Comparing Molecular Variation to Morphological Species Designations in the Deep-Sea Coral Narella Reveals New Insights into Seamount Coral Ranges

Abstract

Recent studies have countered the paradigm of seamount isolation, confounding conservation efforts at a critical time. Efforts to study deep-sea corals, one of the dominant taxa on seamounts, to understand seamount connectivity, are hampered by a lack of taxonomic keys. A prerequisite for connectivity is species overlap. Attempts to better understand species overlap using DNA barcoding methods suggest coral species are widely distributed on seamounts and nearby features. However, no baseline has been established for variation in these genetic markers relative to morphological species designations for deep-sea octocoral families. Here we assess levels of genetic variation in potential octocoral mitochondrial barcode markers relative to thoroughly examined morphological species in the genus Narella. The combination of six markers used here, approximately 3350 bp of the mitochondrial genome, resolved 83% of the morphological species. Our results show that two of the markers, ND2 and NCR1, are not sufficient to resolve genera within Primnoidae, let alone species. Re-evaluation of previous studies of seamount octocorals based on these results suggest that those studies were looking at distributions at a level higher than species, possibly even genus or subfamily. Results for Narella show that using more markers provides haplotypes with relatively narrow depth ranges on the seamounts studied. Given the lack of 100% resolution of species with such a large portion of the mitochondrial genome, we argue that previous genetic studies have not resolved the degree of species overlap on seamounts and that we may not have the power to even test the hypothesis of seamount isolation using mitochondrial markers, let alone refute it. Thus a precautionary approach is advocated in seamount conservation and management, and the potential for depth structuring should be considered.

Population Differentiation and Species Formation in the Deep Sea: The Potential Role of Environmental Gradients and Depth

“…For example, depth related divergence between populations of the coral Eunicea flexuosa appears to be related to strong environmental selection against ecophenotypes from contrasting depths that reduces gene flow and may ultimately lead to speciation [37]. Similar inferences were made for another shallow-water Caribbean coral Faviafragum [124] and for several deep-water corals [101,125,129,131]. In the coral Seriatopora hystrix , reciprocal transplants of depth-segregated, genetically distinct ecotypes implicated post-settlement selection against migrants from parts of the reef formation at different depths [123,132].…”

Use of RAD sequencing for delimiting species

“…First, as we did not find 100% congruence between RAD clades and mtMutS haplotypes, and tested only a restricted set of putative species, mtMutS should still be considered as one of the first steps in an integrative taxonomic loop incorporating more variable markers (for example, Schlick-Steiner et al, 2010;Kekkonen and Hebert, 2014). Second, the evolutionary speed of mtMutS may well vary among octocorals, and its resolving power may therefore vary from one group to another (for example, Baco and Cairns, 2012). Nevertheless, combining mitochondrial markers such as mtMutS and RAD-tag data will without doubt be of tremendous value for testing the large number of outdated species hypotheses within the Octocorallia.…”

Environmental variability and biodiversity of megabenthos on the Hebrides Terrace Seamount (Northeast Atlantic)

“…The presence of internal tides, heterogeneity in seabed substrata, and temperature changes often vary with depth on a single seamount, creating substantial changes in species turnover45 and genetic variability6 even on the same seamount.…”