Photoperiodic Regulation of Gametogenesis in Sea Stars, with Evidence for an Annual Calendar Independent of Fixed Daylength

Abstract

SYNOPSIS. Gametogenesis and gonadal growth in the west coast sea star Pisaster ochraceus normally begins in the fall and leads to large gonads full of gametes in the spring, when spawning occurs. The timing of gametogenesis can be shifted simply by maintaining the animals on a seasonally changing photoperiodic regime out of phase with ambient. When they are kept on a spring-summer photoperiodic regime during the fall and winter, gametogenesis proceeds 6 mo ahead of schedule in the following spring and summer. Gametogenesis can be shifted out of phase even when the eyespots are removed. Short daylengths that normally occur during the fall and winter are not required for gametogenesis to proceed, nor are even the long daylengths of spring and summer that precede the initiation of gametogenesis in the fall. The temporal program is insensitive to fixed daylengths (LD 15:9, 13:11, 12:12, 9:15) and appears to involve an endogenous calendar. Shifting the photoperiodic regime 6 mo out of phase also leads to a shift of the gametogenic temporal program in the sea stars Leptasterias sp. (a brooder) and Asterias vulgaris (from the New England coast), but not in the sea star Patiria miniata. Gametogenic timing also can be switched in the sea urchin Strongylocentrotus purpuratus but the mechanism of the photoperiodic response is fundamentally different; gametogenesis requires short daylengths; continues indefinitely under a repeated short day, fall-winter photoperiod regime, and apparently does not involve an endogenous calendar. As photoperiodic responses are investigated further in these and other marine invertebrates, the models developed primarily from studies on terrestrial organisms may need to be extensively modified or additional new models required.