Vertical and horizontal genetic connectivity in Chromis verater, an endemic damselfish found on shallow and mesophotic reefs in the Hawaiian Archipelago and adjacent Johnston Atoll.

Kimberly A Tenggardjaja,Giacomo Bernardi,Brian W Bowen,Yong-Gang Yao

doi:10.1371/journal.pone.0115493

Kimberly A Tenggardjaja, Giacomo Bernardi + Show 2 more

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https://doi.org/10.1371/journal.pone.0115493

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Abstract

Understanding vertical and horizontal connectivity is a major priority in research on mesophotic coral ecosystems (30–150 m). However, horizontal connectivity has been the focus of few studies, and data on vertical connectivity are limited to sessile benthic mesophotic organisms. Here we present patterns of vertical and horizontal connectivity in the Hawaiian Islands-Johnston Atoll endemic threespot damselfish, Chromis verater, based on 319 shallow specimens and 153 deep specimens. The mtDNA markers cytochrome b and control region were sequenced to analyze genetic structure: 1) between shallow (<30 m) and mesophotic (30–150 m) populations and 2) across the species' geographic range. Additionally, the nuclear markers rhodopsin and internal transcribed spacer 2 of ribosomal DNA were sequenced to assess connectivity between shallow and mesophotic populations. There was no significant genetic differentiation by depth, indicating high levels of vertical connectivity between shallow and deep aggregates of C. verater. Consequently, shallow and deep samples were combined by location for analyses of horizontal connectivity. We detected low but significant population structure across the Hawaiian Archipelago (overall cytochrome b: ΦST = 0.009, P = 0.020; control region: ΦST = 0.012, P = 0.009) and a larger break between the archipelago and Johnston Atoll (cytochrome b: ΦST = 0.068, P<0.001; control region: ΦST = 0.116, P<0.001). The population structure within the archipelago was driven by samples from the island of Hawaii at the southeast end of the chain and Lisianski in the middle of the archipelago. The lack of vertical genetic structure supports the refugia hypothesis that deep reefs may constitute a population reservoir for species depleted in shallow reef habitats. These findings represent the first connectivity study on a mobile organism that spans shallow and mesophotic depths and provide a reference point for future connectivity studies on mesophotic fishes.

Highlights

The majority of coral reef ecosystems studied to date occur at depths shallower than 30 m, yet zooxanthellate corals can extend to depths of over 150 m [1]
Our study addresses two primary issues: 1) vertical connectivity between shallow and mesophotic populations of C. verater and 2) horizontal connectivity across mesophotic populations and across the geographic range of this species
A total of 719 bp of cytb and 394 bp of control region (CR) were resolved for 319 shallow and 153 mesophotic C. verater specimens, including those from Johnston Atoll

Summary

Introduction

The majority of coral reef ecosystems studied to date occur at depths shallower than 30 m, yet zooxanthellate corals can extend to depths of over 150 m [1]. In some areas where shallow reefs thrive, strong thermoclines can prevent the development of mesophotic reefs [4], and the depth at which light is not sufficient to support zooxanthellae defines the lower limit of MCEs [1, 3]. Most connectivity studies on shallow-reef organisms have assessed horizontal connectivity across the range of a given species, whereas vertical connectivity refers to the movement of individuals between depth zones. One of the major motivations for understanding vertical connectivity is evaluating the possibility that mesophotic reefs can seed shallow reefs. Given the vulnerability of MCEs to anthropogenic effects that plague shallow reefs [6], an additional motivation for studying connectivity in these ecosystems is to prevent the loss of potentially unique genetic diversity

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