By Christine A. Parisek

Imagine a time you were standing at the edge of a creek – perhaps small pebbles and cobblestone were stacked along the shallow water edge, aquatic vegetation pushed its way in between, and a light breeze rustled the trees around you as the water swirled and lapped playfully at your feet.

Some people might think fishing would be the best way to enjoy a quiet, picturesque scene like this. But if you’ve never paused to turn a couple of cobblestones over, you could be missing out. If you have, you might have spotted something clinging to the underside of the rock – perhaps a tiny, round, brown “coin”. That’s Eubrianax edwardsii, a water penny beetle larva (lucky pennies, I call them), and they are by far one of my favorite things to find when I’m exploring streams.

Water pennies are aquatic insects that spend their juvenile stage (i.e., larval) seemingly suction-cupped to rocks underwater, moving slowly and deliberately like a tank tread, with measured determination, all the while bracing against the current as they munch and scrape off the algae. When they’re present, you know the water is clean and healthy. As adults, they are terrestrial (like most aquatic insects), and they flaunt an admirable pair of antennae.

I first encountered water pennies while surveying lakes in the Lakes Basin (Plumas–Tahoe National Forest) for my Master’s research, and I was fortunate to continue encountering them for my PhD research in surveying Sierra Nevada lake food webs. I’ve come to really appreciate them, especially as I’ve moved from working solely on aquatic insects toward being an aquatic food web ecologist.

Aquatic insects live in most freshwater environments and inhabit all manner of places, depending on the particular species. That could be the water’s surface (water striders, riffle bugs), the water column (predaceous diving beetles), bottom sediments (stonefly nymphs), or on different microhabitats such as vegetation (damselfly and dragonfly nymphs), mud (burrowing mayfly nymphs, midge larvae), or rocks (clinging mayfly nymphs, case-making caddisfly larvae, water penny beetles). As with different fish taxa, each aquatic insect species has morphology adapted to live in lentic (slow-moving water, e.g., wetland, lake, reservoir) or lotic (flowing water, e.g., streams, rivers) habitats – though not always! That’s actually what spurred my interest in working with water pennies for my Master’s research in the first place – water pennies (adapted to streams) were also in lakes (Parisek 2018; Parisek et al. 2023).

So next time you’re bugging out to visit a lake or stream, consider pausing at the edge and taking a closer look!

About the Author 

Christine A. Parisek is a Postdoctoral Research Scholar at the University of California Davis, a Science Communications Fellow at the Center for Watershed Sciences, and Managing Editor of the California WaterBlog.

Further Reading

Penn, B.M. 2025. A penny for your water quality. https://www.chesapeakebay.net/news/blog/a-penny-for-your-water-quality

Field guide to water penny beetle larvae. By the Missouri Department of Conservation. https://mdc.mo.gov/discover-nature/field-guide/water-penny-beetle-larvae

iNaturalist Project: Critters from lakes and their riparian zones in the Sierra Nevada, California, USA. Curated by Christine Parisek & Sophie Sanchez. https://www.inaturalist.org/projects/sierra-nevada-lakes-riparian-zones?tab=species

Parisek, C.A. 2018. The ecological and evolutionary dynamics of 3 aquatic insects crossing lotic-lentic boundaries in the Lakes Basin, Sierra Nevada, CA. Masters Thesis. California State University – Stanislaus. Published in Freshwater Science:https://doi.org/10.1086/725455.

Parisek, C.A., M.P. Marchetti, and M.R. Cover. 2023. Morphological plasticity in a caddisfly that co-occurs in lakes and streams. Freshwater Science 42(2), pp.161-175. doi 10.1086/725455. EcoEvoRxiv. Pdf.

Parisek, C.A. 2024. Understanding ecosystem and food web processes of freshwater lakes. PhD Dissertation. University of California – Davis. Published in Scientific Reports & Environmental Science & Technology.

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