Geographic differences in vertical connectivity in the Caribbean coral Montastraea cavernosa despite high levels of horizontal connectivity at shallow depths

Abstract

The deep reef refugia hypothesis proposes that deep reefs can act as local recruitment sources for shallow reefs following disturbance. To test this hypothesis, nine polymorphic DNA microsatellite loci were developed and used to assess vertical connectivity in 583 coral colonies of the Caribbean depth-generalist coral Montastraea cavernosa. Samples were collected from three depth zones (≤10, 15-20 and ≥25 m) at sites in Florida (within the Upper Keys, Lower Keys and Dry Tortugas), Bermuda, and the U.S. Virgin Islands. Migration rates were estimated to determine the probability of coral larval migration from shallow to deep and from deep to shallow. Finally, algal symbiont (Symbiodinium spp.) diversity and distribution were assessed in a subset of corals to test whether symbiont depth zonation might indicate limited vertical connectivity. Overall, analyses revealed significant genetic differentiation by depth in Florida, but not in Bermuda or the U.S. Virgin Islands, despite high levels of horizontal connectivity between these geographic locations at shallow depths. Within Florida, greater vertical connectivity was observed in the Dry Tortugas compared to the Lower or Upper Keys. However, at all sites, and regardless of the extent of vertical connectivity, migration occurred asymmetrically, with greater likelihood of migration from shallow to intermediate/deep habitats. Finally, most colonies hosted a single Symbiodinium type (C3), ruling out symbiont depth zonation of the dominant symbiont type as a structuring factor. Together, these findings suggest that the potential for shallow reefs to recover from deep-water refugia in M. cavernosa is location-specific, varying among and within geographic locations likely as a consequence of local hydrology.

The Future of Coral Reefs Subject to Rapid Climate Change: Lessons from Natural Extreme Environments

“…van Oppen et al (2011) found evidence of deep-to-shallow coral recruitment for some reefs (Scott's Reef) on the GBR, but not others (Yonge Reef). Similarly, Serrano et al (2014) found geographic differences in vertical connectivity due to local geophysical conditions. Species-specific differences in population connectivity with depth have also been reported in the Caribbean (Bongaerts et al, 2017).…”

“…This, in part, results from the heterogeneity of coral environments on the scale of individual reefs. For example, local hydrography likely explains why some deep reefs, but not others, are capable of seeding shallow reefs (van Oppen et al, 2011;Serrano et al, 2014). Similarly, differences in local physicochemical conditions see mangroves of the U.S. Virgin Islands considered a coral refuge, while mangrove systems in the Seychelles, Indonesia and in New Caledonia are not (Camp et al, 2016a(Camp et al, , 2017.…”

Theme section on mesophotic coral ecosystems: advances in knowledge and future perspectives

“…However, Serrano et al (2014) observed genetic differentiation by depth (\10, 15-20 and C25 m) in Florida, but not in Bermuda or the USVI. However, sampling only extended significantly into mesophotic depths in Bermuda.…”

“…Previous genetic studies in the Caribbean basin and elsewhere found vertical connectivity in Montastraea cavernasa, a broadcast spawner (Brazeau et al 2013;Serrano et al 2014). However, Serrano et al (2014) observed genetic differentiation by depth (\10, 15-20 and C25 m) in Florida, but not in Bermuda or the USVI.…”

“…Brazeau et al (2013) collected M. cavernosa at shallow (3-10 m), medium (15-25 m), deep (30-50 m) and very deep (60-90 m) depths and found significant genetic differentiation between shallow/medium depths and very deep depths at both Little Cayman Island and the Bahamas, indicative of low vertical population connectivity. Both Serrano et al (2014) and Brazeau et al (2013) invoked selection and asymmetric vertical connectivity from shallow to deeper reefs suggesting that mesophotic reefs may actually be receiving larval subsidies from SWRs. A similar spatial heterogeneity in vertical structure was reported for Seriatopora hystrix, a brooding coral, between northwest and northeast Australia (van Oppen et al 2011).…”