Meet The Ocean Creatures That Use Mesh Of Mucus To Catch Their Food
All animals must eat to survive. If you’ve heard the term “grazer” before, it may bring to mind familiar farm animals, such as cows or sheep munching on pastureland. But the ocean has its own suite of grazers, with very different — even bizarre — body forms and feeding techniques. Instead of teeth, one group of these invertebrates uses sheets of mucus to consume huge quantities of tiny plant-like particles. In our new paper, my colleagues and I suggest a new categorization for this overlooked group: “mucous-mesh grazers,” in recognition of their unusual feeding strategy.
Unlike the mucus in our noses, which appears amorphous and blobby, the mucous sheets of these ocean grazers can be structured into ornate meshes and nets. These mucous sheets can function like a filter to ensnare food as small as bacteria. The grazers themselves are mammoth in comparison: up to 10,000 times bigger than their food. If people ate food that small, you’d be picking salt and sugar grains off your dinner plate.
Marine biologists like me used to think mucous grazing was a “catch-all” feeding strategy – the idea was these guys would just chow down on whatever their mucous sheet caught. But recent technological advances are helping us understand that mucous grazers can be picky eaters. And what they consume — or don’t — influences ocean food webs.
Ornate filtering meshes (amplified 1,000 times and stained bright green in this image) capture particles much smaller than the grazers themselves. Kelly Sutherland, CC BY
How does mucous-mesh grazing work?
Mucous-mesh grazers include salps, pyrosomes, doliolids, pteropods and appendicularians. They are typically centimeters in length, roughly spanning the size of your fingernail to the size of your hand. Some form colonies comprised of many individuals in long chains that can be much longer. These creatures are large and watery compared to their hard-bodied planktonic counterparts. If you stepped on one, it would squish, not crunch. A mostly water body enables them to grow large quickly.
Mucous-mesh grazers are free floating and suited to the open ocean. They live far from shore, where food is scarce and often small. The tiny holes and fibers of their mucous meshes enable them to capture microscopic particles, which they subsequently swallow, sometimes along with the mucus.
[youtube https://www.youtube.com/watch?v=yG2hJoj8b3E?wmode=transparent&start=0]
A chain of salps filtering tiny food particles through an internal mesh.
Unlike spiders that spin their feeding webs, these grazers have a special organ, called an endostyle, that secretes their mucous mesh. Depending on the grazer, the mucous mesh can be located either inside or outside the body. One group, for example, secretes a mucous bubble big enough for the animal to live inside like a house. Another group, nicknamed sea butterflies, secrete mucous webs that attach to their wing-shaped feet. These mucous webs range in size from an inch to over 6 feet.
Historically, scientists assumed mucous-mesh grazers ate anything that passed through the mucous sieve — similar to a strainer in the drain of the kitchen sink catching everything of a certain size that flows in. Recent research by my lab and others challenges this assumption and shows that their feeding may be highly selective. The mucus might capture certain food particles perfectly, while completely rejecting other particles on the basis of their size, shape or surface properties.
For example, when presented with a mixture of rod-shaped and spherical food particles – differently shaped but otherwise similar in size – one species of mucous-mesh grazer preferentially swallows the spherical particles.