A tale of missing fins: Morphological variation among populations of Amargosa Pupfish

Authors: Sean C. Lema

Year: 2013 (xlv)

Abstract

Morphological variation within a species may occur under conditions of spatial environmental heterogeneity. In fish, such intraspecific morphological variation is often linked to conditions of water flow, salinity, or other physical environmental parameters. Recently we identified a population of Amargosa pupfish, C. nevadensis amargosae, in a thermal bore spring outflow near Tecopa, California, where 33.6% of pupfish within the population exhibit either complete (10.1% of fish) or partial (only 1 fin; 23.5% of fish) loss of the paired pelvic fins. Of the fish that show only 1 pelvic fin, 65.7% of the fish have a single pelvic fin on the left side of the body, while 34.3% of fish have the fin on the right side. Compared to C. n. amargosae in the main reach of the Amargosa River where > 99% of fish have 2 pelvic fins, pupfish in this thermal spring population are also smaller in body length and mass, have a larger eye and head length relative to their body length, and a larger relative body depth. Body depth is typically sexually dimorphic trait in Cyprinodon spp. with males exhibiting deeper body morphology than females; this thermal spring population, however, shows a distinct reduction in body depth sexual dimorphism compared to conspecifics in the Amargosa River. Although we are only now being to explore the developmental bases for this morphological variation, the patterns of variation observed point to several plausible mechanisms. For instance, the nearly 2 to 1 left:right ratio of pelvic fin occurrence in fish exhibiting only 1 pelvic fin suggests that the morphological variation may result in part from differences in paired-like homeodomain 1 (pitx1) homeobox gene expression. In addition, previous laboratory work with C. n. amargosae has shown that elevated temperatures during larval life can inhibit pelvic fin development by altering thyroid hormone signaling.