Function and variation of gill rakers in the fish family Catostomidae, with comments on phylogenetic tests of natural selection.

Abstract

Gill rakers in the fish family Catostomidae (Teleostei: Ostariophysi: Cypriniformes) are examined in functional, allometric, and evolutionary contexts. Catostomids are predominantly benthic feeders. Individuals take in a mouthful of substrate and prey, reject the substrate, but selectively retain the prey. Original observations and comparisons with work on <italic>Cyprinus carpio </italic> indicate that during prey processing the branchial basket and rakers of catostomids function by moving toward the palatal organ (muscular pad suspended from the roof of the pharynx) while bulges form on the surface of the palatal organ, pinching prey between palatal organ and rakers. Water currents then wash away the substrate. A dozen raker parameters were measured in <italic>Catostomus commersonii </italic> (19--429 mmSL), <italic>Ictiobus bubalus</italic> (25--309 mmSL), <italic>Erimyzon sucetta</italic> (41--194 mmSL), and <italic> Hypentelium nigricans</italic> (39--197 mmSL). Rakers are added until approximately 100 mmSL. Allometric changes in raker number and size are not correlated with ontogenetic niche shifts. Size of the fish is the best predictor of raker parameters, although the exact ontogenetic trajectory may change when rakers are no longer added. Characteristics of the rakers were analyzed within the framework of a phylogeny and comparisons were made between apomorphic (derived) and plesiomorphic states while taking into consideration hypothesized selective regimes. This Homology Approach takes advantage of the individuality of the Catostomini and Moxostomatini lineages to compare evolution as change over time. Twenty-six catostomid species and two cyprinid species were chosen to represent the phylogenetic, morphological, and ecological diversity. Raker width and length of knobs on the surface of the rakers are correlated with prey size, which is consistent with hypothesized action of natural selection. Distance between rakers and distance between knobs are correlated with substrate size, which is consistent with hypothesized action of natural selection. Unknown factors influenced knob height. Raker length and height are correlated with pharynx size. Lateral projections were found to be hypertrophied in some taxa that could effectively sieve prey from the medium. Multiple factors, including phylogenetically inherited traits and natural selection, influenced the evolution of catostomid gill rakers uniquely in different lineages.