.0, respectively) than those of R. albomaculosus (more than 3.5 and more than 3.0, respectively) ( Figs. 7 C, 8B versus Just 2012a: fig. 19); (5) uropod II peduncles bear one or a few serrulate setae distolaterally and several simple setae mesially, while those of R. albomaculosus lacks setae of any types ( Fig. 7 D versus Just 2012a: fig. 19); (6) uropod III rami bear 3–5 serrulate setae in the present new species, whereas they bear 6–8 in R. albomaculosus ( Fig. 7 E versus Just 2012a: fig. 19).

Figures

FIGURE 1. Geographical distribution of the genus Rhinoecetes Just, 1983. Circles indicate the localities of eight known species.

FIGURE 2. Rhinoecetes spinicaudus sp. nov. A, Holotype (NSMT-Cr 24583, male, 6.9 mm), whole body (pleopods and coxal gills omitted), lateral view; B, Paratype (NSMT-Cr 24591, ovigerous female, 4.0 mm), whole body (pleopods, coxal gills and oostegites omitted), lateral view. Scale bars: 1 mm.

FIGURE 3. Rhinoecetes spinicaudus sp. nov. Holotype (NSMT-Cr 24583, male, 6.9 mm). A, right antenna II flagellum (setae omitted), flexor view; B, C, head, lateral (B) and dorsal view (C). Scale bars: 0.5 mm.

FIGURE 4. Rhinoecetes spinicaudus sp. nov. Holotype (NSMT-Cr 24583, male, 6.9 mm). A, upper lip, anterior view; B, C 1, left and right mandible, medial view; C 2, right mandibular palp article II, medial view; D, lower lip, ventral view; E 1, right maxilla I, dorsal view; E 2, right maxilla I palp (apical robust setae omitted), ventral view; F, right maxilla II, dorsal view; G, maxilliped (setae omitted, with some exceptions), dorsal view. Scale bars: A – C 1, D – G, 0.1 mm; C 2, 0.05 mm.

FIGURE 5. Rhinoecetes spinicaudus sp. nov. Holotype (NSMT-Cr 24583, male, 6.9 mm). A, B 1, right gnathopods I and II (setae omitted), lateral view; B 2, posterior part of gnathopod II carpus, lateral view; C, D, right pereopods III and IV, lateral view. Scale bars: A, B 1, C, D, 0.5 mm; B 2, 0.05 mm.

FIGURE 6. Rhinoecetes spinicaudus sp. nov. Holotype (NSMT-Cr 24583, male, 6.9 mm). A 1, B, C, right pereopods V – VII, lateral view; A 2, carpus of right pereopod V, lateral view; D 1 – 4, left coxal gills on pereonites III – VI, lateral view. Scale bars: A 1, B – D 0.25 mm; A 2, 0.1 mm.

FIGURE 7. Rhinoecetes spinicaudus sp. nov. Holotype (NSMT-Cr 24583, male, 6.9 mm). A, urosome, dorsal view; B, right pleopod II, posterior view; C 1, D 1 and E, right uropods I – III, dorsal view; C 2, distal part of peduncle and rami of uropod I (setae omitted), ventral view; D 2, distal margin of Uropod II peduncle (setae omitted), ventral view. Scale bars: 0.1 mm.