Diapause, quiescence, hatching requirements: What we can learn from large freshwater branchiopods (Crustacea:Branchiopoda:Anostraca, Notostraca, Conchostraca)

The extent to which dormancy in large freshwater branchiopods is controlled endogenously (diapause) or exogenously (quiescence) is not always clear. It is assumed that both processes occur even within the same brood. Based on the effectiveness of common diapause-deactivating processes such as desiccation, hibernation, and resting, it can be stated that diapause is not a general process controlling responsiveness of large freshwater branchiopod eggs. Only in limited cases unequivocal evidence for the positive influence of these treatments is found. With few exceptions, hatching of activated cysts is effected by specific conditions that may even differ among conspecific populations. Generally, each species (or even population) has a specific temperature range or regime for optimal hatching performance. In a suitable thermal environment with sufficient light and oxygen, hatching is invariably invoked by a low osmotic medium. The erratic hatching pattern in most species is thought to be an adaptation to the variable temporary habitat. Hatching is generally spread over several days or even weeks, but the highest peak usually occurs on the first or second day of hatching. Low hatching percentages were found only in subtropical/desert species and may be a reflection of the low chances for successful reproduction. Generation carry-over of propagules in the egg bank by dormancy, and hatching at low conductivity, are, together with obligate oviparity and absence of an asexual life cycle phase, thought to be highly adaptive to the temporary environment. This enabled large branchiopods to survive since the Upper Cambrian.