Microthalestris polaris, sp. nov.

Microthalestris polaris sp. nov.

urn:lsid:zoobank.org:act: 2DD62715-A421-471B-A3A0-83EF1A166B81

Parastenhelia spinosa (Fischer, 1860) sensu Chislenko (1967, 1977), Mielke (1974)

Both M. forficuloides and M. sarsi sp. nov. display the maximum number of eight setae/spines on the female P5 exopod. This number has also been observed in females assigned to M. forficula by Wilson (1932b), and to Parastenhelia spinosa by Chislenko (1967, 1977), Mielke (1974) and Kornev & Chertoprud (2008). Wilson’s (1932b) description is concise, illustrating only P1, P5 of both sexes and male P3 endopod. Differences in body size (distinctly smaller – see above), P1 endopod (proximal segment comparatively longer so that exp-2 is only about 40% the length of enp-1 and the insertion point of the inner seta of enp-1 is located at 17% of the inner margin length), male P3 endopod (with inner seta on enp-3), and the male P5 (with long outer seta on exp-1) rules out conspecificity of the Woods Hole material with M. sarsi sp. nov. Since information about the segmentation of the female antennule, swimming leg armature and morphology of caudal ramus seta V is completely lacking, Wilson’s (1932b) material cannot be attributed distinct specific status at present and is therefore not considered any further. Conversely, the three illustrated reports based on material from Arctic localities (Chislenko 1967, 1977; Mielke 1974) contain sufficient information for a morphological comparison with M. sarsi sp. nov. Based on the descriptions by Chislenko (1967) and Mielke (1974), the Arctic specimens differ from the latter species in the following combination of characters: (a) antennary exopod with two lateral setae on exp-2 (vs one); (b) P1 exopod 75% length of endopod (vs two-thirds); (c) P3–P4 exp-3 with three inner setae (vs two); and (d) male P5 exopod with very long outer seta on exp-1 (vs absent or extremely reduced). They also can be distinguished from M. forficuloides by a number of features: (a) antennary exopod with two lateral setae on exp-1 (vs one); (b) P1 exopod 75% length of endopod (vs two-thirds); (c) P1 exp-2 2.5 times length of exp-1 and 55% length of enp-1 (vs twice and 45%, respectively); (d) P3 exp-1 with inner seta (vs absent); (e) P4 exp-3 with three inner setae (vs two); and (f) male P5 exopod 3-segmented (vs 1-segmented). Chislenko’s (1977) illustrations of the female genital field and P5 (with eight elements on the exopod although its length:width ratio is slightly greater than in the White Sea females) appears to confirm the conspecificity of his Franz Josef Land material with the specimens he had previously reported from the White Sea (Chislenko 1967). Based on the morphological comparison above we are convinced that the Arctic material is sufficiently distinct from both M. forficuloides and M. sarsi sp. nov. to warrant the erection of a new species, which we name M. polaris sp. nov.

Original description. Mielke (1974): 20–22, Abb. 9.

Additional descriptions. Chislenko (1967: 140–144, Figs 45–46); Chislenko (1977: 246, Fig. 6–1, 2).

Type material. The female specimen illustrated by Mielke (1974:Abb. 9A–B) is here designated as the holotype of M. polaris sp. nov. (ICZN Arts 16.4 and 72.5.6). The species can be differentiated by the characters listed in the diagnosis below and those mentioned and illustrated in Mielke (1974) (ICZN Art. 13.1).

Type locality. Svalbard archipelago, Spitsbergen, Longyearbyen; littoral zone.

Differential diagnosis. Microthalestris. Body length 550–800 μm in ♀, 400–420 μm in ♂. Antenna with 2-segmented exopod bearing two setae on exp-1 and two lateral and three apical elements on exp-2; armature of endopod unconfirmed but likely without penicillate spines. P1 exopod about 75% length of endopod; exp-2 elongate, about 2.5 times as long as exp-1, and about 55% length of enp-1; insertion point of inner seta of enp-1 at 25% of inner margin length; exp-3 with two unipinnate spines and two geniculate setae; enp-2 with one minute seta and two non-geniculate claws. Armature pattern of ♀ P2–P4:

P3 endopod ♂ 3-segmented, with apophysis on enp-3, armature pattern [1.1.02 + apo]. P 5 ♀ with elongate exopod (about 2.4 times as long as maximum width), inner margin and proximal half of outer margin straight, with eight elements, proximal outer one long, outer apical one short; endopodal lobe with five elements, innermost one well developed. P 5 ♂ exopod 3-segmented, with seven elements, outer seta of exp-1 very long, longer than or as long as outer basal seta; endopodal lobe with two elements. Armature of P 6 ♂ unconfirmed. Caudal ramus seta V with slightly swollen proximal part.

Etymology. The species name is derived from the Latin polaris, meaning polar, and refers to the Arctic distribution of this species.

Notes. The species appears so far to be restricted to the Arctic Ocean and its marginal waterbodies such as the White Sea. It has been recorded from Kandalaksha Bay (Chislenko 1967), Svalbard (Mielke 1974) and Franz Josef Land (Chislenko 1977). Kornev & Chertoprud (2008: 196–197, Fig. 5.94–A, Б, B) also reported P. spinosa from the White Sea, including an aberrant specimen, and reproduced Chislenko’s (1967) illustrations of the P1 and P5 of both sexes. They mentioned substantial variation in female body length (550–800 μm) but this is a verbatim account of Chislenko’s (1967) measurements. It is also not clear whether the variability included in their armature formulae of P3–P4 (number of inner setae on P3–P4 exp-3 and P4 enp-3; Table 1) is based on their own obervations of White Sea material or reflects variability reported in the literature. Since the authors provided no direct evidence that they were dealing with M. polaris sp. nov., their record is at present to be considered as unconfirmed. This uncertainty is exacerbated by the fact that, despite the reported variability on other segments, the White Sea specimens appear to be consistent in the lack of the inner seta on P3 exp-1 (which is present in M. polaris sp. nov.).

Chislenko (1967) collected two females from washings of the demosponge, Semisuberites cribrosa (Esperiopsidae), in Franz Josef Land but this association is to be regarded as accidental (Huys 2016). Previous records of Thalestris forficula from Arctic localities may refer to M. polaris sp. nov. but in the absence of morphological or other compelling evidence they must remain unconfirmed. These include the records from Lille- Karajak fjord in western Greenland (Vanhöffen 1897), Cape Gertrude in Franz Josef Land (Scott 1899), between Kolguev and Novaya Zemlya (Scott & Scott 1901 – as T. forficulus), Bear Island (Bjørnøya) and Hope Island, Svalbard (Scott & Scott 1901 – as T. forficulus), the Arctic islands north of Grinnell Land, Canada (Sars 1909) and Seydisford in Iceland (Jespersen 1940; Klie 1941 – both as Parastenhelia forficula). Similarly, the Arctic records of Parastenhelia spinosa from Iceland (Ólafsson et al. 2001; Steinarsdóttir et al. 2003; Steinarsdóttir & Ingólfsson 2004), Kandalaksha Bay in the White Sea (Brotskaya 1962; Chertoprud et al. 2005) and from mesozooplankton in the Barents Sea (Dvoretsky & Dvoretsky 2010) require authentication before they can be considered conspecific with M. polaris sp. nov.