Polacanthoderes spp., 2008

Sørensen, Martin V., Macheriotou, Lara, Braeckman, Ulrike, Smith, Craig R. & Ingels, Jeroen, 2025, Antarctic Kinorhyncha: Seven new species from the Antarctic Peninsula, European Journal of Taxonomy 1000 (1), pp. 1-102 : 31-32

Remarks on polymorphism in Polacanthoderes spp.

The description of P. shiraseae also includes a careful re-examination of available P. martinezi specimens (eleven in total), and from both species Yamasaki et al. (2022) reported a relatively high level of intraspecific variation regarding tubes and short acicular spines. Among specimens of P. martinezi they found that a few specimens would have subdorsal tubes on segment 2, subdorsal tubes or short acicular spines on segment 5, and short subdorsal acicular spines on segment 8, whereas the majority of the available specimens would lack these structures. Likewise, they reported the occasional, but yet rather rare, presence of short subdorsal acicular spines on segments 4, 5, and 8 in P. shiraseae .

The new species, P. grzelakae sp. nov., also shows intraspecific variation, which contributes further to the taxonomic challenges. As with P. shiraseae , the polymorphism in P. grzelakae also appears to be expressed mostly in the dorsal series on segments 4, 5, and 8. Of 30 examined specimens, five lacked short subdorsal spines on segment 4, whereas five different specimens had no such spines on segment 5. In addition, another two specimens (of which neither showed spine loss on segments 4 or 5) lacked short laterodorsal acicular spines on segment 8. Additional spine variation was restricted to singletons and included: one specimen with short ventromedial acicular spines on segment 3; one without midlateral tubes on segment 4; one with two pairs of closely positioned, short laterodorsal acicular spines on segment 6; one without short lateral accessory acicular spines on segment 6 (same specimen as the one with double pairs of segment 6 laterodorsal spines); and one specimen with both short sublateral and short lateral accessory acicular spines on segment 7 (all other specimens have no lateral accessory spine on this segment).

The present description of P. grzelakae sp. nov. is based on 30 specimens, whereas Yamasaki et al. (2022) examined around 50 specimens for their description of P. shiraseae . One could obviously always wish for even larger sample sizes, but they still represent enough specimens to provide some hints about where the variation is most expressed and where the differences should be seen as rarities. The data leaves the impression that while all regular acicular spines occur consistently across specimens of Polacanthoderes , nearly all tubes and small acicular spines can potentially vary. However, much of this variation is restricted to singletons and is therefore to some extent neglectable. In both species, higher frequencies of variation are only reached in the occurrence of small acicular spines in the dorsal series of segments 4, 5, and 8. For instance, small subdorsal acicular spines were absent on either segment 4 or 5 in one third of the P. grzelakae specimens.

As mentioned previously, such levels of morphological variation obviously represent a taxonomic challenge for present and future studies of Polacanthoderes , but understanding this polymorphism also makes it easier to comprehend. Thus, we need to acknowledge that eventual future species descriptions of Polacanthoderes spp. have to be based on a sufficiently high number of specimens to be able to detect polymorphic characters. In addition, it can be helpful to keep in mind that we can expect more variation on segments 4, 5, and 8, which in turn makes characters on these particular segments less suitable as species diagnostic.