Date on Senior Honors Thesis

5-2023

Document Type

Senior Honors Thesis

Degree Name

B.S.

Department

Biology

Author's Keywords

convergence; divergence; feeding ecology; morphology; saltwater-freshwater transitions; stingray

Abstract

One of the most fundamental questions in biology is why some groups of organisms are more diverse than others. Classic hypotheses for explaining differences in diversity consider distinctions in time, place, resources, and competitors as the staging grounds for differential diversification. Freshwater and saltwater environments have similar levels of diversity despite significant differences in size, so studying transitions between the two systems can provide insights into evolutionary processes. Despite the challenges associated with this transition, stingrays have invaded freshwater habitats multiple times across different continents, making them useful for better understanding these systems. In this study, I evaluated the frequency of saltwater-freshwater invasions in stingrays, examined three types of diversification among freshwater and saltwater stingrays, and assessed the degree of convergence among freshwater stingrays. I found that, like nearly all other aquatic taxa, stingrays overwhelmingly only transition from saltwater to freshwater. After independent freshwater invasions, river rays did not demonstrate a pattern of increasing morphological or lineage diversification. However, the phenotypic disparity of saltwater stingrays did not follow the Brownian prediction and appeared to spike around two extinction events. Despite not being morphologically distinct from saltwater stingrays, freshwater stingrays do push the boundaries of morphological diversity. Diet guilds did demonstrate morphological differences, with piscivores and molluscivores being distinct from other diet guilds. Freshwater stingrays did not appear to converge morphologically, which may be because there has not been enough time for this to occur among more ancient and more recent freshwater lineages.

Lay Summary

One of the most fundamental questions in biology is why some groups of organisms are more diverse than others. Classic hypotheses for explaining differences in diversity consider factors such as timing, new places or resources, or lack of competition. Freshwater and saltwater environments have similar levels of diversity, despite being significantly different in size, so they are useful to understand what factors influence diversity. Although transitioning from saltwater to freshwater over evolutionary time is challenging, stingrays have done this multiple times and across different continents. In this study, I evaluated how often marine stingrays have invaded freshwater, examined how separate groups of stingrays may be distinct from one another in three different ways, and assessed if freshwater stingrays have evolved to be more similar over time. I found that, like nearly all other aquatic groups, saltwater stingrays are overwhelmingly more likely to invade freshwater than freshwater stingrays are to invade saltwater. After multiple separate invasions, river rays did not have a pattern of increased diversity in the number of lineages or skeletal shapes. However, saltwater stingrays did not change in shape as predicted by a model and had spikes of increased difference in their shape around the same time as two extinction events. Although freshwater stingrays did not have a distinctive skeleton compared to saltwater stingrays, they do push the boundaries of the diversity of skeletal shapes. Stingrays sharing similar diets did not evolve similar skeletons, like how fish-eating and mollusk-eating stingrays were distinct from other groups. I did not find evidence that freshwater stingrays have evolved to be more like one another, which may be because there has not been enough time for this to occur among ancient and more recent freshwater lineages.

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