Selective feeding on nutrient-rich particles by gizzard shad Dorosoma cepedianum does not involve mechanical sorting

Previous field and laboratory studies have concluded that suspension-feeding detriti vorous fish such as gizzard shad Dorosoma cepedianum selectively ingest nutrient-rich particles using either mechanical sorting within the oropharyngeal cavity or behavioral selectivity within the environment, but none have distinguished between these hypothesized selection mechanisms. To determine whether mechanical selectivity occurs within the oropharyngeal cavity, gizzard shad were fed particles of standardized size but different carbon and nitrogen content in homogeneous particle suspensions vs. non-homogeneous particle distributions. By comparing foregut and epibranchial organ contents with the particles available in a homogeneous suspension, we demonstrated that the fish did not use mechanical selection for nutrient-rich particles. Previously published hypotheses for intraoral selection of nutrient-rich particles in gizzard shad using crossflow filtration or gustatory receptors were not supported. However, when particles with different nutrient content were allowed to settle in a heterogeneous distribution, the nutrients in the foregut and epibranchial organs were 1.5 times higher than those of particles in the water and 2.5 times higher than those of settled particles (p <= 0.0001). As a test of one potential behavioral mechanism of particle selection, disturbance of the sediment-water interface resulted in significantly higher organic carbon (p = 0.01) and nitrogen (p = 0.001) within 1 to 2 cm of the bottom compared to the overlying water and the bottom sediment. Thus, future laboratory and field studies should focus on potential behavioral mechanisms of particle selectivity in detritivorous fish suspension feeding on non-homogeneous distributions of small particles (<< 1 mm).