Chimaeradasys oligotubulatus, Kieneke & Todaro, 2021

Kieneke, Alexander & Todaro, M. Antonio, 2021, Discovery of two ‘ chimeric’ Gastrotricha and their systematic placement based on an integrative approach, Zoological Journal of the Linnean Society 192 : -

CHIMAERADASYS OLIGOTUBULATUS SP. NOV.

( FIGS 1, 2)

Zoobank registration: urn:lsid:zoobank.org:act:4871880A-8688-4883-A5DB-C2FE600C389

Type locality: Portugal, Azores, Flores, sandy deep sublittoral bottom at about 6 km to the south-west of the southern coast (station #84-1, 39°19.699’N, 031°16.390’W); collection made on 3 September 2018 with a Shipek Grab lowered to a depth of 257 m from aboard the R / V Meteor during expedition M150 .

Type specimen: The single specimen that was documented microscopically and described herein is the holotype, fixed by monotypy (Article 73.1.2. of the International Code of Zoological Nomenclature, ICZN, 1999). The holotype specimen does not exist any longer (Article 73.1.4. of the International Code of Zoological Nomenclature, ICZN, 1999). Declarations according to recommendations 73G–73J ( ICZN, 2017): the living, anaesthetized specimen was extensively investigated microscopically in order to gain optimal, taxonomy-relevant data. The morphological distinctness and uniqueness is the reason for naming this new taxon now. The specimen was recovered from the microscopic slide and immediately dissolved in order to extract genomic DNA for genetic analyses. The remaining genomic DNA was stored at –20 °C. None of the remaining sediment samples yielded further specimens of the new species. Several gastrotrich taxonomists have been consulted prior to this taxonomic act. The following recommendations regarding taxonomic processing of soft-bodied meiofauna ( Garraffoni et al., 2019) were applied: the specimen was observed and documented using high-resolution objectives and differential interference contrast (DIC), series of highresolution digital images and a video sequence were deposited in MorphDBase (https://www.morphdbase. de/?A_Kieneke_20200503-S-1.1 respectively https:// doi.org/10.20363/mdb.2w1k-mr22 and https://www. m o r p h d b a s e.d e /? A_ K i e n e k e _2 0 2 0 0 5 0 1- M - 7 0.1 respectively https://doi.org/10.20363/mdb.rj1d-8y15). The partial nuclear 18S rDNA and mitochondrial COI sequences have been deposited in the European Nucleotide Archive (ENA) at EMBL-EBI under accession number PRJEB38727 (https://www.ebi. ac.uk/ena/browser/view/PRJEB38727). The individual accession numbers are LR862428 (18S) and LR862427 (COI).

Material examined: Only the single holotype specimen was surveyed using light microscopy with DIC. Furthermore, the genomic DNA was extracted from the single specimen and used as a template for amplification of nuclear and mitochondrial gene fragments .

Ecology: Sediment was made up of fine, organogenous sand (granulometric data not available). Temperature, salinity, oxygen content and irradiation (photosynthetically active radiation from 400–700 nm) of the bottom water at the time of sampling were 14.5 °C/35.93 PSU/218 µmol/kg/160 µmol photons/ m 2 and s, respectively (measured at a depth of 250 m during CTD deployment #77-1 of expedition M150 at 39°19,650’N, 031°16,430’W) .

Diagnosis: A Chimaeradasys of 277 µm in total length and 50 µm in maximum width. Anterior adhesive tubes, one per side, in a row just posterior to the mouth; TbV, a single pair in the anterior pharyngeal region at U15; TbL, one per side at U85; TbVL, five per side, one of which was in the anterior pharyngeal region at U14 and the other four were unevenly distributed along the anterior half of the trunk; TbP, three per side, two at the end of each peduncular branch and one medial, inserting ventrally near the base of the branch. Mouth wide (up to 55 µm in breadth), leading to a shallow, funnel-shaped buccal cavity and surmounted dorsally by a scalloped oral hood. Pharynx 67 µm in length, with pores at U33; PhIJ at U34. Intestine straight, slightly wider at midbody; anal opening at U80. Hermaphrodites, single testis on right body side, beginning well posterior to the PhIJ at U43; caudal organ glandomuscular; frontal organ sac-like, both with spermatozoa inside; ovary and oocytes not seen.

Etymology: The species epithet is composed of the Greek ολιγος, little, and Latin tubulus, a small tube or hose, referring to the low number of ventrolateral adhesive tubes.

