Populations of the coral species Montastraea cavernosa on the Belize Barrier Reef lack vertical connectivity

Abstract

Larval connectivity among and within coral reefs is important for sustaining coral metapopulations, enhancing ecosystem resilience through species and genetic diversity, and maintaining reef ecosystems’ structure and functions. This study characterized genetic structure and assessed horizontal and vertical connectivity among populations of the ubiquitous gonochoric broadcast spawning coral Montastraea cavernosa in Belize. Using nine polymorphic microsatellite loci, we genotyped M . cavernosa colonies from four depth zones at four study sites within Belizean marine management zones. Study sites were selected within South Water Caye Marine Reserve (3 sites) and Glover’s Reef Marine Reserve (1 site). Strong contemporary genetic differentiation was observed between relatively shallow M . cavernosa populations (10 m, 16 m) and relatively deep (25 m, 35 m) populations, coinciding with a transition from reef crest to reef slope. These results were consistent across both marine reserves. Vertical and horizontal migration models suggest that all populations were historically panmictic, with little unidirectional migration. The relative local isolation of shallow and mesophotic M . cavernosa populations in Belize, coupled with the importance of Belize’s upper mesophotic populations as potential larval sources for other areas in the Tropical Western Atlantic, reinforces the need for management strategies that conserve coral populations across all depth zones.

Depth Influences Symbiodiniaceae Associations Among Montastraea cavernosa Corals on the Belize Barrier Reef

“…In these instances, significant differences were observed in the lower mesophotic zone (>60 m). However, in Curaçao there was also a significant shift in symbiont community profile observed at 25 m for M. cavernosa (Bongaerts et al, 2015a), the same depth at which we have seen differences in both symbiont communities and M. cavernosa genetic structure in Belize (Eckert et al, 2019). While there is evidence for Symbiodiniaceae community shifts with increasing depth, these patterns do not appear to be universal.…”

“…Approximately 15 M. cavernosa colonies were sampled at each of the four depth zones (back reef ∼10 m; reef crest ∼16 m; fore reef ∼25 m; upper mesophotic ∼35 m; Figure 1) per reef site (n = 242). Sample collection and initial processing are detailed in Eckert et al (2019). After field collection and processing at Carrie Bow Cay, samples were preserved in TRIzol reagent and initially stored at -20 • C. Samples were then transported to FAU-HBOI on ice and stored at -80 • C until genomic DNA extraction.…”

“…Total genomic DNA was extracted using a modified cetyl trimethylammonium bromide (CTAB) extraction (Mieog et al, 2009) as in Eckert et al (2019). Following DNA extraction, all samples were cleaned with the Zymo Research DNA Clean & Concentrator-5 kit to enhance downstream polymerase chain reaction (PCR) amplification.…”

“…Due in part to the species overlap with shallow reefs, MCEs are hypothesized to be potential refuges for shallow reefs (i.e., the Deep Reef Refugia Hypothesis; Glynn, 1996;Bongaerts et al, 2010;Lesser et al, 2018;Bongaerts and Smith, 2019). Multiple studies have examined the potential for "reseeding" of shallow reefs with larvae from MCE coral counterparts using molecular methods to quantify levels of genetic connectivity between these habitats (Brazeau et al, 2013;Bongaerts et al, 2017;Studivan and Voss, 2018;Eckert et al, 2019). It is important to evaluate symbiont community assemblages as well as coral genetic structure when assessing connectivity of shallow and mesophotic reefs to give insight into the potential barriers to vertical connectivity for depth-generalist scleractinians (Bongaerts et al, 2010).…”

“…These reefs exhibit spur and groove structures to a depth of 20-33 m, a near-vertical step from 30-37 m, and a sloping reef wall continuing to >100 m (James and Ginsburg, 1979; Figure 1). Populations of the depth-generalist, broadcast spawning, scleractinian coral Montastraea cavernosa lack gene flow between relatively shallow (10 and 16 m) and deep (25 and 35 m) populations within the South Water Caye and Glover's Reef Marine Reserves (Eckert et al, 2019). To better understand if the identity of algal endosymbionts associated with M. cavernosa populations in Belize followed similar patterns as observed in genetic structuring of their coral hosts, we characterized the Symbiodiniaceae found within previously genotyped M. cavernosa samples using the highthroughput sequencing of the ITS2 marker and the SymPortal analysis framework.…”