Niche differences in co-occurring cryptic coral species (Pocillopora spp.)

Cryptic species that are morphologically similar co-occur because either
the rate of competitive exclusion is very slow, or because they
are not, in fact, ecologically similar. The processes that
maintain cryptic local diversity may, therefore, be particularly
subtle and difficult to identify. Here, we uncover differences
among several cryptic species in their relative abundance across
a depth gradient within a dominant and ecologically important
genus of hard coral, Pocillopora. From extensive sampling unbiased
towards morphological characters, at multiple depths on the
forereef around the island of Mo’orea, French Polynesia, we
genetically identified 673 colonies in the Pocillopora species
complex. We identified 14 mitochondrial Open Reading
Frame haplotypes (mtORFs, a well-studied and informative species
marker used for pocilloporids), which included at least six
nominal species, and uncovered differences among haplotypes in
their relative abundance at 5, 10, and 20 m at four sites
around the island. Differences in relative haplotype abundance
across depths were greater than differences among sites separated
by several kilometers. The four most abundant species are often
visibly indistinguishable at the gross colony level, yet they
exhibited stark differences in their associations with light
irradiance and daily water temperature variance. The pattern of
community composition was associated with frequent cooling in
deeper versus shallower water more than warmer temperatures in shallow
water. Our results indicate that these cryptic species are not
all ecologically similar. The differential abundance of
Pocillopora cryptic species across depth should promote
their coexistence at the reef scale, as well as promote
resilience through response diversity.