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SynopsisThe influence of the 1ight:dark cycle and temperature on the embryonic development, especially diapause, of the annual fish Nothobranchius korthausae was investigated. The variability of the frequency of diapause II during constant L:D cycle and temperature, but at different times of the year, was also studied. In agreement with previous studies it appeared that diapause I does practically not occur in N. korthausae (and other Nothobranchius species), even under sub-optimal conditions which are known to induce diapause II and III. Only at very low temperatures, a first developmental arrest could be induced during the dispersed phase, as well as during the reaggregation phase. Diapause II and III can be induced more easily, by exposing the embryos to low (sub-optimal) temperatures and short light periods (or constant darkness). Both diapauses are inhibited or terminated at high temperature and long light periods (12L:12D). The occurrence of an indirect 1ight:dark response via the adult fishes could not be demonstrated. At moderate temperatures and in constant darkness (or short light periods) considerable variability in diapause II-frequency was observed. This could be an intrinsic feature of annual fish development, constituting a strategy for better survival of the species.

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Abstract. The effect of salinity on embryonic development of Sepia officinalis (cuttlefish) in the Delta Area (South Western part of The Netherlands) was investigated in 1988/1989, and compared with data concerning the distribution of S. officinalis in the three main parts of this area: Oosterschelde, Westerschelde and Grevelingen. Embryos hatched in water collected at Yerseke (Oosterschelde), Vlissingen (Western part of the Westerschelde) and Bommenede (Grevelingen), i.e., at salinity values above 28.1%o, but not in water sampled at Hoedekenskerke and Hansweert (Middle and Eastern part of the Westerschelde; salinities below 22.0%0). Under laboratory conditions, using diluted Oosterschelde water, the highest hatching percentages of S. officinalis were found at salinities above 29.8%0. Some embryos hatched at a salinity value of 26.5%o but no hatching occurred at salinities below 23.9%o. In embryos exposed to salinity changes during late embryonic development, the developmental rate decreased at salinity values of 28.7%0 or less. Below 22.4%o embryos with morphological malformations were found. It can be concluded that salinity is an important factor limiting the distribution of S. officinalis in most parts of the Delta Area, with the exception of the Western part of the Westerschelde and the Grevelingen.