Abstract
BackgroundThe marine environment is comprised of numerous divergent organisms living under similar selective pressures, often resulting in the evolution of convergent structures such as the fusiform body shape of pelagic squids, fishes, and some marine mammals. However, little is known about the frequency of, and circumstances leading to, convergent evolution in the open ocean. Here, we present a comparative study of the molluscan class Cephalopoda, a marine group known to occupy habitats from the intertidal to the deep sea. Several lineages bear features that may coincide with a benthic or pelagic existence, making this a valuable group for testing hypotheses of correlated evolution. To test for convergence and correlation, we generate the most taxonomically comprehensive multi-gene phylogeny of cephalopods to date. We then create a character matrix of habitat type and morphological characters, which we use to infer ancestral character states and test for correlation between habitat and morphology.ResultsOur study utilizes a taxonomically well-sampled phylogeny to show convergent evolution in all six morphological characters we analyzed. Three of these characters also correlate with habitat. The presence of an autogenic photophore (those relying upon autonomous enzymatic light reactions) is correlated with a pelagic habitat, while the cornea and accessory nidamental gland correlate with a benthic lifestyle. Here, we present the first statistical tests for correlation between convergent traits and habitat in cephalopods to better understand the evolutionary history of characters that are adaptive in benthic or pelagic environments, respectively.DiscussionOur study supports the hypothesis that habitat has influenced convergent evolution in the marine environment: benthic organisms tend to exhibit similar characteristics that confer protection from invasion by other benthic taxa, while pelagic organisms possess features that facilitate crypsis and communication in an environment lacking physical refuges. Features that have originated multiple times in distantly related lineages are likely adaptive for the organisms inhabiting a particular environment: studying the frequency and evolutionary history of such convergent characters can increase understanding of the underlying forces driving ecological and evolutionary transitions in the marine environment.
Highlights
The marine environment is comprised of numerous divergent organisms living under similar selective pressures, often resulting in the evolution of convergent structures such as the fusiform body shape of pelagic squids, fishes, and some marine mammals
Seibel and Carlini [13] tested this hypothesis in a phylogenetic context using a molecular-based phylogeny, and found that correlation between changes in metabolism and depth existed in the pelagic squids
Based on extensive new phylogenetic analyses including most available cephalopod molecular data, we find that transitions between demersal and pelagic habitats were common during the evolutionary history of cephalopods
Summary
The marine environment is comprised of numerous divergent organisms living under similar selective pressures, often resulting in the evolution of convergent structures such as the fusiform body shape of pelagic squids, fishes, and some marine mammals. Related lineages that transition to similar habitats are under the same kinds of selective pressures, often resulting in convergent structures. Adaptive morphological features shared between distantly related organisms inhabiting similar environments provide an opportunity to test for correlation between morphology and habitat to provide new hypotheses about the factors influencing evolution and diversification. Stickleback fish have long been studied for their numerous instances of convergent characters such as body shape, pelvis morphology, skeletal armor, and mating preferences, associated with transitions from marine to freshwater benthic and limnetic habitats Seibel and Carlini [13] tested this hypothesis in a phylogenetic context using a molecular-based phylogeny, and found that correlation between changes in metabolism and depth existed in the pelagic squids
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