Ryocalanus squamatus, Renz & Markhaseva & Laakmann, 2018

RYOCALANUS SQUAMATUS View in CoL SP. NOV.

( FIGS 1–5)

Type material

Holotype: Adult female, dissected, body length 2.17 mm, collection number SMF 37149 About SMF /1–6 (one vial, five slides); Kurile-Kamchatka trench, 40.5808° N, 150.9833° E, station 9–9, project KuramBio, 23 August 2012, above the sea bed at a depth of 5400 m. GoogleMaps

Paratypes: One adult female. Body length 2.05 mm, collection number SMF 37150 About SMF /1–5 (one vial, four slides); Kurile-Kamchatka trench, 43.0303° N, 152.9758° E, station 7–10, project KuramBio, 17 August 2012, above the sea bed at a depth of 5304 m GoogleMaps . One adult male, body damaged, length 1.83 mm, collection number SMF 37151 About SMF /1–6 (one vial, five slides); Kurile-Kamchatka trench, 41.2000° N, 150.0833° E, station 10–12, project KuramBio, 28 August 2012, above the sea bed at a depth of 5251 m GoogleMaps .

Etymology: The specific name refers to the scale-like structures that cover the antennule.

Description: Based on female holotype unless stated otherwise.

Adult female: Total length 2.17 mm; prosome 4.8 times as long as urosome ( Fig. 1A, B). Rostrum ( Fig. 1C) stout and strong, one-pointed. Cephalosome and pediger 1 partly fused ( Fig. 1A), pedigers 4–5 separate; in lateral view, posterolateral corners of prosome extended posteriorly into points, reaching to middle of genital double-somite ( Fig. 1A, B). Pedigers 2–5 covered with fine spinules.

Urosome composed of genital double-somite and three articulated somites ( Fig. 1A, B, D–H). Genital double-somite slightly asymmetrical, with lateral swelling on left side and ventromedial genital opening; in lateral view genital double-somite swollen ventroanteriorly, seminal receptacles of oval shape. Genital double-somite and urosomites covered with rows of fine spinules ( Fig. 1A, B, D–H). Caudal rami symmetrical with two lateral setae (II and III), three terminal setae (IV–VI) and one dorsal seta (VII).

Antennule ( Fig. 1I, K) of 24 free segments and extending to pediger 2, covered with scale-like structures. In holotype armature as follows:

I – 2s + 1ae?, II–IV – 5s + 2ae?, V – 2s + 1ae, VI – 2s + 1ae, VII – 2s + 1ae, VIII – 2s + 1ae, IX – 2s + 1ae; X–XI – 4s + 1ae, XII – 1s + 1ae, XIII – 2s, XIV – 2s + 1ae, XV – 1s, XVI – 2s + 1ae, XVII – 2s + 1ae, XVIII – 2s + 1ae, XIX – 2s + 1ae, XX – 2s, XXI – 2s + 1ae, XXII – 1s, XXIII – 1s, XXIV– 2s, XXV – 2s, XXVI – 2s, XXVII–XXVIII – 5s + 1ae.

Antenna ( Fig. 2A), coxa with 1, basis with 2 setae; endopod segment 1 with 2 setae, segment 2 with 16 setae; exopod incompletely 8-segmented, with 1, 3, 1, 1, 1, 1, 1, 3 setae.

Mandible ( Fig. 2B–D), gnathobase cutting edge with 7 unequal teeth plus ventral seta; exopodal segments with 6 setae; first endopod segment with 2 setae, second with 11 setae; basis with 2 setae. Exopod to endopod ratio 0.82.

Maxillule ( Fig. 2E, F), praecoxal arthrite with 9 terminal spines ( Fig. 2F), 4 posterior and 2 anterior setae; coxal endite with 6 setae, coxal epipodite with 9 setae; proximal basal endite with 4 setae, distal basal endite with 5 setae; endopod with 15 setae; exopod with 11 setae.

Maxilla ( Fig.2G, H), proximal praecoxal endite bearing 4 setae plus attenuation in holotype, 5 setae plus attenuation in paratype, distal praecoxal endite with 3 setae; coxa without outer seta; coxal endites with 3 setae each; proximal basal endite with 4 setae; endites 2–5 with surface spinules, remaining endopod with 8 setae.

Maxilliped ( Fig. 2I), syncoxa with 1 seta on praecoxal endite, 2 setae on proximal coxal endite, 3 setae on middle coxal endite, and 3 setae on distal coxal endite; syncoxa with fine rows of spinules. Basis with 3 distal setae; endopod 6-segmented with 2, 4, 4, 3, 3 and 4 setae.

