In our Research Highlights blog series, we debut newly published fisheries research by our women of fisheries colleagues. If you have research you would like to highlight and share with our readers, submit a nomination form here! 

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McCollough, R., and K. Hecke. 2025. Morphological variation in four common stream fish across an intermittent waterfall in Arkansas, USA. Hydrobiologia

Curiosity about the world around us draws us to science; resilience keeps us there. That’s because scientific journeys rarely follow a straight path. They can be messy and frustrating and fraught with unexpected obstacles, detours, and readjustments. This journey, though, often rewards us with discovery.

As a graduate student at the University of Arkansas Tech University, Risa McCollough conducted nearly a year of fish sampling in Dare Creek in northwest Arkansas to assess movement upstream over a waterfall. Only problem was there was no movement, therefore no data and no study. Although disappointing at first, this would ultimately lead Risa down an exciting new path. “At this point, I had already read various studies about how dams can change the morphology of fish upstream in the reservoir versus downstream in the tailwater, and wondered if I could do something similar, even though it’s a so much smaller scale.” It turns out, there wasn’t much known about the effects of waterfalls on the morphology of warmwater fish species in the Southeastern United States. 

A new study was born. 

Risa collected and photographed individuals of four fish species – Creek Chub Semotilus atromaculatus, Highland Stoneroller Campostoma spadiceum, Redfin Darter Etheostoma whipplei, and Plains Orangethroat Darter E. sp. cf pulchellum I/II. For each fish, she then digitally marked 12 to 13 features called landmarks, such as the center of the eye and base of different fin structures. Geometric morphometrics is an approach that allowed Risa to use these measurements to compare the morphology of fish that were collected above and below the waterfall.  Risa notes, “It allows me to say, yes, fish shape is significantly influenced by how big they are, but also say, after accounting for that difference, there is still significant differences in fish shape according to the reach they are from.”

Indeed, Risa found that the morphology of fish above the waterfall was different than fish below the waterfall. Not only were some species larger upstream, some also exhibited differences in body shapes. These differences in shape could be related to flow or habitat-related changes in the food web, specifically the level of predation and competition present. For example, deeper bodies have been observed in fish inhabiting areas with more variable flow conditions, and fish with more elongated, streamlined heads have been observed in areas with greater predation pressure and complex habitats. Whether these morphological differences could also be the result of genetic divergence remains unknown. Risa notes, “This research honestly left me with more questions, because we don’t know if this is just phenotypic plasticity at work or actual genetic differences between populations.”

As an extension of this work, Risa suggests a closer look into one of her study species, the Plains Orangethroat Darter. “This darter is currently undescribed with two forms, the Red-belly and Blue-belly, which are thought to only occur sympatrically in one watershed, the Illinois Bayou, where I did my work. While genetics would be needed to resolve the descriptions, I think it would be really neat to use geometric morphometric analysis to compare the two forms.”

Risa’s story reminds us that the scientific process doesn’t always go as planned. Yet, if we remain curious and resilient, these unexpected detours can lead to new discoveries and questions that can build upon our understanding of the world.

The full manuscript can be found and downloaded here (open access):
doi.org/10.1007/s10750-025-06039-1