Simons, A. M. | bioRxiv Science

bioRxiv2023

Conspicuous shell shape plasticity across lake-stream habitats in a freshwater mussel (Pyganodon grandis)

2.Hydrodynamic forces and their absence appears to exert differential selection pressure between lake and stream populations, creating a tight fit between organismal phenotypes and their environments. Ecophenotypic variants may be the result from isolated genetic evolution or phenotypic plasticity, where a widespread genotype can produce multiple phenotypes dependent on the environment. Freshwater mussels possess a wide degree of morphological variation that frequently covaries with the environment, making them a good system to understand the mechanisms of ecophenotypic variation across hydrological conditions. We designed a two-year experiment where individuals from the same Pyganodon grandis maternal brood (half-full siblings) were reared at a controlled site and four natural sites involving one lake and three streams. At the end of the experiment, shell shape was quantified for recaptured (N=70), wild (N=206), and zoo-reared (N=305) mussels. Analysis of covariance found significant differences in shell shape between rearing sites, particularly between stream and lake habitats, but no shape differences were detected across the three stream sites. At two of the four sites, shell shape of recaptured individuals matched the morphology of wild populations. Genomic sequencing and parentage analysis identified 27 different fathers among recaptured individuals. Yet, no genetic differences were present between stream and lake habitats and there was no effect of parentage on shell shape. Taken together, phenotypic plasticity, over genetic differentiation, is identified as the primary mechanism of shell shape ecophenotypy. Plasticity may be a key adaptation for freshwater mussels and possibly provide a buffer against their imperilment in degraded habitats. 1. Summary statementStreamlined and obese freshwater mussel shell shapes in stream-lake environments are developed from the same maternal brood and may reflect adaptations to the presence and absence of streamflow.

ecology↗

bioRxiv2021

The evolutionary maintenance of Levy flight foraging: a numerical simulation.

Levy flight is a type of random walk that models the behaviour of many phenomena across a multiplicity of academic disciplines; within biology specifically, the behaviour of fish, birds, insects, mollusks, bacteria, plants, slime molds, t-cells, and human populations. The Levy flight foraging hypothesis states that because Levy flights can maximize an organisms search efficiency, natural selection should result in Levy-like behaviour. Empirical and theoretical research has provided ample evidence of Levy walks in both extinct and extant species, and its efficiency across models with a diversity of resource distributions. However, no model has addressed the maintenance of Levy flight foraging through evolutionary processes, and existing models lack ecological breadth. We use numerical simulations, including lineage-based models of evolution with a distribution of move lengths as a variable and heritable trait, to test the Levy flight foraging hypothesis. We include biological and ecological contexts such as population size, searching costs, lifespan, resource distribution, speed, and consider both energy accumulated at the end of a lifespan and averaged over a lifespan. We demonstrate that selection often results in Levy-like behaviour, although conditional; smaller populations, longer searches, and low searching costs increase the fitness of Levy-like behaviour relative to Brownian behaviour. Interestingly, our results also evidence a bet-hedging strategy; Levy-like behaviour reduces fitness variance, thus maximizing geometric mean fitness over multiple generations. Author summaryIn heterotrophs, incuding animals, survival depends on the net energy gained through foraging. The expectation, then, is that natural selection results in adaptations for efficient foraging that optimize the balance of searching costs and rewards. Levy flight foraging has been proposed as an optimal foraging solution. The hypothesis states, if no information about resource locations are available, and the locations are re-visitable, then selection will result in adaptations for Levy flight foraging, a type of random walk. It has been argued that Levy-like foraging behaviour may simply reflect how resources are distributed, but empirical and theoretical research suggests that this behaviour is intrinsic or innate. However, this research does not address evolutionary mechanisms, and lacks ecological breadth. We extend the current theoretical framework by including evolutionary ecological contexts. We treat an organisms random walk as a heritable trait, and explore ecological contexts such as population size, lifespan, carrying capacity, searching costs, reproductive strategies, and different distributions of food. Our evolutionary simulations overwhelmingly resulted in selection for Levy-like foraging, regardless of the distribution of food, and evidences Levy flight foraging as a bet-hedging strategy. Thus, here we provide some of the first evidence for the evolutionary maintenance of Levy flight foraging.

evolutionary biology↗