Legs 1–4 biramous ( Fig. 3A–E). Exopods and endopods 3-segmented, except leg 1 endopod 1-segmented and leg 2 endopod 2-segmented. Anterior and posterior surface of legs covered with spinules, these spinules much smaller on anterior surface. Coxa of all legs with surface spinules on inner margin; terminal spines on exopod segment 3 finely serrated. Seta and spine formula as in Table 3. Leg 1 ( Fig. 3A), endopod lateral lobe with spinules.

Leg 4 ( Fig. 3D), coxa with medial seta (broken in holotype); basis with row of spinules on distal margin; left leg abnormal in holotype ( Fig. 3E), lacking lateral spines on exopod segment 1 and 3 (no scars), and terminal spine on exopod 3, smaller as right leg.

Adult male: Total length 1.83 mm, prosome 4.6 times as long as urosome ( Fig. 4A, B). Rostrum ( Fig. 4A, C) stout, strong and one-pointed. Cephalosome and pediger 1 separate ( Fig. 4A, B), pedigers 4 and 5 separate. In lateral view, right posterolateral corner of prosome extended posteriorly into points slightly exceeding urosomite 1 ( Fig. 4D). Pedigers 2–5 covered with fine spinules. Caudal rami ( Fig. 4B, E) slightly asymmetrical, with one lateral seta (III?), three terminal setae (IV–VI) plus one dorsal seta (VII).

Left antennule ( Fig. 4F) unmodified, of 24 free segments, extending to pediger 2, covered with scale-like structures; armature as follows: I – 1s + 1ae, II–IV – 5s + 4ae?, V – 2s + 1ae, VI – 2s + 1ae, VII – 2s + 2ae, VIII – 1s? + 2ae, IX – 2s + 2ae, X–XI – 4s + 4ae, XII – 1s + 1ae; XIII – 2s + 1ae; XIV – 2s + 1ae, XV – 2s + 1ae, XVI – 2s + 1ae, XVII – 2s + 1ae, XVIII – 2s + 1ae, XIX – 2s + 1ae, XX– 2s + 1ae, XXI – 2s + 1ae, XXII – 1s + 1ae, XXIII – 1s + 1ae, XXIV – 2s + 1ae, XXV – 2s + 1ae, XXVI – 2s, XXVII–XXVIII – 4s + 1ae.

Right antennule ( Fig. 5A–D) strongly modified for grasping, of 23 free segments; segments XX to XXVI wider than on the left; segments XX to XXII–XXIII with surface spinules; segments XX and XXI with 1 proximal spine each, segment XXV and XXVI with strong lateral attenuations proximally, segment XXII– XXIII fused; hinges occurring between segments XIX and XX, XX and XXI, and XXII–XXIII and XXIV. Armature as follows: segment I – 2s + 1ae, II–IV – 6s + 2ae?, V – 2s + 2ae, VI – 1s + 2ae, VII – 2s + 2ae, VIII – 2s + 2ae, IX – 2s + 2ae; X–XI – 4s + 3ae, XII – 1s + 1ae, XIII – 2s + 1ae, XIV – 2s + 1ae, XV – 1s + 1ae, XVI – 2s + 1ae, XVII – 2s + 1ae, XVIII – 2s + 1ae, XIX – 2s + 1ae, XX – 1s+ 1ae + spine, XXI – 1s + 1ae + spine,

XXII–XXIII – 2s + 1ae, XXIV – 2s, XXV – 2s + 1ae + strong, attenuation, XXVI – 2s + strong, spine like attenuation, XXVII–XXVIII – 5s + 1ae.

Antenna, mandible and maxillule similar to those of female. Maxilla as in female, with proximal praecoxal endite bearing 4 or 5 setae, but without attenuation.

Maxilliped similar to that of female.

Legs 1–4 similar to those of female, but less spinulose. Leg 2 one leg abnormal, lacking lateral spines on exopod segment 1 and 2 (no scars), and terminal spine on exopod 3, exopod 3 with 7 setae, leg small. Leg 4 coxa without strong spinules ( Fig. 5E). Leg 5 ( Fig. 5F) uniramous on both sides, covered with rows of spinules on posterior surface. Right leg with 1-segmented exopod, shorter than left leg, with terminal spine. Left leg with 3-segmented exopod, terminal segment with two spines.

R e m a rk s: T h e n e w s p e c i e s s h a r e s t h e m a i n morphological characters with species of the genus Ryocalanus ( Renz et al., 2013) , which is a 1-pointed rostrum, the armament of the female antennule ancestral segment XXII with 1 seta, a leg 1 endopod with a proximal inner wedge-shaped projection and a male antennule with ancestral segments XXI/XXII–XXIII fused. Both sexes are known only for R. brasilianus Renz, Markhaseva & Schulz, 2013 , while R. spinifrons Shimode, Toda & Kikuchi, 2000 is only known from females and R. bowmani Markhaseva & Ferrari, 1996 and R. infelix Tanaka, 1956 are represented only by males (female of R. infelix is described herein).

