Reproductive Adaptations among Viviparous Fishes (cyprinodontiformes: Poeciliidae).

Within the family Poeciliidae, a wide range of adaptations accompanying viviparity has evolved. The reproductive mechanisms of five representative species were examined and related to the habitats in which they are employed. Poecilia reticulata, the guppy, represents the majority of poeciliids which have a single stage of embryos in the ovary at a time. Most of the nutrients are prepack-aged in the eggs before fertilization; no specialized placental adaptations have evolved. The major variable among species utilizing this mechanism is in the yolk loading time; some eggs are mature and ready to be fertilized immediately after the birth of a clutch of embryos, others take a week or more. The mechanism may be thought of as a generalist-type, being employed throughout the range of poeciliids and in all varieties of habitats. Poeciliopsis monacha has embryos of two different ages occurring simultaneously in the ovary. Its large eggs provide the embryo's entire nutrient supply. Mature embryos weigh 47% less than mature eggs. This species lives in a harsh, montane environment with an unpredictable food supply; its reproductive mechanism is well suited to fluctuating resources. When conditions are favorable, a large clutch of eggs is loaded, but if food is in short supply, clutch sizes are small or only one stage is produced. During starvation conditions, no eggs are produced; but, with restoration of food supply, reproduction resumes quickly. Poeciliopsis lucida has medium-sized eggs and three stages of embryos. At birth, young weigh slightly more than mature ova. This weight gain plus the energy utilized for embryonic metabolism constitutes a significant maternal contribution during development. P. lucida is wide-spread in the three rivers that comprise its range but it does not enter the depauperate rocky arroyos occupied by P. monacha. P. lucida requires a stable food supply characteristic of the fertile and productive downstream regions. Poeciliopsis prolifica carries as many as five stages of embryos which are born at short brood intervals of two to eight days. Ova sizes are small and embryos receive almost all of their nutrients through a placenta. P. prolifica lives in a rich environment where food resources are stable. It is the smallest species in the genus and it produces the smallest young, yet its habitat seems precarious. Living on the steep slope between deep and shallow water it is wedged between deep and shallow water competitors, having to cope with large deep water fish predators as well. Poeciliopsis turneri is the largest of the five species studied and occupies a habitat somewhat unusual for poeciliids. Three stages of embryos are present in the ovary but brood sizes are usually small (ave. 3.6). The ova are small yet the mature embryos are large, weighing almost 19 times more than the ova. Brood sizes in neither P. prolifica nor P. turneri become larger as female size increases. P. turneri is adapted to life in deep swift water where large adults, large offspring and streamlining are important. The other four species can risk the swollen pregnant-look typical of poeciliids, whereas P. turneri cannot. Superfetation (multiple stages of embryos) is advantageous in that it avoids the surge of energy required to load a single large clutch of eggs. The energy for reproduction is sequentially divided among two to five small clutches. Reduction of ova size increases the space available for more stages of embryos. Although embryos with small yolk supplies place a nutrient drain directly on the mother, surges in demand are avoided since embryonic stages enter the growth phase at different times. A continuous food supply to the female is essential; otherwise, embryos will die and the reproductive effort to that point is lost. REPRODUCTION AND DEVELOPMENT IN CHONDRICHTHYAN FISHES