Dispersal

Abstract  Corals provide habitat for numerous marine species and ecosystem services for global human populations. However, they are vulnerable to local and global scale threats, especially climate change. Measuring demographic processes such as dispersal and ecological processes such as niche partitioning are important for predicting their responses to disturbances and environmental change. So far, correlations between coral genetics and the environment or spatial scale have largely been made over large habitat distinctions, such as depth, reef zone, and among islands or geographic regions. Reefs comprise structurally heterogenous landscapes and thus microhabitats may vary considerably, however, we have little understanding of how genotypes are distributed within reefs across fine spatial scales. Dynamics at fine spatial scales are particularly important in corals due to the frequent discovery of genetically divergent but morphologically indistinguishable coral taxa found sympatrically within reefs (i.e., cryptic taxa, often with no obvious environmental distinctions) and evidence that dispersal distances may be small for some taxa. Technological advancements in both genomic sequencing and underwater imaging and computation can help to study fine-scale dispersal and determine whether cryptic taxa are ecologically partitioned. Reduced representation sequencing can be conducted on wild populations and gives access to genomic variation across hundreds of individuals. Structure-from-motion photogrammetry enables the characterisation of structural features of the reef and coral colonies within reefs; thus, it is possible to combine high resolution spatial mapping and micro-environment analyses with genotyped colonies using these two technologies. 

Species of the Caribbean hard coral genus Agaricia (Order: Scleractinia) are arrayed over the entire depth range for photosynthetically dependent organisms, making them an ideal target for comparing mesophotic (>30 m depth) and shallow (<30 m) species, evaluating microhabitat differentiation, and assessing spatial structures across depths. My thesis uses this genus to explore questions related to spatial and environmental differentiation between and within taxa at scales from tens of kilometres to centimetres. The first data chapter (Chapter 2) of my thesis focuses on two mesophotic-occurring species: Agaricia grahamae and A. lamarcki. Despite presuming to be brooders with localised dispersal, no spatial population genetic structure was found over 10s of kms in either species. However, two sympatrically occurring cryptic taxa within each species were found. In A. lamarcki these taxa exhibited incomplete depth partitioning between shallow and mesophotic depths, yet taxa within A. grahamae displayed no obvious environmental distinctions. Demographic histories of all taxa were characterised by gene flow among taxa. This chapter exemplifies the complexities found in corals, where (1) spatial genetic structures do not follow expectations, (2) morphologically similar, sympatric taxa exist both at the same depths and differentiated by shallow and mesophotic depths and (3) gene flow among taxa may be important for the evolution of corals. My second and third data chapters (Chapters 3 and 4) focus on fine-scale characterisation of genotypes across 3D-imaged reefscapes within three depth zones (5, 10, 20 m) and among four sites along the leeward side of Curaçao and spread over ~50 km. Chapter 3 describes the delineation of cryptic coral taxa and investigates dispersal within and between depths among all taxa. The cryptic taxa are defined by divergent genotypic clusters occurring sympatrically and some were found to be associated with particular depth profiles. Disparate spatial genetic structures were found among congeners, where taxa within A. agaricites and A. humilis presented isolation-by-distance and dispersal distances across metres and, in contrast, A. lamarcki taxa presented genetic homogeneity at distances >50 km. This chapter provides one of the few estimates of dispersal distances in corals, which is exceedingly low (across metres), highlights the widespread cryptic diversity within corals and finds substantial differences in dispersal, clonality and genetic diversity among congeners. In Chapter 4, I used photogrammetry to characterise the microhabitat around individually genotyped colonies of Agaricia by deriving novel geometric measures. Environmental niches for sympatric cryptic taxa were determined by describing microhabitats that coral colonies inhabit. Species and cryptic taxa exhibited subtle divergences in their physical microhabitat niches. This chapter tackles the question of how cryptic coral taxa co-occur in seemingly similar environments and demonstrates a novel photogrammetric approach to characterise the microhabitat. 

My thesis applies new technologies and methods to help solve some of the mysteries of corals populations, namely, how far do larvae disperse? And what creates or preserves cryptic taxa? And in doing so, provides insight into coral ecology (interaction with microhabitat, spatial distribution, and dispersal) and evolution (cryptic diversification and hybridisation).

