A redbreast sunfish, Lepomis auritus
I began working with sunfish during my PhD with Tim Higham. During that work we developed a few new approaches for understanding how to quantify the interactions between swimming and feeding, whether this integration changes depending on what the predator is eating, and how these interactions relate to success during prey capture. To do this, we needed a group of model fishes where we already had a good understanding of how swimming and feeding work individually. That's where the freshwater sunfishes and basses (Family Centrarchidae) came in.
Several species have been used to understand how fins work, how muscles work, how jaws work, and how suction works (among other things), creating a strong foundation to build on. Their strong reliance on suction to pull prey into their mouth is particularly interesting since this only works at close distance and for a short time period. This means that coordination with swimming is important for making sure the suction volume is in the right place at the right time. There was also already evidence from Tim's PhD work that this integration existed.
A bluegill capturing prey by coordinating swimming and feeding movements
The first question we wanted to answer was whether there are differences in accuracy between species. However, this is not as simple as it might seem. Fish that use suction actually capture their prey by sucking in the water around the prey, not by grabbing it directly with their jaws. This means that accuracy has to quantified in terms of the volume of water that traps the prey. In a normal video we can see the predator's mouth, but we can't see this volume of water. So we need a way to see the volume of water to know how well the predator is positioning this volume relative to the prey. This involved two parts.
First, Tim developed a method for calculating accuracy based on visualizing particles of water using lasers and reflective beads. Once we can see the water, we can track all the particles that are sucked into a fish's mouth during prey capture. Then we can track these particles back to the beginning of the video and trace a line around the ones that were ingested, so we know what the volume looks like. Finally, we can find the middle of that volume and calculate the distance of the prey from that position as a measure of accuracy.
But this method is intrusive, and restricts the predator to feed in a very small area. It's also tough to measure and videos have to be filmed in a specific way. So we took this method one step further and developed a predictive model that allows us to estimate this volume based on measurements we can get from any video: swim speed and mouth size. Even though it's simple, this model does a good job of estimating the true values that Tim measured, and we found that accuracy differs between 3 species of sunfish, and between two types of prey (see the paper here)!
Differences in ingested volume and predator accuracy between sunfish species feeding on live fish
Figure modified from Kane and Higham 2014
The second question we wanted to answer was whether these differences mean that these predators are coordinating swimming and feeding differently. That work is still in the process of being published so I'll have to keep it secret for now.
A flier, Centrarchus macropterus
At Georgia Southern, my students and I are still interested in these fishes and are working on several questions related to how well this group of fishes survives in our local habitats. For example, how does integration between swimming and feeding affect other kinds of ecological interactions, such as what habitats they are found in. Almost 80% of species in the family of fishes including sunfish and basses can be found within a few hours of Statesboro, so this also makes a great system for asking questions about big-picture patterns, such as how integration changes as species evolve.