Styraconyx robertoi Pérez-Pech & Jesús-Navarrate & Demilio & Anguas-Escalante & Hansen 2020, sp. nov.

Styraconyx robertoi sp. nov.

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Material examined: 20 specimens (PM: 1 female, 1 male, 2 indeterminable gender; ML: 1 indeterminable gender; XK: 10 females, 3 males, 2 indeterminable gender)

Remarks: The population occurring at Xcalak is designated as the type population. See Description of the New Species

Type Material: Female holotype and 13 paratypes (8 females, 2 males, 3 larvae). Morphometric values for the type population are given in Table 2.

Type Locality: reef lagoon, 0.2–1.0 m depth, subtidal zone, Xcalak, Quintana Roo, Mexico

Diagnosis: Styraconyx with subterminal mouth cone, pigmented eyespots present. Asymmetric primary clavae and lateral cirri with common pedestal and surrounded basally by common membrane. Lateral cirri not segmented and with asymmetrically expanded base. Lenticular secondary clavae. Cephalic cirri composed of scapus and tubular, flagellar part only, cirrophore and protruding flagellum absent. Peduncles present only on external digits and reaching claw base, internal digits with proximal pads. In males, internal digits with peduncles present only on leg pair IV. Three-pointed claws with strong primary hook, thinner secondary and accessory hook. Claw sheath not covering tip of claw hooks. Sense organs of first leg two-parted, unsegmented spines on second and third leg. Asymmetric papilla with broad base and terminal spine on fourth leg. Dorsal cuticle with grid-like sculpture and fine punctuation.

Holotype description. Female (Figs. 2; 3B,D; 4 A–C; 7) with roughly cylindrical body form (139 μm in length) broadest between leg pairs II and III (46 μm) and distinctly narrower (36 μm) at the point of the insertion of cirri E. Eye spots (Figs. 3A,E) visible as congregations of brown pigment granules on each side of the pharyngeal bulb slightly posterior to the pedestals bearing the primary clavae. Pillars highly conspicuous in the dorsal cuticle and to a lesser degree in the ventral cuticle. Twenty-two ribs traverse the dorsal surface at right angles to eight equally spaced longitudinal ribs forming a distinct grid-like pattern (Figs. 2A,B; 7), beginning just posterior to the pedestals bearing the primary clavae and extending to a position slightly anterior to the cirri E. Distinct dorso-caudal region without grid-like sculpture. Instead, two longitudinal ribs occur which originate on either side of the median line and curve slightly toward the lateral sides (Fig. 7).

The subterminal mouth cone is short and ends in a smooth, annular dome with a central mouth opening. The buccal tube (16 μm x 2 μm) has a pair of lateral apophyses on each side (Fig. 3E) and stylet supports with a distal swelling. Stylets are long (20 μm) with strong furcae. Pharyngeal bulb (15 μm) with three placoids of equal length (11 μm).

Primary clavae, secondary clavae, and a full set of cephalic cirri are present. The primary clava (10 μm) is highly asymmetric and the form appears different depending on the angle in which it is observed (Figs. 4D,F; 5A). Primary clava and lateral cirrus inserted on a common pedestal and surrounded basally by a short, common membrane. Primary clavae with van der Land’s body present at the base. The lateral cirrus (28 μm) is not segmented and has an asymmetrically expanded base, which is fused to the proximal end of the primary clava (Figs. 4D; 5A,B). Ventral lenticular secondary clava (12 μm) inserted antero-laterally to the external cirrus close to the anterior margin of the head (Figs. 3E; 4E,F; 7). The median cirrus (9 μm) is placed dorsally (Figs. 4A; 6A), well back from the ante- rior margin of the head. Internal cirri (13 μm) are dorsal (Figs. 4B; 6B) and more medial than the ventrally inserted external cirri (12 μm) (Figs. 4C; 6C). Except for the lateral cirri, all cephalic cirri (Figs.4 A–C; 6A–C) are two-parted (which can be difficult to recognize in LM) and each composed of a conical scapus and a tapering, flagellar part with a terminal pore. The flagellum is strictly internal and evident in all cephalic cirri except for the lateral cirri.

Sense organs present on all legs and inserted coxally. Sense organs of leg pair I (9 μm) two-parted and similar to the cephalic cirri having a conical scapus and a tapering, flagellar part with a terminal pore (Figs. 4G; 6D). On leg pairs II and III the sense organs are simple, without any recognizable segmentation, and resemble the shape of curved spines (Figs. 6E,F). Sense organ of leg pair IV (Figs. 4H,I) consists of a smooth papilla (7 μm) with terminal spine (6 μm), which has a terminal pore. The papilla is asymmetrical, having the postero-lateral portion less curved than the antero-lateral portion. It is inserted with a broad base on a short pedestal and appears hemispherical rather than spherical. Dorso-lateral cirrus E (Figs. 4H,I) inserted on a prominent pedestal. It consists of a short (3 μm), oversized cirrophore (Figs. 4I; 7) and a long, tapering spine (23 μm) without evident accordion-pleated portion.

