Spawning of the Tidewater Silverside, Menidia peninsulae (Goode and Bean), in Response to Tidal and Lighting Schedules in the Laboratory

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Abstract
Tidewater silversides,Menidia peninsulae (Goode and Bean) were maintained in 1.3 m diameter holding tanks in identical laboratory recirculating systems. During two weeks under constant conditions (a current velocity of 8 cm per s and continuous illumination, 24L:0D) there was a low relative frequency of arrhythmic spawning. In the subsequent two-week period, fish in one pair of tanks, were maintained under the singular influence of twice daily interruptions in current velocity (8 to 0 cm per s at 0600–0700 and 1800–1900) with continuous illumination. The relative frequency of spawning remained low and there was no evidence of a daily spawning rhythm. However, the mean number of eggs per spawn increased substantially. Fish in the second holding system were subjected to a diel light cycle of 13L:11D with a constant current velocity of 8 cm per s for two weeks. The relative frequency of spawning remained low, and there was no indication of spawning rhythmicity; moreover, there was only a slight increase in the mean number of eggs per spawn. During the third two-week period, fish in the first pair of tanks were provided a 13L:11D diel light cycle, in conjunction with preexisting twice daily interruptions in current velocity; those in the second pair of tanks were provided twice daily interruptions in current velocity in conjunction with the preexisting 13L:11D light cycle. Under the combined influence of interrupted current velocity and diel light cycle cues, there was a marked increase in the relative frequency of spawning in both pairs of tanks. Fish manifested a discernible spawning periodicity, spawns typically occurred between 1800 and 2400; the mean number of eggs per spawn also increased. When fish were returned to constant conditions, current velocity 8 cm per s and 24L:0D for two weeks, the frequency of spawning decreased and there was no indication of a spawning periodicity. Results of another experiment (interrupted current at 1200 to 1300 and 2400 to 0100 with a 13L:11D light cycle) indicated gradual expression of a tidal spawning rhythm during nighttime, 2400 to 0359. Our laboratory results indicate thatM. peninsulae is predominantly a nocturnal spawner and that spawning coincides with interruptions in current velocities.