SEMI-LUNAR AND LUNAR SPAWNING PERIODICITY IN SOME TROPICAL LITTORINID GASTROPODS

Littoraria strigata, Nodilittorina millegrana and N. pyramidalisall spawned regularly in a rock-flanked Malaysian bay in March-June1985, whereas none had spawned there in July-October 1981. N.millegrana showed the least precise timing with some irregularpreponderance during the days between 2d before and 7d afternew or full moon and their associated spring tides. Spawningof N. pyramidalis was more strictly confined to spring tideswith 90% of eggs appearing between 2d before and 5d after newor full moon. There was no apparent difference between alternatespawnings on new and on full moons. L. strigata also spawnedregularly on the spring tides with 89% of eggs appearing between1 and 5 days after new and full moon. In this case, however,full moon spawnings (when spring tides rose highest) yieldedsome 7 times more eggs on average than the weaker spring tidesat new moons. In laboratory aquaria, the three species all released eggs duringboth night and daytime and through periods of both spring andneap tides with no evidence that periodicity persisted in constantconditions. The different degrees of 2-weekly spawning periodicity, togetherwith littorinids which spawn only monthly, may represent a rangeof adaptation to life at increasing shore heights. (Received 26 November 1985;