Higginsarctus, Hansen & Kristensen, 2021

Hansen, Jesper G. & Kristensen, Reinhardt M., 2021, A new genus and five new species of the subfamily Florarctinae (Tardigrada, Arthrotardigrada), European Journal of Taxonomy 762 (1), pp. 149-184 : 152-154

Genus Higginsarctus gen. nov.

urn:lsid:zoobank.org:act:B191A524-2C5F-4DBD-93BA-7DC1ADB82CC7

Type species

Higginsarctus signeae gen. et sp. nov.

Diagnosis

A genus of Florarctinae characterized by large, rigid and backwards bending primary clavae (shared with Ligiarctus ) supported by strong internal cuticular structures at their bases. Secondary clavae occurring as papillae (dome-shaped or spherical) or as flat sacs (round, rectangular or bilobed). Frontal ala, six lateral alae and a single caudal ala present. The proximal part of the alae (except for the frontal) internally supported by procuticle which sends out branching processes (ramuli) into the distal part of the alae. External claws with calcars and with small internal notches; internal claws with accessory spines and small internal notches.

Etymology

The new genus is dedicated to Dr Robert P. Higgins for his contributions to meiofauna research. Higgins +arctus (latinised Greek word), meaning Higgins’s bear.

Remarks

Higginsarctus gen. nov. shares with species of Florarctus , Ligiarctus and Wingstrandarctus the presence of alae, the shape of the cephalic cirri and fourth leg sense organ, the shape of the seminal receptacles, and the presence of internal hook-shaped peduncles. The very characteristic primary clavae, being large, rigid and backward bending, are shared only with Ligiarctus . It is very apparent that this new genus belongs to Florarctinae , perhaps being closest related to Ligiarctus . However, three characters of Higginsarctus gen. nov. set the genus apart from Florarctus , Ligiarctus and Wingstrandarctus , namely the number of lateral alae, the branching processes of procuticle reaching the distal part of the alae, and the presence of a small internal notch both on external and internal claws.

The term ‘caestus’(from Latin = ‘gauntlet’), originally suggested by Renaud-Mornant (1987), has been generally adopted as a term for the supporting procuticle of the alae in species of Florarctus ( Fontoura et al. 2017; Hansen & Kristensen 2020). While the procuticle of the alae in species of Florarctus generally forms proximal lobate processes, it is much different in species of Higginsarctus gen. nov. where the entire proximal half of the alae are supported by a procuticle which sends out branching processes into the distal part of the alae. It is therefore suggested to refer to this special configuration of the supporting structure in the alae of Higginsarctus gen. nov. as ‘ramuli’ (plural form of ‘ramulus’ (Latin) meaning ‘a small branch’).

The six species of Higginsarctus gen. nov. described below differ conspicuously in the morphology of the secondary clava. Further distinguishing characters include differences in alae morphology, genital features and ventral cuticular structures ( Table 1 ).

As in Florarctus and Wingstrandarctus , the distal margin of the lateral and caudal alae in species of Higginsarctus gen. nov. are characterised by small or larger indentations. Although being essential to an accurate description of alae morphology of each species, previous studies have not discriminated between types of indentations. Having studied numerous species of Florarctinae we believe that two morphological distinct types of indentations can be recognised, namely the ‘pointed’ type ( Fig. 1 ) and the ‘arched’ type ( Fig. 1 ). Consequently, the two types are discriminated in the species descriptions below. We also suggest considering both types in all future descriptions of species belonging to Florarctinae .

A new genital structure not observed previously in tardigrades is recognised in 4 out of 6 species. It is situated immediately posterior to the gonopore and will be referred to as the ‘genital stoup’ as it resembles the shape of the holy water holder ( Figs 2 , 4E ). Further investigation is needed to determine the exact function of this structure but it likely plays a role in either copulation or oviposition by facilitating the detention of spermatids/spermatozoa. The presence or absence of a genital stoup in Higginsarctus shintai gen. et sp. nov. and H. laurae gen. et sp. nov. cannot be determined with certainty in the present study. Thus, it might be confirmed as an apomorphy of the new genus, pending a closer study of new material.

