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.

Figure 1. Map indicating regions of seamount collections.

Overlaid is the distribution of haplotypes for the NCR1 gene. For clarity, only the three most common haplotypes are shown. Haplotype 1 occurred in Narella in Hawaii, but only in Parastenella and Primnoa in the GOA. Satellite imagery: GoogleEarth. Date accessed: 05 Jan 2011. Co-ordinates: approx. 18 to 61°N, 168°E to 114°W.

Figure 2. Geographic distribution of combination haplotypes found in Alaskan waters.

A. Distribution for combination haplotypes that occur on Derickson Seamount. No combination haplotypes on Derickson were shared with the GOA Seamounts, overlap with Hawaii is shown. Satellite imagery: GoogleEarth. Date accessed: 05 Jan 2011. Co-ordinates: approx. 18–58°N, 168°E to 150°W. B. Geographic distribution of combination haplotypes found in the Gulf of Alaska. Numbers within a circle indicate number of individuals with that haplotype when greater than 1 for a feature. Satellite imagery: GoogleEarth. Date accessed: 05 Jan 2011. Co-ordinates: approx. 52° to 59°30′N, 153°50′ to 143°20′W.

Figure 3. Geographic distribution of combination haplotypes found in the Hawaiian Archipelago.

The upper panel provides a geographic context for the features of interest. Land is black and the 1000 m isobath is provided. The lower panel provides a depth scale to demonstrate the difference in depth range between haplotypes of a given morphotype(s). Base map prepared in ESRI ArcMap 10.0 using bathymetric data from GEBCO http://www.gebco.net/(30 arc second version) and terrestrial data from http://www.naturalearthdata.com/. Accessed 01 July 2011.

Figure 4. Depth distribution of Narella combination haplotypes sorted by minimum depth of occurrence.

Combination haplotypes, as designated in Table 1, are given in the columns, along with the first three letters of the species name. 100 m depth bins are provided in the rows. Dark blue indicate an actual depth for a given haplotype, light blue indicates possible range for Haplotype 3 and is used to fill in the depth range for other columns. Numbers indicate number of individuals in a given depth range with that haplotype when the value is greater than 1. ? - indicates mean of possible range of depths for specimens for which depth was not recorded.

Figure 5. Maximum likelihood tree for taxa that amplified for all markers, based on 3348 nt alignment of combination haplotypes.

Values above the line are Bayesian posterior probabilities and below the lines are maximum likelihood bootstrap values as percent of 1000 bootstraps. CH numbers correspond to combination haplotype designations as given in Table 1. Narella species names in bold indicate specimens collected in Alaska.