Ryocalanus squamatus sp. nov. differs from all other Ryocalanus species by the shape of the rostrum, which is short and stout, compared to the long and slender rostrum possessed by all other Ryocalanus species and an only slightly asymmetrical genital segment, which is strongly asymmetrical in R. brasilianus , R. spinifrons and R. infelix . Pedigers 2–5, as well as the anterior part of the legs, are covered in fine spinules, which are absent in other Ryocalanus females, and the coxa of leg 4 lacks the robust spines that are usually found in all other ryocalanid females of the genera Ryocalanus and Yrocalanus . The antennule is covered with scale-like structures, a character not detected so far in any other species of Ryocalanus .

There are significant morphological transformations in the Ryocalanus male right ancestral antennule segments distal to segment XIX, and evidence that ancestral segments XXI/XXII–XXIII are fused, with the main hinge located between fused segments XXII–XXIII and segment XXIV. This is unlike the earlier interpretation of the R. infelix male right antennule ( Ohtsuka & Huys, 2001). Consequently, the male right antennules of Ryocalanus species are distinct to the male right antennules in the genus Yrocalanus , where the main hinge can be found between segment XXII and fused segments XXIII–XXIV. For further comments, see also remarks for the description of a R. infelix female with additional comments for the male. The right male antennule of Ryocalanus squamatus sp. nov. differs from that of other Ryocalanus males in the shape of segment XXV, which has a strong lateral attenuation, being longer than the segment itself. This attenuation is absent in other Ryocalanus species. Furthermore, segments XX–XXIII are equipped with surface spinules in R. squamatus sp. nov., while present on segments XIX–XXIII in R. infelix (Tanaka, 1954) and segments XIV–XXI in R. brasilianus ( Renz et al., 2013) . These spinules are lacking in the R. bowmani male right antennule. The lateral teeth observed on the male antennule segments XXVI and XXIV in Ryocalanus infelix and R. brasilianus and the combed spines on segments XXIV and XXV in R. bowmani are absent in R. squamatus sp. nov..

The antenna exopod to endopod ratio in the new species is 0.82, while in all other Ryocalanus species it is close to 1, a character hypothesized to be diagnostic for the genus Ryocalanus ( Renz et al., 2013) . The mandible basis carries 2 setae (vs. 3 setae in all other Ryocalanus species), the mandible first endopod segment carries 2 setae (vs. 4 setae in all other Ryocalanus species) and the maxilliped endopod segment 5 outer seta is missing (this seta is present in all other species).

Males of Ryocalanus squamatus sp. nov. differ from their congeers in the shape of the rostrum, the spinules covering pedigers 2–5 and the scale-like structures covering the antennule (the latter two not present in other Ryocalanus species), the shape of ancestral segments on the right antennule with a large, hook-like extension on the posterior border of ancestral segment XXV, and the number and morphology of spines and segments of leg 5. From the armature of the 2-segmented right leg 5 it cannot be distinguished if the segments comprise a fused coxo- and basipodite with a one-segmented exopod or if the exopod is reduced.

Some variability was observed in the armature of the proximal praecoxal endite of the maxilla between right and left limb of individuals in both female and male specimens, i.e. the presence of 4 or 5 setae. Furthermore, this endite was always equipped with a short attenuation in females, a feature not reported before in other ryocalanid species. However, a re-examination of the holotype of Yrocalanus bicornis ( Markhaseva & Ferrari, 1996) (Smithsonian Institution, Catalogue No. USNM 264034, as Ryocalanus bicornis Markhaseva & Ferrari, 1996 ) showed that this short attenuation was present in the type material from Volcano 7. In both sexes of R. squamatus sp. nov., the distal spine on the maxillule praecoxal arthrite had a cavity at its tip, presumably a specific adaptation connected with feeding. This character was also found during the re-examination of the holotype of Ryocalanus bowmani from Volcano 7 (Smithsonian Institution, Catalogue No. USNM 268291; Fig. 6). A morphologically similar structure can be found in the mandible of Heterorhabdidae , although it is not clear from the analysis by light microscopy if this structure serves the same function during predatory feeding as suggested for this family ( Ohtsuka et al., 1997).

Abnormalities were observed in the formation of a swimming leg of Ryocalanus squamatus sp. nov., as in one female, as well as the male specimen, exopods of leg 4 and 2, respectively, lacked lateral spines, and exopod segment 3 lacked the terminal spine and deviated in the setation typical for this taxon.