Independent Transitions to Freshwater Environments Promote Phenotypic Divergence, Not Convergence, in Stingrays

Area of Science:

• Evolutionary Biology
• Ecology
• Vertebrate Zoology

Background:

• Convergent evolution, where similar traits evolve independently, offers insights into evolutionary processes.
• Vertebrate transitions from saltwater to freshwater habitats provide natural experiments for studying adaptation.
• Stingrays (Dasyatidae) have repeatedly colonized freshwater environments globally, presenting a unique case study.

Purpose of the Study:

• To investigate the frequency of saltwater-to-freshwater invasions in stingrays.
• To compare ecological and phenotypic diversification between freshwater and saltwater stingray lineages.
• To assess the extent of morphological convergence among freshwater stingray species.

Main Methods:

• Analysis of stingray invasion frequencies across continents and clades.
• Comparative analysis of ecological traits and morphology in freshwater versus saltwater lineages.
• Morphospace analysis to evaluate convergence among freshwater species with similar niches.

Main Results:

• Freshwater stingrays, while not morphologically distinct from marine counterparts, expand the overall morphological diversity of the group.
• Trophic specialists like piscivores and molluscivores occupied distinct regions of morphospace.
• Limited morphological convergence was observed among freshwater stingrays, both as a group and across different lineages sharing similar niches.

Conclusions:

• The lack of strong morphological convergence in freshwater stingrays may be due to insufficient time for evolution or the influence of ancestral body plans (bauplans).
• Contingency in evolution, driven by paths of least resistance and weak selection, could explain the observed diversification patterns.
• Stingray freshwater invasions highlight the complex interplay of time, ancestry, and selection in shaping evolutionary trajectories.