Cyaegharctus kitamurai gen. et, 2020

Fujimoto, Shinta & Jimi, Naoto, 2020, A new marine tardigrade genus and species (Arthrotardigrada, Styraconyxidae) with unique pockets on the legs, Zoosystematics and Evolution 96 (1), pp. 115-122 : 115

Cyaegharctus kitamurai gen. et sp. nov. Figs 1 , 2 , 3

Styraconyxidae gen. sp. ( Fujimoto et al. 2017): partial sequence of the nuclear 28S rRNA available at DDBJ/GenBank under accession number LC103153 and voucher micrographs provided by MorphoBank ( O’Leary and Kaufman 2012) at http://doi.org/10.7934/P2234.

Material examined.

Holotype: KUZ Z2624 (on microscope slide in glycerol), an adult female collected from Daidokutsu (type locality) on 28 July 2019 by KY. Paratypes: KUZ Z2625 (on microscope slide in glycerol), an adult female collected on 13 May 2019 by SF; KUZ Z2626 (on microscope slide in glycerol), an adult female collected on 26 August 2013 by KY; KUZ Z2627 (on SEM stub), a four-claw juvenile collected on 13 May 2019 by SF. All paratypes were collected from the type locality.

Type locality.

Muddy bottom of Daidokutsu (28 m below sea level), off Iejima Island, Okinawa Islands, Ryukyu Archipelago, Japan (26°43'N, 127°50'E).

Diagnosis.

Cyaegharctus with median, internal and external cirri each with basal swelling (cirrophore?) and short subdivision; lateral cirri each with short subdivision; cirri E on cirrophores arising from round lateral processes between legs III and IV; seminal receptacle ducts run postero-laterally with U-shaped bend (vesicles unknown); leg I sensory organs spine-like, each with distal subdivision and terminal pore; legs II and III sensory organs spine-like without subdivisions; leg IV sensory organs papillate with short distal tubes.

Description of holotype.

Adult female (Figs 1 , 2A-J ). Dorso-ventrally flattened body 202 μm in length and 117 μm wide at level of leg III (Figs 1A , 2A ). Outer epicuticle loose from pillar layer as artefact of formaldehyde fixation (for further information see Remarks on paratypes). Fine epicuticular pillars recognised.

Cephalic region (head) (Figs 1A , 2B-D ) with unpaired median cirrus, paired internal cirri, paired external cirri, paired lateral cirri, paired primary clavae, paired secondary clavae and antero-ventral directed mouth. Median, internal and external cirri (7 μm, 10 μm and 10 μm, respectively) with proximal swelling (cirrophore?) and short, distal subdivision terminating in pore. Lateral cirri (19 μm), each with short, distal subdivision with terminal pore. Primary clavae ovoid (6 μm) with distal pore and van der Land’s body at base. Secondary clavae compact and conical (4 μm wide at base). Internal cirri at anterior margin of head and dorsal median cirrus at slightly posterior position. External cirri latero-ventral to internal cirri. Secondary clavae between internal and external cirri. Dorsal lateral cirri and ventral primary clavae share common base at antero-lateral margin of head. Buccal apparatus consists of buccal tube (38 μm), paired stylets, paired stylet supports (inserted 29 μm from mouth opening), three simple placoids (11 μm) and pharyngeal bulb. Observation of buccal apparatus incomplete due to dissolved structures after permanent preparation and difficulty in recognising pharyngeal bulb’s margin after squeezing of specimen.

Paired spine-like cirri E (38 μm) (Figs 1A , 2E ) on cirrophores arise from round lateral processes at level between legs III and IV.

Rosette-like gonopore 9 μm anterior to anus (Figs 1A , 2E ). Gonopore morphology disturbed when cleaning specimen and only contour recognisable after permanent preparation (8 μm in diameter). Anus 16 μm wide at posterior most part of body between legs IV and consists of paired longitudinally elongate lobes. Paired seminal receptacles open immediately anterior to gonopore. Seminal receptacle ducts first run laterally, bent in U-shape and run postero-laterally. Terminal vesicles of seminal receptacles not recognised.

