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Development of a master filter feeder: Ontogeny and functional morphology of the invasive silver carp (Hypophthalmichthys molitrix) Open Access

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Silver carp are an invasive and exceedingly destructive species of Asian carp that have become widespread within the upper Mississippi River Basin. Silver carps’ ability to outcompete native filter feeders may be attributed to their complex trophic anatomy. The trophic apparatus of silver carp is composed of three anatomically discrete structures that work together: modified filtering plates (which filter small particles), paired epibranchial organs (which aggregate particles), and a large, modified palatal organ (which may play a role in moving particles from the filtering plates into the epibranchial organ). In silver carp the palatal organ is comprised of large muscular folds that interdigitate with the filtering apparatus. The filtering apparatus of silver carp is characterized by biserial, fused filtering plates used to capture a broad range of particle sizes. Early in ontogeny silver carp gill rakers are reminiscent of other stereotypical teleostean rakers, characterized by individual rakers that are tightly packed along the entirety of the branchial arches. At 30mm SL secondary epithelial projections and concomitant dermal ossification begin to stitch together individual gill rakers. By adulthood, the stitching of bone and complete fusion of the overlying epithelium creates rigid filtering plates with morphologically distinct faces. The inner face of the plates is organized into a net-like matrix while the outer anatomy of these plates is characterized by long parallel channels riddled with openings of different sizes that change in orientation throughout the length of plate. Hydrodynamic studies reveal that the filtering plates in silver carp induce organized vortical flow that maximizes fluid interaction with the filtering substrate, thus augmenting particle capture and retention. Moreover, during filtering fluid ultimately undergoes a dramatic change in direction towards the epibranchial organs which may also help move particles towards this aggregating organ. Epibranchial organs are particle-aggregating structures found across teleostean fishes that range in complexity from small slits on the dorsal face of the pharyngeal cavity to exceedingly intricate spiraling structures. Despite this morphological diversity and broad distribution, little is known of the epibranchial organ’s functional anatomy. Moreover, the epibranchial organ in silver carp is distinct from others previously described. In silver carp the epibranchial organ is created by five independent pharyngeal involutions that form spiraling epithelial tubes, surrounded by circumferential muscle. This significant addition of surface area within the epibranchial organ likely explains how silver carp can process huge amounts of phytoplankton, outcompeting both invertebrate and vertebrate filter feeders. A functional model for this novel aggregating organ is proposed.

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