Not by Sieving Alone: Observations of Suspension Feeding in Daphnia
Abstract:Fluorescent dyes and particles, epifluorescent microscopic video image analyses and high-speed microcinematography were used to determine fluid flow and particle capture by Daphnia magna and D. pulex. These fluorescent techniques detected processes that could not be seen or differentiated by direct observation with light microscopy due to their high speed or the lack of light inside the filtering chamber. We conclude from our analyses that: (1) The majority of water and particles entering the filtering chamber flow tangentially across the surface of the “filtering combs” of appendages 3 and 4. (2) Water travels in distinct streams through the carapace cavity and does not separate into diffuse clouds, as would be expected under strict sieving through the setules. (3) Particles accelarate rapidly upon entering the carapace and reach the food groove within one half cycle of the metachronal beating of the feeding appendages. (4) In keeping with what is known from previous observations, the majority of particle retention occurs on appendages 3 and 4. They may be captured directly in the ventral food groove, or on the distal portions of the setae (near the food groove and body wall) of limbs 3 and 4. However, particles may also be retained on appendages 2 and 5. (5) We propose that the flow of filtered water passes under the flexible setal tips near the body wall and around the bases of appendages 3 and 4 before it is expelled posteriorly. (6) The first appendage pair can move independently of the metachronal beat pattern to dislodge particles that are stuck to it to reject particles from under the labrum. (7) Manipulation of captured particles in the food groove and bolus is primarily with the setae of the second appendage. (8) A set of setae on the mid-ventral margin of the carapace separates incurrent from excurrent flow in D. magna. (9) A major flow of incurrent water is expelled from the carapace on the fore stroke of the appendages and is, therefore, not processed to remove particles. The other major incurrent flow passes through the medial space between the appendages toward the food groove, where particles are extracted from it. Image analysis allowed us to determine actual speeds of particle movement to make calculations of particle inertia and energy: (10) Inertial impaction is an unlikely mechanism of particle capture. (11) The energy required to force water through the filtering combs is calculated to be 20 to 45% of the animal's metabolism, making sieving an energetically costly process. These calculations and our observations of fluid and particle motion lead us to conclude that sieving through the setular mesh of limbs 3 and 4 is an unlikely process for routine particle collection by Daphnia.
Document Type: Research Article
Publication date: November 1, 1988
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