. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. DAYLENGTH. DEGREES. AND DIAPAUSE 43 100r M Q> O w D O 9) C CO 15 3 CO. 20 • 10 11 12 13 Daylength (h) 14 15 16 Figure 1. The timing of the switch from production of subitaneous (immediately hatching) eggs to diapausing eggs by Diaplomus sanguineus from Bullhead Pond. Rhode Island. The continuous line connecting error bars shows the mean trajectory- over 9 years (± 95% CI) for copepods in the lake, expressed as a function of photoperiod on the date that the animals were collected. The line connecting individual open data p
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. DAYLENGTH. DEGREES. AND DIAPAUSE 43 100r M Q> O w D O 9) C CO 15 3 CO. 20 • 10 11 12 13 Daylength (h) 14 15 16 Figure 1. The timing of the switch from production of subitaneous (immediately hatching) eggs to diapausing eggs by Diaplomus sanguineus from Bullhead Pond. Rhode Island. The continuous line connecting error bars shows the mean trajectory- over 9 years (± 95% CI) for copepods in the lake, expressed as a function of photoperiod on the date that the animals were collected. The line connecting individual open data points shows the photoperiod response of females reared in the laboratory at 9°C (from Hairston and Olds. 1986). The line connecting closed data points shows the photopenod response of females reared at the temperatures prevailing at the daylengths indicated (this study). thermal inertia of water. The temperature of the medium in which they live is a more reliable indicator of the passing of the seasons than is the case on land. We might expect, then, that temperature plays a more significant role in regulating timing of diapause in aquatic animals than in their terrestrial counterparts. Among pelagic copepods. significant temperature modification of photoperiod re- sponse has been found in every instance investigated, both for cyclopoid copepods with a late-instar diapause (Wat- son and Smallman. 1971: Alekseev, 1990) and for cal- anoid copepods with egg diapause (Marcus, 1982: Walton. 1985; Hairston el «/., 1990: Ban. 1992). Furthermore, Alekseev (1990) has pointed out that the effect of tem- perature in delaying critical photoperiod is about twice as great in pelagic copepods ( hours per C°) as it is in terrestrial insects ( hours per C°). An enhanced effect on critical photoperiod is only one way in which temperature might be expected to influence the timing of diapause. In some species, diapause response to changing photoperiod can be gradual rather than a dis
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