Abstract
BackgroundEcological processes are increasingly being viewed as an important mode of diversification in the marine environment, where the high dispersal potential of pelagic organisms, and a lack of absolute barriers to gene flow may limit the occurrence of allopatric speciation through vicariance. Here we focus on the potential role of ecological partitioning in the diversification of a widely distributed group of marine protists, the planktonic foraminifera. Sampling was conducted in the tropical Arabian Sea, during the southwest (summer) monsoon, when pronounced environmental conditions result in a strong disparity in temperature, salinity and productivity between distinct northern and southern water masses.ResultsWe uncovered extensive genetic diversity within the Arabian Sea planktonic foraminifera, identifying 13 morphospecies, represented by 20 distinct SSU rRNA genetic types. Several morphospecies/genetic types displayed non-random biogeographical distributions, partitioning between the northern and southern water masses, giving a strong indication of independent ecological adaptations.ConclusionsWe propose sea-surface primary productivity as the main factor driving the geographical segregation of Arabian Sea planktonic foraminifera, during the SW monsoon, with variations in symbiotic associations possibly playing a role in the specific ecological adaptations observed. Our findings suggest that ecological partitioning could be contributing to the high levels of 'cryptic' genetic diversity observed within the planktonic foraminifera, and support the view that ecological processes may play a key role in the diversification of marine pelagic organisms.
Highlights
Ecological processes are increasingly being viewed as an important mode of diversification in the marine environment, where the high dispersal potential of pelagic organisms, and a lack of absolute barriers to gene flow may limit the occurrence of allopatric speciation through vicariance
This study investigates the biogeographical distributions of planktonic foraminiferal small subunit (SSU) rRNA genetic types in the Arabian Sea mixed layer during the SW monsoon, when pronounced environmental conditions lead to a distinct disparity in temperature, salinity and productivity between adjacent northern and southern water masses
From our study of the tropical Arabian Sea, we have demonstrated that biogeographical distributions of the planktonic foraminiferal morphospecies/genetic types can be influenced by adaptations to differing hydrographic conditions
Summary
Ecological processes are increasingly being viewed as an important mode of diversification in the marine environment, where the high dispersal potential of pelagic organisms, and a lack of absolute barriers to gene flow may limit the occurrence of allopatric speciation through vicariance. We focus on the potential role of ecological partitioning in the diversification of a widely distributed group of marine protists, the planktonic foraminifera. The foraminifera are an important group, used frequently for paleoceanographic studies, and as a proxy for past climate change Their utility is owed to an exceptional fossil record, spanning over 180 million years (Ma), and to the fact that individual “morphospecies” (identified by shell morphology) display characteristic environmental preferences, which are reflected in their spatial and temporal distribution in the oceans, and in the chemistry of their calcite shells. High levels of sequence variation have been found in the small subunit (SSU) ribosomal (r) RNA gene of the planktonic foraminiferal morphospecies, indicating the presence of numerous ‘cryptic’ genetic types [4,5,6,7,8,9,10,11,12,13,15], with mounting evidence indicating that these individual genetic types may display non-random geographical distributions, indicative of distinct ecological adaptations (ecotypes) [4,5,6,7,8,9,10,11,12,13,15]
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