Four pairs of legs (Figs 1 , 2F-J ) each with usual leg sensory organ on dorsal side of femur’s proximal portion, pocket organ on dorsal side of femur’s distal margin and four digits terminating in claws. Legs I each with spine-like sensory organ (10 μm) with distal subdivision. Legs II each with spine-like sensory organs which lack subdivisions (length not measured due to bad orientation). No sensory organs recognised on legs III but presence likely (recognised in paratypes; see Remarks on paratypes). Legs IV each with sensory organ (7 μm) consisting of papilla (4 μm) with distal tubular portion (2 μm). Pocket organ consists of small opening, cavity and apparent dense body. No difference recognised amongst legs I-IV pocket organs besides protruded opening of leg II pocket organ. Leg IV pocket organ in good orientation for observation of whole structure (Fig. 1B ). Internal digits each with proximal pad and thin peduncle. External digits each with broad peduncle that do not reach claws, subdivided into proximal and distal portion. Internal digits longer than external digits. Each claw of both internal and external digits with three points: primary hook and less-developed, secondary and accessory hooks. Digits and peduncles slightly longer than those of preceding leg pairs (legs I: internal digits 13 μm (peduncles 3 μm), external digits 10 μm (peduncles 5 μm); legs II: 14 μm (3 μm), 10 μm (5 μm); legs III: 15 μm (4 μm), 10 μm (5 μm); legs IV: 16 μm (4 μm), 10 μm (6 μm)).

Remarks on paratypes.

Fixation of specimens using formaldehyde seems to have introduced an artefact in the cuticle, i.e. the detached (or loose) outer epicuticle. When the specimens were sorted in distilled water before fixation, the outer epicuticle did not look loose at 63 × magnification (all four specimens) and also at 400 × magnification (only observed for KUZ Z2627) as it would appear in Tholoarctus . Although these are only brief observations and we did not conduct any experiment to test this artefact, we consider the outer epicuticle’s detached state as an artefact and excluded this character state from the diagnoses of this genus and species.

The adult female paratype KUZ Z2625 revealed the presence of a cuticular ring surrounding the rosette gonopore (Fig. 2K ) and the presence of the spine-like leg III sensory organs with no subdivisions. In the adult female paratype KUZ Z2626, the legs I-III sensory organs, claws and peduncles were orientated better than the holotype and the other paratype for observation (Fig. 2L, M ). However, the pocket organs were not recognised for this specimen, probably due to the excessively-squeezed state. Other character states, recognised in these two paratypes, confirmed the observation of the holotype. The morphometrics are as follows ( KUZ Z2625, Z2626): body length (159 μm, 210 μm), body width (83 μm, 110 μm), median cirrus (5 μm, 8 μm), internal cirrus (10 μm, 12 μm), external cirrus (9 μm, 12 μm), lateral cirrus (17 μm, 19 μm), primary clava (6 μm, 6 μm), secondary clava width (4 μm, 6 μm), cirrus E (33 μm, 33 μm), leg I sensory organ (7 μm, 12 μm), leg II sensory organ (8 μm, 7 μm), leg III sensory organ (10 μm, 11 μm), leg IV sensory organ (4 μm, 7 μm), gonopore (6 μm, -), anus length (-, μm), anus width (-, -), gonopore-anus distance (5 μm, -) (- represents 'not measured’).

