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Unsolved Problems in Cell Biology Biology 446 Sept 21, 2016 Albert HarrisWater flow in spongesSponges are "Filter-Feeders": They use flagella to suck water through their bodies, and get their nutrition by catching small micro-organisms, such as bacteria and one-celled algae. Every textbook in Invertebrate Zoology contains at least one diagram what almost everyone has long accepted must be the pathway of this water flow through the sub-group of sponges called "Leuconoid" sponges. Observations of transparent living sponges have convinced Calhoun Bond and me that this textbook version is incorrect. Because past researchers studied cross sections through fixed (killed) sponges, they could not see the locations and directions of water flow. Guesses made over a century ago became so rigidly frozen in the minds of experts, that they no longer consider any possibilities other than what the textbooks claim. There are two related mistakes: #1) Textbooks claim that water enters sponge bodies through incurrent canals. Actually, there are no separate incurrent canals; they don't exist; the real inward path of water is through the sponge's extracellular matrix, where the skeleton, collagen fibers and phagocytic cells are located. #2) The directionality of water flow is claimed to be produced by the orientation of the power strokes of the flagellated cells ("Choanocytes"), toward the excurrent canals. As an analogy, imagine lots of rowers sitting along both sides of a stream, and propelling water past themselves, in the direction of their power strokes. Incidentally, this really is how clams and oysters, and also sea-squirts, use flagella to pull water through their bodies and filter out microscopic food. Although excurrent canals actually do exist, the flagellar beat is not like that of a ciliated epithelium, but instead like what occurs in the category of kidneys called nephridia, in which water is pulled from one side to the other of a flagellated epithelium.
Sponge flagella beat in waves, away from choanocyte cell bodies, and toward the tip of these flagella. What experiments could prove (or support) or disprove one or the other of these theories about the cause and pathway of water flow through the interior of sponges? Please invent as many experiments as possible. They can use fluorescent dyes to track water flow; or individual flagella can be observed, or choanocytes detached from surrounding cells; or holes can be punched through the surface of sponges, to determine how this effects paths of fluorescent or otherwise labeled water; Or fluorescent water could be injected into different parts of the interior of sponges. If you can think of any way to answer theses questions by using PCR or in situ hybridization labeling or gel electrophoresis, that will be OK.
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