Cyaegharctus, Fujimoto & Jimi, 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 gen. nov.

Type species.

Cyaegharctus kitamurai gen. et sp. nov.

Diagnosis.

Styraconyxidae with dorso-ventrally flattened body; cuticle smooth; epicuticular pillars present; cephalic region with complete set of cephalic cirri, ovoid primary clavae and compact conical secondary clavae; median cirrus and internal cirri at anterior margin of cephalic region; external cirri latero-ventral to internal cirri; lateral cirri and primary clavae sharing common base at antero-lateral position of cephalic region; secondary clavae between internal and external cirri; buccal apparatus with stylet supports; cirri E spine-like; seminal receptacle ducts opening anterior to gonopore; terminal anus with pair of large longitudinally elongate lobes; dorsal side of each leg with usual sensory organ on proximal part of femur and pocket organs at distal margin of femur; internal digits each with proximal pad and thin peduncle; external digits with proximal developed peduncles; claw sheaths present; adult female with three-pointed claws on all digits; four-claw juvenile with three-pointed claws on internal digits and single-pointed claws on external digits; three-pointed claws each with accessory and secondary hooks less developed compared to primary hook.

Etymology.

The genus name is masculine and derives from two words, Cyäegha, a deity of darkness and caves in the Cthulhu Mythos from Eddy C. Bertin’s short story Darkness, My Name Is and arctus, a Latinised Greek word meaning bear.

Remarks.

The presence of pocket organs distinguishes the new genus and species from all other members of the phylum (see Description of holotype and Discussion for further information on the new structure). For this uniqueness, it is difficult to infer the new taxon’s taxonomic position, based solely on this morphology. Here we use other comparable morphological characters to infer its taxonomic position. The new taxon’s three-pointed claws and peduncles indicate its affinity to the following four genera of Styraconyxidae : Raiarctus , Styraconyx , Tetrakentron and Tholoarctus (van der Land 1975; Kristensen and Higgins 1984; Jørgensen et al. 2014).

The new taxon’s epicuticular pillars are not developed to the degree of Raiarctus 's characteristic pillars ( Jørgensen et al. 2014). In addition, the new taxon has its seminal receptacles opening near the gonopore, in contrast to those of the latter genus opening laterally apart from the gonopore ( Jørgensen et al. 2014).

Styraconyx harbours species covering a wide range of character states and has been regarded as a non-monophyletic group ( Kristensen and Renaud-Mornant 1983; Kristensen and Higgins 1984). Due to this problematic state, many of the new taxon’s character states fall within the range of Styraconyx , i.e. morphology of cephalic cirri, primary clavae, usual leg sensory organs, claws and digits (including peduncles) and seminal receptacles ( Kristensen and Higgins 1984). However, the new taxon’s compact conical secondary clavae differ from Styraconyx 's flat sac to dome-shaped ones ( Kristensen and Higgins 1984). Further, although it is not included in the generic diagnosis ( Kristensen and Higgins 1984), it is apparent from all the original descriptions of Styraconyx species that they have a cylindrical body shape differing from the new taxon’s dorso-ventrally flattened body and also the anus is not known to be developed as the large bi-lobed anus of the new taxon ( Thulin 1942; Robotti 1971; Kristensen 1977; Renaud-Mornant 1981b; Pollock 1983; Kristensen and Higgins 1984; D’Addabbo Gallo et al. 1984, 1989; Chang and Rho 1998; Bartels et al. 2015; Pérez-Pech et al. 2020).

The new taxon and Tetrakentron both have a dorso-ventrally flattened body, but the latter genus has short legs with strongly developed claws (van der Land 1975). The latest diagnosis of Tetrakentron by van der Land (1975) is brief and we here present some other differences, based on information that van der Land (1975) did not implement in the diagnosis and additional information provided by subsequent literature ( Kristensen 1980; Kristensen and Hansen 2005). One difference is that, in contrast to the new taxon, Tetrakentron has unique peduncles, which van der Land (1975) called ‘drop-shaped’ (already illustrated in its original description ( Cuénot 1892); for detailed drawing see van der Land (1975)). Another difference is that the new taxon and Tetrakentron have differently arranged cephalic appendages: i) the new taxon’s primary clavae are positioned laterally apart from the secondary clavae, whereas the two clavae are situated close together in Tetrakentron and ii) the new taxon’s secondary clavae are positioned posterior from the anterior margin of the cephalic region, whereas those of Tetrakentron are at the antero-lateral margin (see detailed drawings and SEM in Kristensen (1980)). The presence of epicuticular pillars in the new taxon further differentiates it from Tetrakentron without pillars ( Kristensen 1980). On the other hand, the new taxon and Tetrakentron have similar secondary clavae morphology and they both have their seminal receptacles opening anterior to their gonopores ( Kristensen and Hansen 2005). It is also noteworthy that these two taxa, as well as Angursa , have a terminal anus ( Kristensen and Hansen 2005; Fujimoto and Hansen 2019). Whether these morphological similarities between the new taxon and Tetrakentron are phylogenetic signals or not need to be verified with molecular data.

The new taxon has a dorso-ventrally flattened body differing from Tholoarctus 's cylindrical body with a large detached outer epicuticle ( Jørgensen et al. 2014), Further, the new taxon has ovoid primary clavae and conical secondary clavae, in contrast to Tholoarctus with cone-shaped primary clavae and large secondary clavae surrounding the internal and external cirri ( Jørgensen et al. 2014).

For the morphological differences indicated above, the new taxon’s designation to any of the four genera is not plausible and the erection of a new genus and species is justified.