A special kind of leaf-like structures are present on the ventral cuticle in the two species Higginsarctus signeae gen et sp. nov. ( Fig. 2 ) and H. laurae gen. et sp. nov. ( Figs 14 , 16C ). Such structures have not been discussed or named previously in Florarctinae although they are present in Florarctus heimi Delamare-Deboutteville & Renaud-Mornant, 1965 (see Gąsiorek et al. 2021: fig. 6), and will tentatively be referred to as ‘folia’ (plural form of ‘folium’ (Latin), meaning a thin leaf-like structure or lamella, a leaf, especially a thin leaf or plate (geometry)).

Figures

Fig. 1. General features of the lateral alae of Higginsarctus gen. nov. The procuticle sends out branches (ramuli) to the edge of the alae. Drawing by Stine B. Elle.

Fig. 2. Higginsarctus signeae gen. et sp. nov. Ventral overview. Holotype, ♀ (NHMD-293899). Abbreviations: see Material and methods. Drawing by Stine B. Elle.

Fig. 4. SEM-micrographs of Higginsarctus signeae gen. et sp. nov., paratype, ♀ (NHMD-293902). A. Ventro-lateral overview. B. Close up on the head, lateral view showing cephalic cirri and secondary clava. C. Internal cirrus. D. External cirrus. E. Gonopore and genital stoup. F–G. Scapus of cirrus E broken in two (note the unique small papillae of scapus). H. Primary clavae (broken) and lateral cirrus. Abbreviations: see Material and methods.

Fig. 14. Higginsarctus laurae gen. et sp. nov., holotype, ♀ (NHMD-293905). Ventral overview. Abbreviations: see Material and methods. Drawing by Stine B. Elle.

Fig. 16. DIC-micrographs of Higginsarctus laurae gen. et sp. nov. A. Paratype, ♀ (NHMD-293907). Overview. B–D. Paratype, ♀ (NHMD-293908). B. Close up on the head, ventral view showing the external cirri and secondary clavae. C. Close up on the caudal region, ventral view showing the folia and gonopore. D. Close up on the bucco-pharyngeal apparatus. Abbreviations: see Material and methods.

Tables

Table 1. Differential table of major morphological characters in species of Higginsarctus gen. nov.

	H. signeae gen. et sp. nov.	H. shintai gen. et sp. nov.	H. lassei gen. et sp. nov.	H. martini gen. et sp. nov.	H. laurae gen. et sp. nov.	H. alatus gen. et comb. nov.
Secondary clava	Flat, rectangular with sligthly protruding contours	Flat, comma- shaped	Flat, oval	Ovoid, dome- shaped	Spherical	Flat, bilobed
Antero-lateral ala	Unilobed without indentation	Unilobed without indentation	Unilobed without indentation	Bilobed with medial pointed indentation	Trilobed with pair of medial pointed indentations	Unilobed without indentation
Medio-lateral ala	Unilobed without indentation	Unilobed without indentation	Bilobed with medial pointed indentation	Bilobed with medial pointed indentation	Bilobed with medial pointed indentation	Unilobed without indentation
Postero-lateral ala	Bilobed, rectangular with medial arched indentation	Bilobed with medial pointed indentation	Bilobed with medial arched indentation	Trilobed with anterior pointed indentation and posterior arched indentation	Bilobed with medial pointed indentation	Bilobed with medial arched indentation
Caudal ala	Quadrilobed, rectangular with 3 arched indentations	Quadrilobed, rectangular with 3 arched indentations	Quadrilobed, rectangular with 3 arched indentations	Quadrilobed, round with 3 arched indentations	Bilobed, round with medial pointed indentation	Bilobed, rectangular with medial arched indentation
Leg sensory organs I–III	Similar in length	Similar in length	Similar in length	Similar in length	Similar in length	Highly different in length
Genital stoup	Present	–	Present	Present	–	Present
Genital papilla	Absent	Absent	Absent	Absent	Absent	Present
Folia	Present	Absent	Absent	Absent	Present	Absent