Meeting Abstract
The complex niche of a fish species is difficult to define, but morphological traits are useful indicators of niche dimensions. Fish morphology partly reflects the evolutionary history and the influence of environmental conditions, such as prey selectivity and availability. This study compares two closely related species, sand shiner (Notropis stramineus) and silver jaw (Notropis bucatus) minnows, in terms of the morphological shape changes of the upper, lower, and pharyngeal jaws over ontogeny. These two species of minnows feed on small invertebrates and mostly midge larvae. The fish were collected locally in Kickapoo Creek, IL. We measured and photographed 40 individuals of each species binned in two different size classes (large and small). The traditional morphometrics measured, standard length, snout-to-vent length, eye diameter, upper jaw length, lower jaw length, and gape, were regressed onto total length to test for allometry. Digital pictures were processed with tpsDig and further analyzed with MorphoJ with a regular geometric morphometrics procedure using principle component analyses. We found that jaw variables show a positive allometric relationship with increasing body length. This is most likely related to a prey shift, from midge larvae to small invertebrates, as the fish grows due to gape limitations. In proportion to body size, the eyes of the small individuals are much larger than that of the larger individuals in both species. This is expected as smaller fish tend to rely on their eyes more for feeding and escaping predators. Geometric morphometrics revealed shortening and bulking of the anterior jaw elements and elongation of the pharyngeal elements with ontogeny. An increased reliability on the pharyngeal apparatus could be driving this shift.
