4.4 Sea surface temperature as a major diversification driver in
marine organisms?
Conversely to pelagic fishes that show no or low inter-ocean structure
(Díaz-Jaimes et al. 2010, Ely et al. 2005) and no intra-ocean structure
(Nomura et al. 2014, Taguchi et al. 2015), sea mammal’s gene flow is
shaped by sea temperature which drives structuration between ocean
basins and among breeding areas (Alexander et al. 2016, Jackson et al.
2014, Richard et al. 2018, Fontaine et al. 2014, Viricel & Rosel 2014).
SST plays also a role in the diversification of sea turtles, since only
cold-adapted species are able to exchange genes among oceans (Dutton et
al. 1999). Finally, organisms with a pelagic larval phase show globally
low structuration (Kelly & Palumbi 2010) but when detected,
structuration is often linked to sea temperature (Benestan et al. 2015;
Teske et al. 2005, 2018). Therefore, SST appears as a generic driver of
diversification in marine organisms, though their patterns of
structuration are generally considerably weaker (see Bowen et al. 2016
for a review) than those found here. We suggest that the reason for this
discrepancy lies in the fact that these seabirds are central place
foragers, i.e. they still depend on terrestrial habitats for breeding,
the latter being impacted for instance by glaciations. They are
therefore highly sensitive to any latitudinal change of SST in
comparison to island distribution, which acts as a constraint since an
optimal area in regard to SST may lack any island for breeding. Indeed,
in marine organisms with low dispersal abilities, patterns of
structuration and divergence are more similar to the patterns found here
on shearwaters. For instance, timing of population divergence between
Atlantic and Indian Ocean lineages and within the Atlantic in the
seahorse Hippocampus ’kuda complex’ (Floeter et al. 2007) fits to
our estimates. This species has no planktonic larval duration (Lourie et
al. 2005) and long dispersal events are considered rare and implying a
few individuals (Teske et al. 2005). Moreover, Cape Agulhas is known to
be a phylogeographic break among several costal species, due to the
difference of currents and sea temperatures between the two oceans
(review in Teske et al. 2011). On the other extreme, seabirds do not
compare either to terrestrial organisms living on islands, despite being
highly philopatric, simply because they can disperse easily, if an
island is available to being colonised.
In this small shearwater complex, geographical barriers and/or isolation
by distance may have been a major driver of differentiation at large
scale (typically, between Oceans) while SST has been a more important
driver at smaller scale (within Oceans), with shearwaters shifting their
breeding latitudes with a changing SST. However, since these seabirds
depend on the geographical distribution of their breeding islands and
because these seabirds are not the best seabird fliers, this
distribution becomes a major constraint resulting in the present
geographical structure, promoting local adaptation to small scale
ecological constraints and reducing gene flow. Therefore, petrels and
shearwaters present an interesting case study where diversification
processes rely more (or at least equally) on ecological factors, in
particular sea surface temperature, rather than distance or continental
barriers, in contrast to either “true” marine organisms or terrestrial
organisms. Strict marine organisms can disperse far more, or
alternatively are unconstrained by island distribution, and thus show
much less geographical structure within taxa. The terrestrial organisms
tend to disperse far less, and isolation-by-distance tends to be a main
driver of population differentiation (Meirmans 2012; Vekemans & Hardy
2004). Indeed terrestrial organisms, such as lizards or birds in
Macaronesia (AlmalkI et al. 2017; Brehm et al. 2003), geckos in Cape
Verde (Arnold et al. 2008) or birds in America (Patel et al. 2011), have
revealed strong splits between islands with no shared haplotypes for the
same mitochondrial markers.