Each leg with evident coxa, femur, tibia and tarsus. Tarsus of all legs bearing four digits of different length with external digits being shorter than internal digits. Peduncles (Figs. 3B) present only inside the external digits and are of the same type as Styraconyx craticulus (Pollock, 1983) reaching the claw bases (see Kristensen & Higgins, 1984). They have a strongly curved basal portion and an enlarged medial portion with two small lateral points (Fig. 8A). The internal digits have thin, proximal pads. All digits bearing three-pointed claws (Figs. 3D,F; 5C) with primary hook, secondary and accessory hook. The primary hook is the strongest. The secondary hook is thinner but slightly longer than primary hook, and accessory hook is the shortest. The tips of all three hooks are free of the claw sheath (Fig. 8A).

Seminal receptacles are not evident. The gonopore (Figs. 2C; 5D) is relatively large (6 μm) and consists of a rosette of six cells situated close to the anus. The anus is an undulating slit and composed of two larger lateral lobes and a small anterior lobe (Figs. 2C; 5D).

Males. Only 2 males were found. The smaller one measures 112 μm in body length, the larger measures 154 μm. Both males display a peculiar combination of digit characters in having only two peduncles on each foot of leg pairs I–III (similar to the females) and four peduncles on each foot of leg pair IV. On leg pair IV the peduncles of internal digits are rod-shaped and extend from the base of the digits, through the proximal pads, to the claw bases (Fig. 8B).

The testes are not evident. The male gonopore is a simple, oval opening situated close to the anus. No other differences between males and females were observed.

Larvae. Three larvae were found, all with four claws on each foot and with body lengths of 95 μm, 112 μm and 114 μm. The specimens display larval characters in the shape and size of the primary clava and in the overall body form (Fig. 3C). The primary clavae are asymmetric but shorter (6–8 μm) and less slender than in adults. The head is not well-differentiated from the trunk and the frontal margin appears blunt, whereas in adults it is protruded (Fig. 3A). One of the specimens has four peduncles on the digits of leg pair IV and so is considered to be a male larva.

Scanning electron microscopy. The SEM observations of a single specimen confirmed most of the diagnostic features of this species and aided to a better understanding of their true nature. In LM, the claws, and especially their hooks, can look very different depending on their orientation in the preparation (Figs. 3D,F), whereas the three-dimensional SEM image leaves no mistake of interpretation. This applies also to the sense organs where segmentation and terminal pores can be difficult to recognise by LM, but clearly evident with SEM. In this case where both the primary clava and the base of the lateral cirrus are asymmetric, their true shape is only recognizable with SEM (Figs. 5A,B). Not visible in LM but revealed by SEM is a pair of large pores (Fig. 5D) posterior to the gonopore. While their function is unknown, the position close to the gonopore suggests that they are openings of the seminal receptacles.

Differential diagnosis. The 14 previously described taxa (13 species and 1 subspecies) of the genus Styraconyx can generally be divided into two major groups (Kristensen & Higgins 1984; D’Addabbo Gallo et al. 1989; Chang & Rho 1998; Bartels et al. 2015); the S. hallasi group and the S. sargassi group. Species in the S. hallasi group have peduncles on two digits (the external) of each leg and claws with reduced accessory hooks, while species in the S. sargassi group (including the type species S. haplocerus Thulin, 1942) have peduncles on all four digits and claws with three hooks of nearly equal size. While females of Styraconyx robertoi sp. nov. should be placed within the S. hallasi group due to the presence of peduncles only on the external digits of legs I–IV and claws with reduced accessory hooks, the unusual digit configuration of the males, i.e. peduncles only on the external digits of legs I–III and peduncles on all four digits of leg IV, complicates the assignment of the new species to either of the two groups. Within the S. hallasi group only Styraconyx kristenseni sensu lato Renaud-Mornant, 1981 has asymmetric primary clavae that are similar to those of the new species. However, S. kristenseni differs from S. robertoi sp. nov. in hav- ing equal sized claw hooks, slender papilla on leg IV with evident cirrophore, and by lacking secondary clavae and the characteristic grid-like dorsal sculpture. Interestingly, the grid-like sculpture of the dorsal cuticle of S. robertoi sp. nov. bears an immediate resemblance to that of S. craticulus and S. craticuliformis Chang & Rho, 1998, both of which belong to the S. sargassi group. This sculpture is the main decisive character differentiating these three species from all other Styraconyx. S. craticuliformis and the new species both differ from S. craticulus by having reduced accessory hooks, a thin common membrane at the base of the primary clavae and lateral cirri, asymmetric primary clavae, and a smooth papilla on leg IV. In summary, Styraconyx robertoi sp. nov. is most similar to S. craticuliformis but can be readily distinguished from it by the number of peduncles, the presence of distinct secondary clavae, and the shape of the papilla on leg IV, which in S. craticuliformis is more spherical.

Type Repositories: Holotype (NHMD-633434) and seven paratypes (2 females NHMD-633435–633436, 2 males NHMD-633437–633438, 3 larvae NHMD-633439–633441) are deposited in the Tardigrada collection of the NHMD, Copenhagen. Six paratypes are deposited in the collection at ECOSUR, Quintana Roo, Mexico.

Etymology: The species is dedicated to our valued friend, Dr. Roberto Guidetti of the University of Modena and Reggio Emilia, Italy in recognition of his remarkable contributions to the study of tardigrades.