Incipient Speciation Across a Depth Gradient in a Scleractinian Coral?

Abstract

A few marine cases have demonstrated morphological and genetic divergence in the absence of spatial barriers to gene flow, suggesting that the initial phase of speciation is possible without geographic isolation. In the Bocas del Toro Archipelago of the Atlantic Coast of Panama, we found two morphotypes of the scleractinian coral Favia fragum with opposing depth distributions. One morphotype fit the classical description of F. fragum and was most abundant at 3 m depth. A second morphotype was distinguished by raised corallites and was restricted to Յ 1 m depth. The two morphotypes overlapped in distribution at 1 m depth. Multivariate analysis of polyp-level characters (shape and distribution of septa within corallites) divided samples into two groups corresponding to initial qualitative observations of colony shape and corallite relief. To determine whether reduced gene flow maintains morphological variation, we measured the frequencies of alleles at five allozyme loci in both morphotypes at three sites 1-2 km distant. While there were significant differences in allele frequencies between morphotypes within sites, there were also frequency differences among sites at most loci, with the exception of nearly fixed alleles at the PGM locus. Extremely low heterozygosity permitted us to use haplotypes to compare genetic distance between morphotypes and among sites. Comparisons between haplotype data and a null model assuming gene flow between morphotypes showed that the two morphotypes shared significantly fewer haplotypes than expected, and average genetic distance between morphotypes was significantly greater than expected. Partitioning haplotype variation with analysis of molecular variance demonstrated that 35% of the variation was explained by morphotype, whereas 28% of the variation was explained by site. Two PGM heterozygotes and several individuals homozygous for rare PGM alleles are consistent with hybridization, and perhaps introgression by selfing within morphotypes. We consider three hypotheses for this morphological and genetic divergence in F. fragum: (1) intraspecific polymorphism, (2) incipient species, (3) biological species; and discuss the role of reproductive characters in a divergence-with-gene flow mechanism of speciation.

Long prereproductive selection and divergence by depth in a Caribbean candelabrum coral

“…However, ecological segregation is also widespread on Caribbean coral reefs. Genetic differences have been detected between ecomorphs with overlapping ranges (11)(12)(13)(14)(15), and thus habitat diversity may aid marine speciation. For example, the most common coral on Caribbean reefs was regarded as one species, but fertilization, genetic, and morphological data suggest it is a complex of three species segregated in part by depth (5,16).…”

Inviable immigrants drive diversification in the sea

“…The authors sought to explain how reproductive isolation arises in species-corals-that possess features that seemingly render the allopatric model of speciation untenable (12): corals are typically long-lived, geographically widespread, and characterized by widely dispersing larvae. Different species of coral often inhabit different depths, suggesting that populations living at different depths may, in fact, be incipient species (12).…”

Two distinct, geographically overlapping lineages of the corallimorpharian Ricordea florida (Cnidaria: Hexacorallia: Ricordeidae)

“…Other relevant ecological data (e.g., depth, exposure, salinity, temperature) and morphometric characters (number, size and color of polyps, number and size of marginal and discal tentacles, etc.) should be recorded to test for possible ecological specialization to different habitats (Knowlton and Jackson 1994;Carlon et al 2002;Prada et al 2008) and sympatric divergence. Disruption of gene flow in proximate populations and sympatric or parapatric speciation by ecological differentiation (Doebeli and Dieckmann 2003) is possible in the marine environment where very steep environmental gradients exist (e.g., depth, light, salinity).…”