While gill rakers have no role in gas exchange, the predominant function of gills, they do perform an equally important function for filter-feeding fish – food acquisition. These bony projections serve as a sieve to trap food particles. They vary greatly in length and number and these characteristic differences have evolved with different feeding strategies for fish. A plankton feeder, for example, such as American Shad Alosa sapidissima, has very tightly-packed, comb-like, gill rakers to efficiently filter their food from the water column. An omnivore or piscivore, on the other hand, has shorter, more widely-spaced gill rakers, better for larger prey items. In fact, the morphology of gill rakers is so diverse that they are often used as a taxonomic tool to identify and classify fish species (e.g., gill raker counts can differentiate species on a dichotomous key).
A meristic is a countable trait, such as number of gill rakers or number of dorsal fin spines. Morphometrics examines the size and shape using a measurable trait, such as standard length or wet weight, which can be gauged as a length, mass, angle or ratio of other measurements. Meristic traits and morphometrics are often used to classify taxa, sometimes down to the species level or sub-species level. In dichotomous keys, these counts and measurements can help identify a particular species of fish. Prior to modern genetic techniques, meristics and morphometrics were the principal foundation for fish taxonomy and systematics. Even today, meristics and morphometrics are commonly used for species identification and ground-truthing genetic analyses with phenotypic traits.