The SEM of a four claw juvenile (Fig. 3 ) confirmed the results of light microscopy and also provided further detail. However, this SEM specimen also seems to have its outer epicuticle detached. If the outer epicuticle is attached to the underlying layer, a pattern is recognised on the surface of the body due to the pillar layer, but no such indentations were found, suggestive of the detached state of the outer cuticle (for ultrastructure of cuticle, see Kristensen and Neuhaus 1999). The proximal part of each leg has an inflated appearance (Fig. 3A ) not recognised in light microscopy. The view of the cephalic region (Fig. 3B, C ) revealed the three-dimensional morphology and the arrangement of the cephalic appendages and also confirmed the presence of terminal pores on the cephalic cirri and the primary clavae. The conical shape of the secondary clavae seems not as evident as in light microscopy probably due to the overlying outer epicuticle. The large anus is not on the ventral surface and rather direct posteriorly (Fig. 3D ). The leg sensory organs were recognised (Fig. 3E-H ) and those of legs I and IV revealed to have terminal pores. The pocket organs (Fig. 3I, J ) were recognised on all legs, however, with slightly different degrees of protruded appearances. The protruded state might be an artefact caused during specimen preparation since specimens on microscope slides do not always have these appearances (Figs 1B , 2F, I, J ) or the pocket organs are capable of moving. In the latter case, since no muscles seem to be attached to the structures, a passive movement is likely. Regarding claw/digit morphology, the peduncles (internal structures) were not recognised but the three hooks of the internal claws and the single-pointed external claws were recognised (Fig. 3J ).

Etymology.

The species is dedicated to Prof. Dr. Akihisa Kitamura (Shizuoka University), who has been studying Daidokutsu and its bivalve assemblage to understand the paleo-environment (e.g. Kitamura et al. 2007).

Figures

Figure 1. Drawings of Cyaegharctus kitamurai gen. et sp. nov., holotype KUZ Z 2624. A. Habitus (ventral view). B. Leg IV pocket organ. an anus, bt buccal tube, ca cavity, cE cirrus E, db dense body, ec external cirrus, go female gonopore (detail not available), ic internal cirrus, lc lateral cirrus, mc median cirrus, pc primary clava, pl placoid, op opening, sc secondary clava, soI, IV legs I and IV sensory organs, sr seminal receptacles, ss stylet support, st stylet.

Figure 2. DIC and PhC micrographs of Cyaegharctus kitamurai gen. et sp. nov., adult female. A. Habitus (dorsal view). B. cephalic region (ventral view). C. Lateral cirrus. D. Buccal apparatus. E. Caudal region (ventral view) (epicuticle pillars visible). F. Leg I sensory organ and pocket organ. G. Leg II pocket organ (arrowhead indicates protruding portion). H. Leg II digits and claws. I. Leg III pocket organ. J. Leg IV pocket organ. K. female gonopore. L. Legs I-III sensory organs. M. Leg III digits and claws. A-J. holotype KUZ Z 2624, K. paratype KUZ Z 2625, L, M. paratype KUZ Z 2626. an anus, bt buccal tube, ca cavity, cE cirrus E, db dense body, ec external cirrus, go gonopore, ic internal cirrus, lc lateral cirrus, mc median cirrus, pc primary clava, pei, e peduncles of internal and external digits, pl placoid, poI-IV legs I-IV pocket organs, pp proximal pad, sc secondary clava, soI-IV legs I-IV sensory organs, sr seminal receptacles, ss stylet support, st stylet.

Figure 3. SEM micrographs of Cyaegharctus kitamurai gen. et sp. nov., four-claw juvenile paratype KUZ Z 2627. A. Habitus (lateral view). B. Cephalic region (frontal view). C. Cephalic region (ventral view). D. Anus. E. Leg I sensory organ. F. Leg II sensory organ. G. Leg III sensory organ. H. Leg IV sensory organ. I. Leg IV pocket organ. J. Leg III digits and claws. ah accessory hook, an anus, cE cirrus E, ec external cirrus, fe femur, ic internal cirrus, lc lateral cirrus, mc median cirrus, pc primary clava, ph primary hook, poIII, IV legs III and IV pocket organs, pp proximal pad, sc secondary clava, sh secondary hook, soI-IV legs I-IV sensory organs, ti tibia.