Secrets of Filter-Feeding Sharks and Rays
by Misty Paig-Tran, Ph.D
Imagine a fish that is longer than a school bus. Now imagine that fish feeds on animals smaller than your pinky nail! How is this incredibly large fish able to filter these tiny organisms from the water? My research at the Friday Harbor Labs focused on answering how some of the largest fishes in the ocean filter tiny planktonic prey from the water column. To answer these questions, I set off with my colleague Silvia Hinojosa to the Yucatán Peninsula (near Cancun) to swim with whale sharks and manta rays. During these trips we had four main objectives: 1) to measure the swimming speed of whale sharks and manta rays while they were actively feeding in a plankton bloom, 2) to determine the types and sizes of plankton that the whale sharks and mantas were feeding on, 3) to obtain video of individual feeding events so that we could get an idea of whether water flow through the mouth of these fishes was turbulent or laminar (streamlined), and 4) to attach satellite tags onto the wings of manta rays so that we could test whether long term movement patterns could be predicted based on where plankton blooms were occurring throughout the year. We found, that although whale sharks and manta rays feed in the same plankton blooms, they were filtering out different types and sizes of plankton.
Following these trips into the field, I ventured back to FHL to begin teasing apart the mechanisms behind filter-feeding. I began by creating physical models of a filtering fish in the lab. A model fish allowed me to adjust individual parameters that were likely to affect filter performance separately (larger and smaller gill slits, swimming speed, etc.). I found that the anatomy of a fish and the speed that a fish swims while feeding will change the type and size of plankton being captured. In other words, these fishes can target specific types of prey rather than just collecting all of the species of plankton in the bloom. The speed that a fish swims through a bloom is incredibly important for what sizes of prey are being collected. After revisiting our field data, we noted that whale sharks and manta rays do in fact feed while swimming at different speeds; which likely affects what sizes and types of prey they are targeting.
The anatomy of the filter structure was also incredibly important to filtering different prey types in the model; therefore, it was likely that the filters between each of the 14 species would also be very different. I set off on a museum tour to collect filters from each of these fishes. I brought these filters back to FHL to look at them under our electron microscope. I noted that there was a wide variation in the anatomy of the filters between species; some were very smooth, others had cilia, and others were completely covered with tooth-like structures! I predicted that the commonality between these filters would likely be some kind of mucus that would coat the filter structure to help collect particles. To investigate the presence of mucus producing cells, I turned to histology. To my surprise I found that only three of the 14 species actually had mucus cells lining the filter. How then do these animals collect tiny food particles if there is no sticky surface for the particles to adhere to? The answer appears to be cross-flow filtration. Cross-flow filtration works as a self-cleaning mechanism that pushes any particles that collect on the filter back toward the esophagus to be swallowed, no mucus necessary. This mechanism of filtration alleviates clogging of the filter, allowing the animal to feed for longer periods without closing its mouth to process the food particles.
I presented these data at the Friday Harbor Labs during my Ph.D. dissertation defense in November 2012. If you are interested in filter-feeding, please don’t hesitate to contact me at firstname.lastname@example.org.
Misty Paig-Tran, Ph.D
Saddleback College, CSU Fullerton