Embryonic diapause

Abstract: Annual fishes can maintain permanent populations in temporary aquatic habitats since the population survives dry seasons in the form of diapausing eggs. Populations persist even though subject to erratic environmental cycles and recurrent ecological catastrophes. Developmental arrest occurs at one or all of the following stages: Diapause I (Dispersed cell phase); Diapause II (Long somite embryo); Diapause III (Prehatching). In Austrofundulus, Diapause I is facultative. Diapause II and III are obligate, long and variable (105 ± 20 days each). Subpopulations (about 10%) of “escape eggs” bypass Diapause II and/or Diapause III. Pterolebias and Rachovia eggs behave much like those of Austrofundulus. In Rachovia, the duration of Diapause II is 80 ± 33 days and of Diapause III at least 61 ± 23 days. Subpopulations of “escape eggs” are present. Cynolebias and Nothobranchius can undergo facultative arrest at Diapause I and II and enter obligate arrest at Diapause III. Annual species of Aphyosemion can undergo facultative arrest at Diapause I and II, and an obligate arrest at Diapause III. A few species may also experience obligate arrest at Diapause II. A short term arrest phenomenon, “retarded hatching,” sometimes is encountered among non-annual Aphyosemions and other non-annual cyprinodonts. Survival strategy is based on the “multiplier effect”; i.e., interposition into the developmental pathway of three branch points containing diapause stages of prolonged, variable duration generates eight different distributions of total developmental time. Thus, a single egg population of identical age can generate several subpopulations, all of which develop according to different schedules. A developmental program is established which permits the repeated loss of individual eggs under conditions which may initiate hatching but do not allow for maturation and successful reproduction. The “multiplier effect” augmented by other adaptations guarantees that some portion of the egg population will survive to reproduce.