Description: The body measures 277 µm in total length ( Figs 1A, B, 2A–C). The habitus shows a clear widening in the region of the mouth opening (U10) that is surmounted by a scalloped oral hood. The mouth measures 43 µm in diameter and continues in a shallow, funnel-shaped buccal cavity ( Figs 1B, 2B, C). The region of the pharynx (U10 to U34) is slightly narrowed followed by the slightly widened trunk until approximately U55. The rear trunk continuously narrows until U87 and between U87 and U97, it is just a narrow peduncle that splits into a pair of short appendages (peduncular branches) at U97 ( Figs 1A, B, 2B). Body width is 50/36/50/41/30/18/8 µm at U10/20/55/70/80/85/95, respectively. The trunk is dorsoventrally flattened with a flat venter and a moderately arched dorsum.

The adhesive tubes occur as anterior (TbA), ventral (TbV), lateral (TbL), ventrolateral (TbVL) and posterior (TbP) groups ( Figs 1B, 2B, C). There is only a single pair of TbA. At U10, these two ventral TbA are placed close to the ventral (posterior) mouth rim ( Figs 1B, 2C). The distance between the insertions of both tubes is 26 µm. Each TbA is about 7.5 µm long. At U15, a single pair of TbV is placed midventrally ( Figs 1B, 2C). Each of the two TbV is about 5.5 µm long and the distance between both tubes is 13.5 µm. A single pair of TbL inserts at U85 ( Figs 1A, 2B, C); TbL are about 18 µm long and project more posteriorly than laterally at an angle of less than 40° against the longitudinal axis of the animal. It has to be stressed that this single pair of TbL could, in principle, alternatively be defined as a fourth pair of TbP (see ‘Taxonomic affinities’). There are five pairs of TbVL: the first pair inserts in the anterior pharyngeal region at U14, while the others are unevenly distributed from about the pharyngeo-intestinal region to about half the trunk at U37, U46, U59 and U62, respectively ( Figs 1B, 2C). The most anterior TbVL are just 7.5 µm long, followed by the second pair with up to 12.5-µm long tubes, while all other TbVL measure between 16 and 18 µm in length. All TbVL project laterally and slightly towards posterior at an angle between 48° and 60° against the longitudinal axis of the animal. In total, there are six bilaterally arranged TbP, all confined to the peduncular branches ( Figs 1B, 2C). In fact, each peduncular branch bears two tubes at its apex, arranged in a V-shaped manner, and a third tube along the medial margin, inserting ventrally near the base. Distal tubes measure 8.5 µm in length, while the proximal tube is 10.5 µm long. The ventral locomotory cilia start at U12 shortly posterior to the ventral mouth rim and end posterior to the anus at about U85. In the pharyngeal region, the cilia cover the entire ventral surface and are weakly arranged in transverse rows. At about U40, locomotory cilia seem to split into two separated fields, each c. 10 µm in width and 20 µm spaced from each other at U50. Around U60, both fields fuse again and form a more or less uniform field of more sparsely placed cilia ( Figs 1B, 2C). Length of ventral locomotor cilia varies between 11 and 14 µm. At least ten presumptively sensory cilia, up to 20 µm long, insert on the dorsal surface of the oral hood, roughly arranged in pairs. Further putative sensory cilia insert dorsolaterally along the trunk, arranged roughly pairwise ( Figs 1A, 2A). Some of these cilia are associated with epidermal glands. Along the dorsal (anterior) and lateral parts of the mouth rim, there are seven evenly spaced and presumptively sensory papillae, each one measures about 4µm in height ( Figs 1A, B, 2B). No cilia where directly linked to the papillae.

The holotype specimen had five pairs of bilaterally arranged epidermal glands at U10/22/30/64/79, respectively ( Figs 1A, 2A). The globular to slightly pear-shaped glands had a diameter of 5 to 6 µm and appear with granular content. At least for the second and third pair, we were able to observe putative sensory cilia associated with the epidermal glands. In the area of the trunk, between roughly U33 and U62, glands seemed to be missing.

