Life-history variation of Gambusia nobilis, Pecos Gambusia, at Bitter Lake National Wildlife Refuge

Authors: Alyssa Hopkins

Year: 2012 (xliv)

Abstract

Life history characteristics are shaped by differential investment of limited resources that maximizes lifetime fitness under variable selective pressures. The live bearing Pecos Gambusia, Gambusia nobilis, is an ideal organism for studying ecological effects on life history because reproductive investment is made throughout embryonic development and they occupy isolated habitats with varying ecological conditions. The goal of this research is to describe seasonal and population variation in life history and ecological effects on these characteristics. Specimens for dissection were collected monthly (May 2011 through April 2012) from four populations at Bitter Lake National Wildlife Refuge, Roswell NM, (sinkholes 7, 27 South, 31, and 37). Females were gravid between April and August with brood sizes ranging from 2-49 with a mean of 16. Maternal brood reproductive effort (brood mass/eviscerated mass) increased from April to May and declined from June to July. G. nobilis did not strongly exhibit the brood size/average egg mass trade-off commonly observed in fishes. Brood size only accounted for 3.41% of the variation in egg size (R²=0.0340, p=0.0569). Brood Size showed an inverse U distribution from April through August while average egg mass increased throughout the reproductive season. Increasing offspring size may maximize survival under changing environmental conditions and shortened growth period prior to overwintering. No strong correlations were found between these life history characteristics and ecological conditions except for a weak positive correlation between relative abundance of the predator Plains killifish, Fundulus zebrinus, and maternal brood reproductive effort (R²=0.0416, p=0.0349). Egg mass and brood size appear most correlated to seasonal variables including water temperature, day length, and pH. Between populations, no significant difference was found in maternal brood reproductive effort, egg mass, or brood size when pooled across months. Preliminary results may also suggest unique reproductive strategies of matrotrophy and superfetation. Egg mass was significantly greater in eyed vs un-eyed embryos (possibly through matrotrophic provisioning) and females with embryos of different developmental stages were observed, supporting the hypothesis of superfetation.