Livebearing fishes of the Family Poeciliidae exhibit a variety of developmental strategies. In the guppy, young develop in the ovary from the energy stored in large, yolky eggs (lecithotrophy), whereas in the least killifish, the eggs are very small, with little yolk, and most energy for development and growth is transferred from the mother to the embryo through a placenta-like structure (obligate matrotrophy). Other species exhibit a mixed strategy in which energy stored in relatively large eggs is supplemented by maternal transfer of nutrients during embryonic development (supplemental matrotrophy). The type of strategy exhibited by a given species has been suggested to reflect the productivity of the environment that it inhabits. Obligate matrotrophs are predicted to occur only in highly productive environments where food for the mother is abundant and predictable. In less productive or less predictable environments, lecithotrophy is the expected strategy. Species that exhibit supplemental matrotrophy may inhabit unpredictable environments but be capable of exploiting excess food if it occurs and passing extra energy on to developing offspring (facultative supplemental matrotrophy).
The largespring gambusia has traditionally been considered to be strictly lecithotrophic, but recent studies have demonstrated mother-to-embryo transfer of nutrients in this species despite the absence of an obvious placental structure. This study proposes to use radiolabeled nutrients to quantify matrotrophic contribution in largespring gambusia in order to address the following questions: What is the nature of the mother-embryo transfer surface and how does it limit the amount or types of nutrients transferred? To what extent does the largespring gambusia exhibit matrotrophy under natural conditions? How does matrotrophic contribution vary with environmental conditions within a given population and among different populations? Understanding the occurrence of facultative matrotrophy, both across habitats and temporally within habitats, may help explain the variety of developmental strategies within the Family Poeciliidae and the relationship of those strategies to environmental productivity. Furthermore, the strategy of facultative matrotrophy itself may reflect an adaptation to unpredictable environments that allows species exhibiting this strategy to occupy habitats that undergo natural environmental variation. This strategy, then, may provide an important buffer against human-induced environmental changes.