The digestive system starts anteriorly with the ample mouth opening that continues in a funnelshaped buccal cavity. Fold-like structures at the dorsal epithelium of the buccal cavity and along the lip-like mouth rim indicate a certain contractile property of the whole oral basket ( Figs 1B, 2C). The pharynx is about 67 µm in length with pharyngeal pores that open ventrolaterally at U31. The width of the musculoglandular pharynx is 42/16/11/14/22/11 µm at U10/15/20/25/31/33, respectively. The pharyngeointestinal junction is at U34 ( Figs 1A, 2B). The intestine spans from U34 to U80, where it opens externally with an inconspicuous ventral anus ( Fig. 2C, F). It has a maximum width in its middlethird and tapers considerably toward the anus. Widths are 16/19/22/12/8/5 µm at U35/45/55/60/70/75, respectively. Only a slight regionalization of the intestine is recognizable with a higher density of presumptively enzymatic vesicles in the middlethird and bigger, vacuole-like digestive vesicles in the posterior-third ( Fig. 2B). The nature of the gut content was not ascertainable.

There is a single, 80-µm long right testis spanning from U43 to U73, including the posteriorly directed vas deferens ( Figs 1A, 2B, D–F). However, since the actual termination of the sperm duct could not be determined with certainty (see below), the length measurement is just an approximation. The maximum width of the single testis is 15 µm at U60. Lateral to the posterior testis, between U62 and U76, there is a thick-walled, muscular hollow structure, presumptively the caudal organ ( Figs 1A, 2B, D–F). It is roughly club-shaped and narrows posteriorly (maximum width is about 11 µm at the muscular section) into a kind of outlet duct that passes the intestine ventrally and forms a slight ‘swelling’ (9.5 µm in diameter) on the left side of the posterior intestine ( Fig. 2D–F). Dorsal to the intestine, between U60 and U 68 in a median position, there is a sac-like, 23 by 11 µm structure, the frontal organ ( Fig. 2D). All three reproductive structures, testis, frontal and caudal organs, are filled with filiform spermatozoa with clearly spiralled sperm heads ( Fig. 2D–F). While sperm lies in more or less parallel bundles inside the testis and the caudal organ, they are curled inside the frontal organ.

Remarks on external anatomy and reproductive structures: The apparent absence of maturing oocytes in the studied specimen of Chimaeradasys oligotubulatus is a likely indication that the specimen has not reached full adulthood. While we acknowledge that fully mature specimens may reach a larger size and may develop some additional adhesive tubes of the anterior and ventrolateral series, the well-formed spermatozoa inside the testis and caudal organ, and the motile spermatozoa inside the frontal organ, indicate that the size of fully grown adults will not be much different from that of the studied specimen. Likewise, the number of adhesive tubes will not undergo ample variations.

Furthermore, the studied specimen of C. oligotubulatus shows a heterogeneous organization of the ventral locomotory ciliation, i.e. cilia are organized in transverse bands in the anterior and posterior body regions, but as paired longitudinal bands at mid-trunk. Such an inhomogeneous organization is so far unreported among Gastrotricha. Considering that in C. polytubulatus (see below) the locomotory cilia are organized in transverse rows all over the ventral surface, we think the unusual organization of locomotor cilia observed in C. oligotubulatus represents an artefact rather than the natural condition, probably due to excessive compression of the specimen. Consequently, C. oligotubulatus should be considered to possess ventral cilia distributed in transverse rows that cover the entire ventral surface.

There are some uncertainties concerning the reproductive system of the new species. We were not able to unambiguously detect external and/or internal pores of either the testis, or the frontal and caudal organs. Furthermore, the connectivity between the three organs could not be clarified with certainty. If the function of these organs matches those of the closest phylogenetically allied species (see below), as the incomplete information we currently have seems to suggest, most likely the vas deferens and the caudal organ open independently on the ventral side each with its own pore or via the anus (for a summary see, for example: Ruppert & Shaw, 1977; Ruppert, 1978; Kieneke & Schmidt-Rhaesa, 2015). Likewise, a direct internal connection between frontal and caudal organ (see: Ruppert, 1978; Kieneke et al., 2009) is possible, but could not be demonstrated unequivocally. Neither a large egg nor any developing oocytes were observed in the single specimen of the new species. On the left side of the intestine, more or less in the same region as the frontal organ (between U60 and U72), there is an accumulation of droplets and globular material of presumptively glandular property, about 7 µm wide at U65 ( Fig. 2B). We were not able to definitely clarify whether this is just vesicular mesodermal tissue or secretory material of an as yet undescribed gland.

Variability and remarks: Information on character variability among individuals of the new species is currently not available.

Taxonomic affinities: The taxonomic affinities of the new genus and both new species are discussed collectively below, following the description of the second new species.