Parastenhelia gracilis Brady 1910
Creators: name: Huys, Rony
name: Mu, Fanghong
Publisher: name: Zenodo
Year: 2021
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Hexanauplia
Harpacticoida
Parastenheliidae
Parastenhelia
Parastenhelia gracilis

description: Parastenhelia gracilis Brady, 1910 and other subantarctic records of Parastenhelia
Microthalestris gracilis (Brady, 1910) comb. nov. belongs to a close-knit group of subantarctic species characterized by the position of the inner seta on P1 enp-1 which originates from the middle third of the segment. This subantarctic group includes M. gracilis, M. antarctica (Scott, 1912) comb. nov., M. costata (Pallares, 1982) comb. nov., M. santacruzensis sp. nov. and M. variabilis sp. nov. In all other members of the genus the inner seta of P1 enp-1 is situated further proximally, at around 25% the length of the inner margin of the segment. Interestingly, the latter condition is also displayed by M. minuta (Pallares, 1982) comb. nov. from Tierra del Fuego and M. campbelliensis sp. nov. from Campbell Island, suggesting the independent colonization of the subantarctic region by two different lineages. Microthalestris gracilis comb. nov. is known from a single female collected in the Baie de l’Observatoire (Observatory Bay) along the northern shore of the Golfe du Morbihan in the Kerguelen (Brady 1910: 513; Textfig. VIII). A second record by Jayabarathi (2016) from seagrass beds in South Andaman is highly questionable. The original description includes illustrations of the antennule, P1, P2, P5 and caudal rami but is deficient in many aspects. The armature is unknown for P3–P4 except for Brady’s (1910) dubious claim that their distal endopodal segments have only three elements. The P2 is shown to have two inner setae on exp-3 which is also questionable. Since within the Parastenheliidae this condition is only found in members of Thalestrella and Foweya (Table 1) it may indicate that Brady’s (1910) illustration is either based on an observational error or in reality refers to P3 or P4. Alternatively, the possibility that P. gracilis represents a member of Foweya cannot be excluded but this would require information on the male and the sexual dimorphism in this species. Another unusual feature of P. gracilis is the endopodal lobe of P5 which is remarkably small and displays only four setae. Lang’s (1948) concept of P. spinosa grouped together all the various forms in which the middle segment of P1 exopod was at least four times as long as broad, with the exception of P. gracilis which was maintained as a distinct species mainly on the difference in the site of origin of the inner seta of P1 enp-2 and the shape of the female P5.
Scott (1912: 561; Plate IV, figs. 25–33) described a second subantarctic species, P. antarctica, from Scotia Bay (60°43’42” S, 44°38’33” W) in the South Orkney Islands and compared the female (males are unknown) with P. gracilis. Gurney (1927: 544) suggested that P. antarctica appeared to be identical with P. gracilis and Lang (1934: 23) formally treated them as synonyms but neither author gave a justification for this claim. This course of action appears premature since significant differences can be observed in the morphology of P1 (length of exopod, in particular exp-2; length of claws on enp-2) and P5 (shape of exopod), both of which were dismissed as irrelevant by Lang (1934, 1936a, 1948). Although an unusual degree of intra-individual and intra-population variability has been recorded in the armature patterns of P2–P 4 in some species of Parastenhelia (e.g. Mielke 1974, 1990) such variability is not expressed in the morphology of either P1 or P5. Lang (1936a: 53; Figs 36–37) reported two females from Berkeley Sound, Port Louis in the Falkland Islands which he attributed to P. gracilis. His illustration of the P1 leaves no doubt that he was dealing with the geographically closer P. antarctica since it is fully compatible in the relative lengths of its rami, exp-2 and claws on exp-3 with Scott’s (1912) original description. Thomson (1883, 1913) recorded numerous Thalestris -like specimens from Otago Harbour, Dunedin in the South Island of New Zealand and assigned them to Claus’s (1863) T. forficula [see also Brady (1899) who found it abundantly in the same location]. Lang (1934, 1936a) initially listed Thomson’s record under the synonymy of Parastenhelia forficula var. littoralis and subsequently, again without giving any specific justification, classified it as conspecific with P. spinosa (Lang 1948). Wells et al. (1982) suggested that the Otago Harbour material may belong to P. megarostrum but were unable to make a firm recommendation for this assignment on morphological grounds (see below). Thomson’s (1883) illustrations (Plate X, figs 16–21) show several similarities with the original description of P. antarctica, including (a) the general outline and proportional segmental lengths of the female antennule; (b) P1 exopod with elongate exp-2; (c) P1 endopod showing similar L:W ratio for enp-1; claws on enp-2 elongate as in P. antarctica; and (d) female P5 setation and general shape of exopod. Based on these similarities we consider it likely that Thomson’s (1883) T. forficula belongs to P. antarctica, the only main difference being the position of the inner seta on P1 enp-1 which inserts more distally. The female antennule of Thomson’s specimens was described as 8- segmented but reinterpretation of his illustration (Plate X, fig. 17) suggests it has nine segments (as in P. antarctica). The presence of four setae on the proximal segment indicates that it represents the second segment and that the real first segment was not observed or lost during dissection. The aesthetasc that originates from a ventral pedestal of the fourth segment is shown to insert on the anterodistal corner of this segment (in reality the fifth) in Thomson’s drawing; this can be explained by assuming that the author observed the antennule in dorsal aspect. Based on the arguments above, Microthalestris antarctica (Scott, 1912) comb. nov. is here reinstated as a valid species. It is very close to M. costata comb. nov. in the morphology of the P1 and the foliaceaous ♀ P5 exopod but differs from it in the armature of the distal endopodal segment of P4 (Table 1).
description: Published as part of Huys, Rony & Mu, Fanghong, 2021, Johnwellsia, a new intertidal genus of Parastenheliidae (Copepoda, Harpacticoida) from the Taiwan Strait, China, including a review of the family and key to genera, pp. 236-318 in Zootaxa 5051 (1) on pages 278-279, DOI: 10.11646/zootaxa.5051.1.13, http://zenodo.org/record/5572417