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.

The influence of photoperiodicity on hatching of Sepia officinalis was investigated under different experimental light-dark (LD) conditions. The results are viewed in relation to some relevant properties of the perivitelline fluid (PVF) and the egg capsule during embryonic development. In embryos of S. officinalis the transition from light to dark ap-pears to act as a 'Zeitgeber' or synchronizer. The embryos consistently hatched during periods of darkness, even when the duration of the dark period was short (1–4 h) and replaced part of the natural light period. The hatching rhythm was independent of the embryonic stage at which the experimentwas started. Embryos that developed under a given LD rhythm did not hatch at that rhythm if it was changed or eliminated. In the absence of an external LD rhythm the time to hatching increased. Lack of pigmentation in the egg envelope appeared to reduce the time to hatching. If embryos were exposed to a single dark period of 1–4 h significantly more hatched during darkness while a dark-pulse of ten minutes resulted in no hatching. At the end of embryonic development the egg capsule of S. officinalis becomes thinner due to the expanding PVF. Absence ofthe envelope did not affect embryonic development but dramatically increased mortality and prema hire hatching (96%). Spectrophotometrical investigations indicated that light between 200 and 900 nm is absorbed similarly by the envelope and by female ink. The function of pigmentation in the envelopes remains obscure.

The influence of photoperiodicity on hatching of Loligo forbesi and Loligo vulgaris embryos was investigated under different experimental light-dark (LD) conditions. The transition from light to dark stimulated hatching and functions as a ‘Zeitgeber’ or synchronizer. Independent of the timing and duration of the dark period most embryos hatched soon after termination of the light period. Embryos which had developed in constant light, showed no hatching rhythm at all. If these embryos were exposed to a dark shock most embryos hatched soon after the onset of darkness. A twilight shock, in which the light was reduced by 50% (i.e. 50 μE s−1 m−2), could not stimulate hatching. Embryos which were kept from stage X on in an artificially controlled LD cycle, preferentially hatch in a period which coincides with the period at which darkness usually occurred when placed in constant illumination from stage XX onwards.

The influence of copper on embryonic development and hatching of Sepia offinalis was investigated. Copper exerts a profound effect on both hatching stage and time-to-hatching. At high copper concentrations (50-200 ppb Cu2+), the embryos hatch earlier than the controls but have a lower survival potential. No external morphological malformations were found. Whereas copper does not accumulate in the embryo or in the vitellus, it is absorbed by the envelope and/or the chorion.

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