Myxicola, Koch

Putignano, Matteo, Langeneck, Joachim & Giangrande, Adriana, 2024, The forgotten diversity of the genus Myxicola (Polychaeta: Sabellidae) in North America: redescription of historical taxa and description of two new species, Journal of Natural History 58 (37 - 40), pp. 1393-1427 : 1419-1424

American Myxicola species

All the members of Myxicola species examined herein can be distinguished from each other on the base of important morphological differences ( Figure 17 ). A first group is composed of M. bushae sp. n. and M. affinis , characterised by smaller size, tubular ventral lips and abdominal uncini of similar morphology, always showing at least one long tooth around half length of main fang. However, members of those species can be easily separated from one another on the basis of the dorsal radiolar appendages (long and filiform in members of M. affinis , not visible in M. bushae sp. n.), thoracic uncini (which differ between species in the length and curvature of the main fang and the number and disposition of the teeth above it) and arrangement of teeth on abdominal uncini. In fact, it is remarkable how all Pacific American Myxicola species previously synonymised with M. infundibulum show teeth above abdominal uncini arranged in such a way that secondary teeth are apical to the primary one (ie M. affinis , M. conjuncta and M. pacifica , although the latter show smaller teeth flanking secondary tooth; see Putignano et al. 2023 and Figure 16E, F ); in contrast, members of taxa from the Atlantic coast show teeth lying on the same plane, similarly to European and Mediterranean taxa ( Putignano et al. 2023).

A second group includes M. boki sp. n. and M. conjuncta , both larger species, sharing well-developed and low ventral lips. However, members of the former taxon possess more open ventral lips and dorsal lips with conspicuous dorsal radiolar appendages, while specimens of the latter have ventral lips closer to each other and dorsal radiolar appendages are not visible. Also, the abdominal uncini seem to have similar morphology, as members of both species exhibit the presence of a vestigial handle and a main fang distinctly shorter than breast, but the disposition and number of apical teeth, the thickness of the main fang and the shape of the breast differ between members of these species, as does the morphology of the peristomial triangular lobe and radiolar tips.

Myxicola glacialis accounts for a third group, well separated from all the other taxa by the number of thoracic chaetigers. Members of this species are most similar to specimens of M. conjuncta , especially those of the ‘long form’. In addition, members of the two species have almost identical morphology of both thoracic and abdominal uncini; however, they drastically differ in the number, length and arrangement of apical secondary teeth, which are placed in a row in members of M. glacialis , resembling the pattern of Atlantic American and European taxa. In addition, individuals belonging to this species show a peculiar structure of lips and radiolar appendages, so far never found in Atlantic, Pacific or Mediterranean taxa.

Generally, the number, shape and disposition of radiolar skeleton cells are not identical for any of the analysed taxa. The highest similarity concerning these characters can be observed between members of M. boki sp. n. and M. conjuncta , of which specimens belonging to the former show a radiolar skeleton composed of 7 cells arranged radially, in cross section, while cross sections of the radioles of members of the latter species show 8 rows of skeletal cells arranged around a central row (9th cell).

(drawings of M. pacifica from Putignano et al. 2023).

Interestingly, the number of radiolar skeleton cells clearly separates members of the American Myxicola species from their Mediterranean and Eastern Atlantic congeners, as these latter have no more than 2 cells arranged in a row ( Darbyshire 2023; Putignano et al. 2023). A similar observation can be made for the presence of some sort of vestigial handle, a feature completely absent in the Myxicola species from the Old World. Despite these evident differences, which are consistently considered taxonomically informative for the family Sabellidae , and despite the redescription by Putignano et al. (2023), Darbyshire (2023) still included Myxicola pacifica among the synonyms of M. infundibulum . The present work shows that the differences highlighted by Putignano et al. (2023) between M. pacifica and the European M. infundibulum group can be generalised to all North American species here examined. Further studies are needed to ascertain whether these differences can also be considered phylogenetically informative for the genus Myxicola .

Lastly, it should be noted that the photographs included in Bush (1905) show specimens quite similar to the ones analysed herein, supporting the hypothesis (already backed by the original labels) that they correspond to type material. Interestingly, the long form of M. glacialis reported by Bush shows a different colouration respect to the ‘short form’, making the longer one appear much more similar to M. conjuncta . In our study we could analyse only specimens of the short form; thus, further analyses should be taken to ascertain that both forms belong to the same species.

Figures

Figure 16. Photographs of thoracic and abdominal uncini, respectively, of American Myxicola species: A, B. M. affinis; C, D. M. glacialis; E, F. M. pacifica.

Figure 17. Summary of the main characters used in the morphological analysis of American Myxicola taxa