Obura-Journal_of_Biogeography

D. O. Obura
glaring gap is the absence of Palaeogene and Neogene coral
fossils from the WIO, to complement those found in the
Asian coastlines of the Indian Ocean (see Wilson & Rosen,
1998; McMonagle et al., 2011). It is possible this gap may
never be filled, as it appears extensive carbonate deposits
were not created in the WIO, whether due to tectonic inactivity
(affecting shallow deposits) or due to other factors
such as marine climate (see Peterson & Backman, 1990),
deepening the ‘Palaeogene gap’ for Indian Ocean reef-coral
biodiversity (Johnson et al., 2015).
Patterns of coral and reef fish diversity are among the
most extensively studied for tropical marine taxa, and are
widely used as evidence for general patterns that may also be
found in other taxonomic groups (Bellwood & Hughes,
2001; Roberts et al., 2002; Reaka et al., 2008; Veron et al.,
2009; Tittensor et al., 2010; Briggs & Bowen, 2012, 2013;
Bowen et al., 2013). This suggests that the hypotheses presented
here, of deep and shallow evolutionary influences on
coral biogeographical pattern, could be considered in refining
biogeographical classifications for other taxa as well (e.g.
Spalding et al., 2007; Briggs & Bowen, 2012). This is relevant
to current interest in marine conservation, as evolutionary
diversity is not uniformly spread among species, and attention
to old, relict lineages with more unique genetic diversity
can be an important criterion in biodiversity conservation
and management (Jetz et al., 2014; Curnick et al., 2015).
Analyses would be enriched by considering diverse taxa
showing a range of evolutionary rates, from slow to fast (e.g.
Brown et al., 1979; Shearer et al., 2002), to ensure different
processes and periods of genetic differentiation are
addressed.
The tectonically inactive WIO appears to act as a stable
‘museum’ for species, and Obura (2012a) suggests that currents
in the Mozambique Channel, particularly in the north,
accumulate and preserve species in a second hotspot for shallow
marine biodiversity after the Coral Triangle. The present
configuration of currents in the Channel, of energetic mesoscale
eddies in both cyclonic and anticyclonic directions,
result in profound ecosystem and productivity consequences
within the channel, including high connectivity and larval
recruitment (Ternon et al., 2014). The eddies are driven by
vorticity induced in the South Equatorial Current when it is
forced around the northern tip of Madagascar (Backeberg &
Reason, 2010), a feature that has likely persisted throughout
the Neogene and was perhaps also present even during the
Eocene and Oligocene when Africa and Madagascar were further
south, as was the main equatorial current from the east
(Brass et al., 1982). The Mozambique Channel and stable
African continental slopes to the north and south are also
where the coelacanth Latimeria chalumnae has persisted, having
disappeared from the fossil record across the globe at the
Pg/T extinction, marking the start of the Cenozoic (Smith,
1939). This provides additional corroboration for this
hypothesis that the Mozambique Channel forms a tectonically
and oceanographically stable region that preserves old
lineages reliant on continental shelf habitats. Thus this centre
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of diversity (Obura, 2012a) may act as a centre of accumulation,
with complex feedbacks to the centres of origin hypothesized
here being likely (see Bowen et al., 2013).
Finally, two questions emerge on present and future
dynamics. First, will the present condition of high connectivity
across the Indian Ocean lead to greater homogenization
of the Indo-Pacific fauna in the W&NIO above current
levels? The fate of the W&NIO Tethyan relict species is likely
to be eventual loss, though their competitive inferiority with
younger species (Ricklefs, 2011) is belied by their persistence
over tens of millions of years. The contribution of new species
created through ongoing speciation in sub-regions of the
W&NIO (Bowen et al., 2013) should, by contrast, persist.
Second, given the inevitable obliteration of the IAA by continued
continental collision, will the tectonically inactive
WIO become a museum for IAA lineages and W&NIO endemics
as the next biodiversity hotspot for shallow marine species
establishes in a new region of tectonic collision?
ACKNOWLEDGEMENTS
The original work on this hypothesis was supported through
a research grant (MASMA/OR/2008/05) and then a writing
grant (MASMA/books/02/12) from the Marine Science for
Management (MASMA) programme of the Western Indian
Ocean Marine Science Association (WIOMSA). The ideas
presented here have benefited from discussions and common
regional interests with colleagues, in particular with Francesca
Benzoni and Allen Chen, and the work of their collaborators
and students establishing some of the lines of
evidence supporting these hypotheses, and with Melita
Samoilys. My thanks especially go to two anonymous referees
and the editor, whose comments greatly improved the manuscript.
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