DIAPAUSE, QUIESCENCE, HATCHING REQUIREMENTS - WHAT WE CAN LEARN FROM LARGE FRESH-WATER BRANCHIOPODS (CRUSTACEA, BRANCHIOPODA, ANOSTRACA, NOTOSTRACA, CONCHOSTRACA)

The extent to which dormancy in large freshwater branchiopods is contr olled endogenously (diapause) or exogenously (quiescence) is not alway s clear. It is assumed that both processes occur even within the same brood. Based on the effectiveness of common diapause-deactivating proc esses such as desiccation, hibernation, and resting, it can be stated that diapause is not a general process controlling responsiveness of l arge freshwater branchiopod eggs. Only in limited cases unequivocal ev idence for the positive influence of these treatments is found. With f ew exceptions, hatching of activated cysts is effected by specific con ditions 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 enviro nment 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. Hat ching is generally spread over several days or even weeks, but the hig hest peak usually occurs on the first or second day of hatching. Low h atching percentages were found only in subtropical/desert species and may be a reflection of the low chances for successful reproduction. Ge neration carry-over of propagules in the egg bank by dormancy, and hat ching at low conductivity, are, together with obligate oviparity and a bsence of an asexual life cycle phase, thought to be highly adaptive t o the temporary environment. This enabled large branchiopods to surviv e since the Upper Cambrian.