Food in the form of fine particulate matter suspended in the water is. very abundant. It can come in the form of:
Actually eating it requires three things:
Methods of food capture:
Sieving: large particles trapped with fine-meshed collecting apparatus. This seems to be the primary method used by suspension-feeding ray-finned fish such as menhaden.
Problem: Many suspension feeders are able to trap particles much finer than the mesh of their filtration apparatus. How is this possible? Aerosol filtration theory - the basis for technologies for removal of industrial waste from the air - provides four additional methods that, it turns out, are employed by suspension feeders:
Direct intercpetion: low-density particles adhere to collecting elements due to mucus and/or electrostatic attraction. Once attached, it can be transported to the mouth. E.G.: Tunicates and Branchiostoma, which arrest particles on strands of mucus which are transported to the mouth by means of ciliary action
Inertial impact: high-density particles impact on collecting elements then fall out of suspension.
Motile particle capture : small, motile organisms preferentially collide with collecting elements.
Gravitational deposition : large, dense particles drop onto collecting elements.
Aerosol filtration theory and empirical data show that different filtration methods are more effective at different current speeds. Thus, suspension feeders may vary their feeding strategy depending on the type of food particle available, even to the point of changing their swimming speed.
Suspension feeding effectiveness varies with the structure of the feeding apparatus, This must meet two basic requirements:
Crinoids: (Ord. - Rec). Very common and successful suspension feeders of the Paleozoic. Hit very hard by the Permian extinction event. Now occur in two forms:
Captured food with an array of arms that radiate from the main body. These are lined with rigid pinnules. Running down the middle of each arm and branching onto the pinnules, are ambulacra (sing. ambulacrum) lined with tube-feet. Pinnules and arms reduce the velocity of water currents so that tube-feet can snag particles by direct interception. they then pass them hand-over-hand to the mouth, in the center of the body.
Two basic strategies for food capture:
Graptolites: (Cam. - Sil). First known as enigmatic dark markings in early Paleozoic rocks, they quickly became the index fossils of choice for the Ordivician - Silurian. Almost always, they were two-dimensional carbonized remains. Discovery of three-dimensionally preserved specimens led to realization that graptolites were colonies of zooids that secreted a proteinaceous hard structure very similar to that of living pterobranchs. Each zooid was a minute suspension feeder, using a lophophore (cilia-covered feeding appendage). Each colony begins with the theca of an ancestral zooid which ends in a long thread, the nema. Graptolite morphology: Sparing you the details of their actual taxonomy, we note that graptolites come in two general morphs:
Vertebrates: Vertebrates engage in ram-feeding in which the animals swims through particle-laden water while filtering. Many living groups do this:
NOTE: Ram-feeding may be continuous as in basking sharks, or intermittent as humpback whales.
Fossil ram feeders include:
Leedsichthys, the Jurassic ecological equivalent of a basking shark, but a ray-finned fish.
Stomatosuchus, the Cretaceous ecological equivalent of a baleen whale, but a crocodylian.