Connectivity of two scleractinian corals in the south west Indian Ocean.
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Date
2010
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Abstract
Generations of hard corals have built the complex reef ecosystems that harbour a huge diversity of
sea-life in the world’s shallow tropical oceans. These undergo both sexual and clonal reproduction,
and may contain signatures in their genomes which help to decipher the riddles of past population
dynamics and evolutionary history. Two species of coral, Acropora austera and Platygyra
daedalea, were collected from sites along the east African coastline from Kenya in the north to
Maputaland, South Africa in the south, and from the Chagos Archipelago. Sequences of two
different DNA regions were tested, in a preliminary study, for their potential ability to elucidate
connectivity and differentiation among these coral populations. These were the nuclear ribosomal
ITS region of P. daedalea populations, and a previously-unused marker, the carbonic anhydrase
3/550 nuclear intron of A. austera. These molecular markers indicated high levels of connectivity
amongst populations in a preliminary study based on limited sample sizes and a subset of
populations. It was decided to further explore the variability of the carbonic anhydrase 3/550
intron, which showed evidence of subdivision and structuring within Mozambique populations
relative to South African populations, in a study in which both the sample size per site and the
number and range of sampled sites were increased. ITS sequences, although highly variable,
revealed no population differentiation in P. daedalea; STR markers were used in subsequent studies
of population differentiation in this species. Populations of both A. austera and P. daedalea showed
signs of high connectivity along the region of the coastline sampled in this study. However, there
appeared to be a disjunction in ecological connectivity between reefs in Maputaland, South Africa
and those in southern Mozambique, between Durban and Maputo where the Agulhas Current
originates. This was reinforced in A. austera populations which displayed a region of genetic
discontinuity between Inhaca Island and Maputaland reefs of the central reef complex, in the region
of Rabbit Rock. Northern reef complexes also harboured unique haplotypes in contrast to southern
reefs which shared all haplotypes with those in the north, an indication that northern reefs have
seeded the southern (Maputaland) reefs. P. daedalea populations appeared evolutionarily panmictic
over scales relevant to this study. Evidence for fine-scale structure indicated that populations were
separated from one another over ecologically relevant time-scales. These populations were defined
by both their habitats and their sampling location. There was a possibility that the Platygyra
species complex included cryptic species that were not distinguishable from P. daedalea. However,
the disjunction in the connectivity between northern and southern population groups was also
evident in the population structure of P. daedalea. There was a net immigration of propagules of
both P. daedalea and A. austera into populations north of the disjunction between groups, where the prevailing current regime is dictated by the Mozambique Channel eddies. In contrast populations to the south of the disjunction (the southern population group) which are subject to the swiftly flowing Agulhas Current, showed a net emigration of propagules from Maputaland reefs. These emigrants were likely to be lost to inhospitable habitat south of the marginal Maputaland region. Although there was evidence for migration of both Platygyra and Acropora propagules between the Bazaruto Archipelago reefs and certain Maputaland reefs, genetic exchange between Mozambique and Maputaland reefs appeared to be limited and may have occurred primarily at evolutionary rather than demographic levels. Managers may need to treat the regional Maputaland reefs as separate stocks and manage them accordingly, as the relative isolation of these corals in the central and southern reef complexes in Maputaland, South Africa, means that they are at risk to losing species to evolutionary extinction. It is also important that reef health in northern Mozambique and Tanzania is maintained as, despite evidence of a break in demographic connectivity, between reefs in these regions and those in Maputaland, there was evidence to suggest that reefs were connected at evolutionary scales, thus maintaining levels of genetic diversity on southern African reefs.
Description
Thesis (Ph.D.)-University of KwaZulu-Natal, Westville, 2010.
Keywords
Coral reefs and islands., Acropora., Daedalea., Scleractinia., Theses--Environmental biology.