Speciation‐by‐Depth on Coral Reefs: Sympatric Divergence with Gene Flow or Cryptic Transient Isolation?

The distributions of many sister species in the sea overlap geographically but are partitioned along depth gradients. The genetic changes leading to depth segregation may evolve in geographic isolation as a prerequisite to coexistence or may emerge during primary divergence leading to new species. These alternatives can now be distinguished via the power endowed by the thousands of scorable loci provided by second‐generation sequence data. Here, we revisit the case of two depth‐segregated, genetically isolated ecotypes of the nominal Caribbean candelabrum coral Eunicea flexuosa. Previous analyses based on a handful of markers could not distinguish between models of genetic exchange after a period of isolation (consistent with secondary contact) and divergence with gene flow (consistent with primary divergence). Analyses of the history of isolation, genetic exchange and population size based on 15,640 new SNP markers derived from RNAseq data best support models where divergence began 800K BP and include epochs of divergence with gene flow, but with an intermediate period of transient isolation. Results also supported the previous conclusion that recent exchange between the ecotypes occurs asymmetrically from the Shallow lineage to the Deep. Parallel analyses of data from two other corals with depth‐segregated populations (Agaricia fragilis and Pocillopora damicornis) suggest divergence leading to depth‐segregated populations may begin with a period of symmetric exchange, but that an epoch of population isolation precedes more complete isolation marked by asymmetric introgression. Thus, while divergence‐with‐gene flow may account for much of the differentiation that separates closely related, depth‐segregated species, it remains to be seen whether any critical steps in the speciation process only occur when populations are isolated

Host-dependent variation in density of corallivorous snails (Coralliophila spp.) at Curaçao, southern Caribbean

Snails of the genus Coralliophila (Muricidae: Coralliophilinae) are common in the Caribbean as corallivores that feed on a large range of host species. The present study concerns the distribution of two Coralliophila snails, C. caribaea and C. galea, at 5-m and 10-m depths at Curaçao (southern Caribbean), as associates of the common scleractinians Orbicella annularis and Pseudodiploria strigosa. Coralliophila galea was abundant on both host species, while C. caribaea was represented only by a single individual on a colony of P. strigosa. No significant differences in shell length were found between snails associated with O. annularis and P. strigosa. The distribution of C. galea on both host species deviated significantly from a random distribution. The snails were most abundant at 5-m depth, particularly on larger colonies of O. annularis, with > 60 % of large colonies colonized by snails, while snails were absent on small colonies. This distinction was not significant in P. strigosa at the same depth or in O. annularis at a depth of 10 m. The results suggest that host preference should be considered in assessments of reef health in connection to damage caused by Coralliophila spp

Genetic and morphological variation in corallivorous snails (Coralliophila spp.) living on different host corals at Curacao, southern Caribbean

Snails of the genus Coralliophila (Muricidae: Coralliophilinae) are common corallivores in the Caribbean, feeding on a wide range of host species. In the present study, the morphological and genetic variation in C. galea and C. caribaea were studied in relation to their association with host coral species at Curaçao. Differences in shell shape among snails living on different hosts were quantified using geometric morphometric and phylogenetic relationships were studied using two mitochondrial markers (12S and COI). Based on these analyses, a new species, C. curacaoensis sp. nov., was found in association with the scleractinian coral Madracis auretenra. Both C. galea and C. caribaea showed host-specific differences in shell shape, size, and shell allometry (i.e. changes in morphological development during growth). Shell spire variability contributed foremost to the overall variation in shell shape. In C. caribaea minor genetic differences existed between snails associated with scleractinian and alcyonacean corals, whereas in C. galea such intraspecific variation was not found. These results shed more light on morphological and genetic differences among coral-associated fauna living on different host species