Phenotypic plasticity in the marine environment

Phenotypic plasticity refers to the ability of an organism to change its phenotype in response to changes in the environment and it is integral in influencing an organism’s survivability when confronted with habitat variation. Phenotypic plasticity encompasses all types of environmentally induced changes (e.g. morphological, physiological, behavioural, phenological) that may or may not be permanent throughout an individual’s lifespan. As with any organismal trait, the way in which an individual responds to environmental changes is subject to evolutionary change and phenotypic plasticity on a whole. These environmental changes can have negative impacts as they can destabilise homeostasis and development, and disrupt the match between an organism’s phenotype and the environment, thereby lowering fitness.

Organisms counter environmental variation with their own adaptive variation of two types: between- and within-generation variation 2. Between-generation variation is mostly genetic and can result in adaptive change within a population facilitating natural selection, as a result of mutation, genetic drift etc. Within-generation variation is almost always non-genetic, occurs in individuals, and is frequently adaptive, because it allows individuals to adjust to environmental variation in real time, otherwise known as phenotypic plasticity 1

During the first half of the 20th century, evolution was viewed to be primarily based on the mutation of genes, however it is now known that natural selection selects for phenotypes not genotypes 1. Genetic Assimilation is a term used to describe the process by which a phenotype originally produced in response to an environmental condition, later becomes genetically encoded via artificial selection or natural selection. It is an important idea because it suggests that acquired, phenotypic-plastic traits can become genetically fixed  inferring that environmental induction can initiate evolutionary change.

Numerous studies have been conducted in the last century investigating phenotypic plasticity in terrestrial species. However it is marine species in particular that have been shown to exhibit remarkable observable phenotypic plasticity, often due to the extreme environmental gradients that some marine species inhabit. Plasticity is thought to be integral to the survival of many marine organisms inhabiting the intertidal zone where strong environmental gradients occur. Previous studies on species of intertidal marine gastropods have shown strong evidence for phenotypic plasticity occurring from the lower shore (where the impact of wave action is higher) to the upper shore (where heat, osmotic stress and predation are more prominent stressors), in the form of shell polymorphism.

One such study, conducted by García et al 2013 3, investigated the role of phenotypic plasticity in the intertidal marine gastropod Melarhaphe neritoides to explore whether shell polymorphism is occurring. With samples of M.neritoides being collected from both the lower and upper shore, morphometric analysis determined samples collected at exposed sites showed similar convergent polymorphism to that which has been observed in other gastropod species 4, with individuals possessing a more rounded shell and a larger aperture than those collected at protected sites. This is due to gastropods requiring a larger muscular foot to secure to the substratum on the wave-exposed lower shore, thus favouring a larger shell aperture. Samples of sexually immature M.neritoides were also collected from both shore heights, encompassing both shell morphologies, and reared in a lab for 9 months under conditions absent of the stressors found in the intertidal zone. After 9 months, their shell morphology differences were maintained suggesting a genetic basis for this polymorphism, thus inferring that genetic assimilation of the plastic shell morphology has occurred.   

Similarly, a study conducted by Geoffrey Trussell in 1997 5 investigated the morphological plasticity of Littorina obtusata before and after a catastrophic storm in New England. He investigated the shell length and relative shell height as well as shell aperture area on littorina populations in 2 protected and 1 wave exposed site. Snails sampled after the storm had relatively squatter shells than those sampled before the storm which was consistent with shell height patterns found in natural populations. This rapid change in shell height as a result of hydrodynamic stress (high wave action) suggests a phenotypically plastic response to this hydrodynamic stress inflicted by the storm. The relative aperture area of the snails was shown to have decreased after the storm, which is surprising as an increase in shell aperture usually occurs with increased hydrodynamic stress to support a larger muscular foot. A decrease in shell length at the wave exposed site was observed and is likely due to the impacts of the storm being more severe at this exposed location. It is proposed by Trussell that the shifts in shell morphologies after the storm were mediated by the ability of the snails to avoid free-stream flows by hiding in sheltered crevices. Decreases in shell height, length and aperture area may prove advantageous when attempting to fit into sheltered crevices to shelter from wave action. These results suggest that this plasticity of shell morphology in the species shows a remarkable increase during extreme weather events, and thus may play a prominent role in selection and thus evolution, especially in areas of constant extreme weather.

Due to these strong environmental gradients that impact the habitats of marine animals, our understanding of phenotypic plasticity is ever increasing, as plasticity in marine organisms can be observed with ease (for example the intertidal environment), more so than in terrestrial habitats. Other instances where plasticity can be observed in the marine environment is in temperate species, where thermal plasticity is responsible for their survival in areas that experience a greater range of temperatures, and thus may be expected to have evolved the capacity to deal with such changes 6.

We live in a world where anthropogenic induced environmental changes are continuously occurring. Global climate change resulting from increasing concentrations of green house gases is warming the oceans and the organisms that inhabit the marine environment must continually adapt to these rising temperatures. Many marine species are currently undergoing significant range shifts and exceedingly rapid changes in phenotype, driven potentially by warming, ocean acidification, and human-induced evolution. There is considerable debate over whether these changes are the result of rapid evolution or physiological responses to changes in environmental variables. However,an increasing amount of research into the plasticity of marine phenotypes is being conducted so as to gain a better understanding of how these organisms will cope in an environment experiencing dramatic changes.

– JK


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Sources:

  1. Whitman and Agrawal (2009): http://www.eeb.cornell.edu/agrawal/pdfs/whitman-and-agrawal-2009-Ch_1-Phenotypic-Plasticity-of-Insects.pdf
  2. Meyers, JJ Bull. Trends in Ecology & Evolution 17 (12), 551-557, 2002
  3. Garcia, Sara D.; Diz Angel P.; Sa-Pinto, Alexandra; Rolan-Alvarez, Emilio, (2013): https://eurekamag.com/research/037/458/037458893.php
  4. Carvajal-Rodríguez et al. (2005): http://mollus.oxfordjournals.org/content/71/4/313
  5. Trussell, G. (1997) http://www.int-res.com/articles/meps/151/m151p073.pdf
  6. Stillman, J. (2003) Acclimation capacity underlies susceptibility to climate change. Hopkins Marine Station, Stanford University, Ocean-view Boulevard, Pacific Grove, CA 93950, USA. Science (Impact Factor: 31.48). 08/2003; 301(5629):65. DOI: 10.1126/science.1083073

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