. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 136 HAMEL ET AL. Small tubules Large tubules. 0 40 80 120 160 0 40 80 120 160 Gonadal tubule wall thickness (|im) Figure 10. Psolusfabricii Frequency distributions! 10 nm size classes) of the thickness of the gonadal tubule wall for small and large tubules for males collected from May 1988 to August 1989. The arrows indicate the mean thickness for each sampling date. to °C between 5 and 6 June. These variations were due to the semidurnal tides in the Estuary (Demers el al. 1986). During 1988. the maximum temperature
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 136 HAMEL ET AL. Small tubules Large tubules. 0 40 80 120 160 0 40 80 120 160 Gonadal tubule wall thickness (|im) Figure 10. Psolusfabricii Frequency distributions! 10 nm size classes) of the thickness of the gonadal tubule wall for small and large tubules for males collected from May 1988 to August 1989. The arrows indicate the mean thickness for each sampling date. to °C between 5 and 6 June. These variations were due to the semidurnal tides in the Estuary (Demers el al. 1986). During 1988. the maximum temperature was reached in mid July, and the autumnal decrease began in late August. In 1989, warming began in March, attained a peak in late June, and then varied around 6°C until the end of the study (2-4°C daily fluctuations were frequent). The accelerated gonadal growth, which began in March or April, occurred at about the time that the vernal warming began. There was no spawning, either during the warming phase or during marked temperature variations in May and June; rather, spawning occurred in late July when temperatures were more stable. Thus, although spawning occurred at about the same temperature in the two years (5-6°C), the point in the temperature cycle was quite dif- ferent (Fig. 11). Photoperiod. In our study, the renewal of gametogenesis in January coincided with the period when day length and daily bright sunshine were beginning to increase, and the gonadal peak was attained at the photoperiod maxi- mum (Fig. 11). Although Psolusfabricii spawned near the photoperiod maximum in both years, the 1988 spawning occurred at the beginning of this maximum and the 1989 spawning one month later, when photoperiod was just beginning to decline. Freshwater run-off'and the predicted timing of the phy- toplankton bloom. The period during which freshwater run-off decreased in the Estuary was much later in 1989 (July) than in 1988 (late May) (Fig. 11). Nevertheless, spawning in both
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