Pragmatic Assignment of Species Groups Based on Primary Species Hypotheses: The Case of a Dominant Component of the Southern Ocean Benthic Fauna

Abstract

Ecological studies that enhance our understanding of the structure and function of the natural world rely heavily on accurate species identification. With rapid sample accumulation and declining taxonomic expertise, cladistics, phylogenetics and coalescent-based analyses have become key tools for identification or discrimination of species. These tools differ in effectiveness and interpretation depending on researcher perspective and the unique evolutionary histories of the taxa. Given the cost and time required for taxonomic assessment of ambiguous species groups, we advocate a pragmatic approach to clarify species assignment. We carried out a case-study on species from the diverse ophiuroid genus Ophiacantha common in shelf habitats around the Southern Ocean. Although several of the species are formally described with clear and distinctive morphological characters and reproductive strategies (O. vivipara, O. pentactis, O. densispina, O. antarctica, and O. wolfarntzi), recent molecular data has highlighted issues with these morphospecies, the characters that formally define them and their evolutionary histories. Here we provide evidence that key morphological features of species can be deceptive and show that six-armed O. vivipara, for example, is not a widely distributed Southern Ocean species as currently accepted, rather, three disparate clades. Ophiacantha pentactis, described as having five arms, frequently has six arms and the six-armed form is mistakenly classified as O. vivipara. All six-armed specimens collected from the Antarctic continental shelf fall into the O. pentactis species clade. Molecular tools designed for species delimitation appear to fail to reflect the “true” species composition. Rather than rely on a single tool for species recognition, we advocate an integrated approach using traditional detailed taxonomic morphology, summary statistics of molecular sequence data from populations, robust phylogenies, sufficient geographical sampling and local biological knowledge to ensure that species hypotheses can be built on mutually supporting lines of evidence.

Genomic insights of evolutionary divergence and life history innovations in Antarctic brittle stars

“…Emerging evidence has also highlighted that contrasting life histories can be found between closely-related ophiuroid species in the Southern Ocean, with brooding strategies mainly found around Antarctic islands and broadcasting strategies mainly observed on the Antarctic continental shelf (Jossart et al, 2019;Sands et al, 2021; this study). For example, there are strong parallels in the case of 5 vs 6 arms and brooding vs broadcasting between Ophiontous and Ophiosabine (Sands et al, 2021). Brooding and broadcasting in congeneric species have also been reported in other extant echinoderms in the Southern Ocean, including the Astrotoma agassizii complex (Jossart et al, 2019).…”

“…Finally, cluster 3 (of K = 3) is mostly observed around the Antarctic continental shelf (with one exception individual in Bransfield Mouth), suggesting genetic homogeneity around the shelf. Connectivity around the Antarctic continental shelf has also been observed in other Southern Ocean benthic species linking to the ACC (Hemery et al, 2012; Matschiner et al, 2009; Raupach et al, 2010; Sands et al, 2015; Sands et al, 2021). Alternatively, such circularity could also be linked to the eastward‐flowing Antarctic Slope Current (Thompson et al, 2018).…”

“…The Ross Gyre is a cyclonic regional gyre in the Southern Ocean that connects the ACC to the Ross Sea continental shelf, which would explain the presence of cluster 2 connecting Adélie Land, the Balleny Islands, Scott Island and the Ross Sea. Matschiner et al, 2009;Raupach et al, 2010;Sands et al, 2015;Sands et al, 2021). Alternatively, such circularity could also be linked to the eastward-flowing Antarctic Slope Current (Thompson et al, 2018).…”

Not All That Glitters Is Gold: Barcoding Effort Reveals Taxonomic Incongruences in Iconic Ross Sea Sea Stars

“…Molecular tools differ in effectiveness and interpretation in relation both to the research question and the unique evolutionary histories of the taxa [27]. They are proving particularly useful and efficient in the case of Antarctic sea stars.…”