Electro Sensory and Turbidity: American Paddlefish’s Decline and Outlook

Polyodon spathula

Figure 1: American Paddlefish (image courtesy of U.S. Fish and Wildlife Service)


The American Paddlefish (Polyodon spathula) or spoonbill as they are often referred to due to their paddlelike snout, is a large and ancient fish that has survived relatively unchanged since the late cretaceous period around 68 million years ago when dinosaurs walked the earth. They can be found throughout North American in the Mississippi River Basin. While they are a protected and rare state threatened species in Ohio, they are found most commonly in Ohio River tributaries downstream of the first dams; particularly in the Scioto River south of Columbus (Rice and Zimmerman 2019). In Ohio, they can grow to a maximum size of 5′ and 184lbs, however fish ranging from 2-4′ and around 20-30lbs are much more common (Rice and Zimmerman 2019). The cause for their decline was attributed to increased turbidity, or suspended particles like clay and silt, and being cutoff from suitable spawning areas to reproduce due to the construction of dams (Rice and Zimmerman 2019). In order to understand why turbidity has impacted these fish so much and why their populations are beginning to rebound as water quality improvements t0 the rivers has been made, you must first understand how they feed.

Filter Feeding:

Despite their massive size and tough exterior, these fish are filter feeders that prey mostly on small plankton crustaceans, which are small organisms that float freely in the water. You may be wondering how it is possible for the paddlefish to eat enough of these organisms to grow to such a large size. Luckily for the paddlefish, they are excellent filter feeders like Humpback Whales. Having been described by early naturalists as “A living plankton net” these fish have hundreds of gill rakers that form a tight mesh that collects plankton as they swim around with their large gaping mouths open as seen in Figure 2.

Figure 2: Paddlefish gill rakers (Courtesy of John Lyons of Virigina Tech)

But what purpose does the paddle serve?


The paddle or rostrum was once thought to have been used to dig through the stream substrate foraging for food, but research has proven that it is actually an electro sensory organ covered in sensory pores (Figure 3) used by the paddlefish to detect the weak electrical currents plankton give off (Wilkens and Hofmann 2007). In a study published in 2007, researchers ran feeding experiments on juvenile paddlefish where they placed the paddlefish in dark tanks and witnessed them able to seek out single small crustaceans that were placed in the tank (Wilkens and Hofmann 2007). They also noted that they would avoid metallic objects in the tank, even in the dark, as this likely interfered with their perception of electrical currents which may also hint at a further impact locks and dams could have on paddlefish by disrupting their ability to navigate rivers and access spawning habitat after dams were constructed (Wilkens and Hofmann 2007). If paddlefish can locate food even in the dark or muddy waters, then why has increased turbidity impacted them so much?

Figure 3: Paddlefish rostrum and closeup of electro sensory pores (Helfman et al. 2007)


The issue with increased turbidity is largely due to their ability to finely filter feed. Some studies have shown that 50% of the gut content of some paddlefish was detritus (decomposing organic material like small leaves) and sand (Pyron et al 2019). Under highly turbid conditions, paddlefish gill rakers may become clogged with materials that are suspended in the water, making them unable to filter feed plankton effectively. In addition to not being able to filter feed as effectively, other studies have linked high turbidity to reduced growth of plankton species (Kirk and Gilbert 1990). Despite the paddlefish’s ability to find food solely using electroreception, they’d still struggle to find food as plankton growth can be reduced, leaving them less food in water with high turbidity.


At present, water quality in Ohio has been steadily improving after legislation like the Clean Water Act in 1972 and many fish species have since begun to bounce back after having been impacted by habitat and water quality degradation (Pyron et al 2019). As water quality continues to improve and many old and outdated dams are removed; granting these fish access to places to spawn, we can be hopeful that conservation efforts to protect this ancient Ohio fish will be successful and future paddlefish will be free to filter feed all they want.



Helfman, G., Collette, B. B., Facey, D. E., & Bowen, B. W. (2009). The diversity of fishes: biology, evolution, and ecology. John Wiley & Sons.

Kirk, K. L., & Gilbert, J. J. (1990). Suspended clay and the population dynamics of planktonic rotifers and cladocerans. Ecology71(5), 1741-1755. Lon A. Wilkens, Michael H. Hofmann, The Paddlefish Rostrum as an Electrosensory Organ: A Novel Adaptation for Plankton Feeding, BioScience, Volume 57, Issue 5, May 2007, Pages 399–407.

Pyron, M., Mims, M. C., Minder, M. M., Shields, R. C., Chodkowski, N., & Artz, C. C. (2019). Long-term fish assemblages of the Ohio River: Altered trophic and life history strategies with hydrologic alterations and land use modifications. Plos one14(4), e0211848.