For marine reserves to function as effective harvest refuges for exploited species, the reserve must protect a substantial proportion of the population for an indefinite period of time. Because most marine reserves are space-limited, the buildup and equilibrium population sizes of mobile species will be influenced by the size and boundary conditions of the refuge. A logistic rate model was used to predict equilibrium population sizes in a marine harvest refuge, based on species-specific dispersal dynamics and the spatial configuration of the refuge. The model parameters were derived for Caribbean spiny lobsters and queen conch in an isolated marine reserve at Glover’s Reef, Belize, and were compared to observed population change over a 5-yr period. Spiny lobsters and queen conch, the two most heavily exploited species in the Caribbean, differ in larval recruitment rates (immigration) and mobility of adults (emigration). The expected increase in the population size of spiny lobsters in this refuge was 250% and queen conch was 420% over that of the initial fished population. The observed densities of lobsters and conch in the refuge approached the predicted estimates within three years. To further explore the impact of alternative spatial configurations on refuge populations, the model was run on the same populations in two hypothetical refuges. In a refuge of the same area but 50% less absorbing boundary (adjacent to intensively fished areas), the spiny lobster population was expected to be 30% larger than the equilibrium population size in the original refuge, whereas the queen conch population was not expected to change from that in the original refuge. In a refuge that was 50% larger and with 50% less absorbing boundary, the spiny lobster population was expected to increase 110% and the queen conch population was expected to increase 50% over the equilibrium population size in the original refuge. Relatively minor changes in refuge area and boundary conditions may thus result in major population-level responses by exploited species, depending on dispersal dynamics and habitat availability. This simple model may be applicable for rapid assessment of the potential efficacy of proposed harvest refuges. 

A three-year field study (January 2011–December 2013) of the Odonata of Curaçao, supported by photos and exuvial collections, recorded a total of 21 species from the island, almost doubling its previously known fauna. The lists of Odonata known from Aruba and Bonaire were also updated by specimen and photo records, and 24 species are now known from these three islands. During the period of the study, odonates decreased in abundance and diversity in Curaçao, apparently because heavy rains just before the study began led to colonization of the island by several nonresident species that subsequently declined and disappeared as wetlands diminished during a period with normal rainfall.

Abstract:

The ability of coral reefs to recover from natural and anthropogenic disturbance is difficult to predict, in part due to uncertainty regarding the dispersal capabilities and connectivity of their reef inhabitants. We developed microsatellite markers for the broadcast spawning gorgonian octocoral Eunicea (Plexaura) flexuosa (four markers) and its dinoflagellate symbiont, Symbiodinium B1 (five markers), and used them to assess genetic connectivity, specificity and directionality of gene flow among sites in Florida, Panama, Saba and the Dominican Republic. Bayesian analyses found that most E. flexuosa from the Florida reef tract, Saba and the Dominican Republic were strongly differentiated from many E. flexuosa in Panama, with the exception of five colonies from Key West that clustered with colonies from Panama. In contrast, Symbiodinium B1 was more highly structured. At least seven populations were detected that showed patterns of isolation by distance. The symbionts in the five unusual Key West colonies also clustered with symbionts from Panama, suggesting these colonies are the result of long-distance dispersal. Migration rate tests indicated a weak signal of northward immigration from the Panama population into the lower Florida Keys. As E. flexuosa clonemates only rarely associated with the same Symbiodinium B1 genotype (and vice versa), these data suggest a dynamic host–symbiont relationship in which E. flexuosa is relatively well dispersed but likely acquires Symbiodinium B1 from highly struc- tured natal areas prior to dispersal. Once vectored by host larvae, these symbionts may then spread through the local population, and/or host colonies may acquire different local symbiont genotypes over time. 

Abstract: 

The mechanisms by which algae disperse across space on coral reefs are poorly known. We inves- tigated the ability of four common Caribbean herbivorous fish species to disperse viable algal fragments through consumption of macroalgae and subsequent defecation. Fragments of all major algal taxa (Phaeophyta, Rhodophyta, and Chlorophyta) were found in 98.7 % of the fecal droppings of all fish species; however, the ability to survive gut passage and reattach to a substrate differed between algal taxa. While survival and reattachment approached zero for Phaeophyta and Chlorophyta, 76.4 % of the fragments belonging to the group Rhodophyta (mostly species in the order Gelidiaceae) survived gut passage, and were able to grow and reattach to the substrate by forming new rhizoids. Our results thus show that Gelidid algal species are dispersed by swimming herbivores. While the relative contribution of this mechanism to overall algal dispersal and recruitment in a wider ecological context remains unknown, our findings illustrate a previously undescribed mechanism of algal dispersal on coral reefs which is analogous to the dispersal of terrestrial plants, plant fragments, and seeds via herbivore ingestion and defecation.