Rice, D. L., Zimmerman, B.. (2019). A naturalist’s guide to the fishes of Ohio. Ohio Biological Survey.


Microplastics in Fish: Not a Microproblem

Plastics are everywhere. We humans use them in everything – from toys to food packaging to medical supplies and beyond. As a manmade material, plastics do not readily break down in the environment once we are finished using them. To make matters worse, there is an abundance of teeny, tiny pieces of plastics in our world. These tiny bits can occur as a byproduct of the production of plastic goods, or they may be a result of litter in the environment breaking down into smaller and smaller pieces (Barboza et al., 2020). Because they are so small, they spread easily. Today, little pieces of plastic are found everywhere: in our soil, on our beaches, in rivers and oceans, and even scarier… inside the bodies of animals.

“Microplastic” by Oregon State University is marked with CC BY-SA 2.0. To view the terms, visit https://creativecommons.org/licenses/by-sa/2.0/?ref=openverse

These miniscule bits of plastic measuring less than 5 mm are also known as microplastics, and they truly are ubiquitous in today’s environment. (So much so that they are now considered a contaminant of concern on a global scale (Barboza et al., 2020).) Ingestion of microplastics has been documented in over 700 marine animal species, including sea turtles, whales, dolphins, and fish (Wootton, 2021). This either happens when animals mistake microplastics as food and ingest them by accident, or they ingest another smaller prey organism that also has microplastics inside of its body (Wootton, 2021). Fish may also take in microplastics passively as they filter contaminated water through their gills. As a result, microplastics have been found in the digestive tracts, muscle tissues and even the gills of fish (Barboza et al., 2020).

So why should we care if fish are accumulating small particles of plastic in their bodies? Recent studies have demonstrated that there are multiple toxic impacts of microplastic ingestion in fish. These impacts include impaired development, decreased feeding and body mass (Naidoo and Glassom, 2019), damage to cells, changes in behavior, impaired reproductive capacity, and even death (Barboza et al., 2020). There are documented instances of neurotoxicity (or damage to the nervous system) as a result of microplastic ingestion in fish. Oxidative stress, or an imbalance of antioxidants and free radicals in the body, has also been found to result from accumulation of plastics in the body, and may lead to cell and tissue damage in fish (Barboza et al., 2020). All of these impacts have the potential to harm the overall population of a particular species of fish, and ultimately alter food webs.

Conceptual model illustrating capture, retention and internalization of microplastics by fish species (Barboza et al., 2020).

If none of that grabbed your attention, perhaps this will: since microplastics have been found in the edible muscle tissues of fish, humans are also at risk of accumulating small bits of plastic in their bodies after eating a fish meal. Further studies into human risk assessment of microplastic ingestion are warranted, and perhaps microplastic daily intake limits may be in our future once the research is more solid (Barboza et al., 2020). Also, if food webs are altered enough by reduced populations of fish impacted by microplastics, maybe your favorite type of fish will be a lot harder to come by in the grocery store years down the line. For now, take note next time you are on a walk around your neighborhood and see the tiny pieces of a broken up plastic bottle cap – think about the impacts to the fish in a nearby waterway once a heavy rain washes those microplastics downstream. If nothing else, perhaps this thought will motivate each of us to choose to use less plastic in some capacity in our daily lives.



Barboza LG, Lopes C, Oliveira P, Bessa F, Otero V, Henriques B, Raimundo J, Caetano M, Vale C and Guilhermino L. (2020) Microplastics in wild fish from North East Atlantic Ocean and its potential for causing neurotoxic effects, lipid oxidative damage, and human health risks associated with ingestion exposure. Science of the Total Environment 717:1-14.

Naidoo T and Glassom D. (2019) Decreased growth and survival in small juvenile fish, after chronic exposure to environmentally relevant concentrations of microplastic. Marine Pollution Bulletin 145:254-259.

Wootton N, Reis-Santos P and Gillanders BM. (2021) Microplastic in fish – A global synthesis. Rev Fish Biol Fisheries 31:753-771.

Hypoxia: A “choose-your-own-adventure” story with fish

Below are screen shots of the Twitter thread I created to engage people with how fishes respond to hypoxia. Hypoxia is the depletion of oxygen, and this can occur naturally in bodies of water or as a result of excess nutrients from human activities (e.g., farming, urban waste) that are flushed into bodies of water. Although fish do not explicitly think about their potential options in coping with changes to their habitat, fish are faced with options that are often not as advantageous for them compared with if the water had not become hypoxic. To engage people with this issue of conservation physiology, I created multiple Twitter surveys in which I asked people to imagine they were a fish in a body of water that had just become hypoxic. The post contains a good deal of gifs that are not nearly as entertaining when they are screenshot, so I would encourage you to see the whole tread on my Twitter (@almeida_zoe).