Experimental examination of interpopulation variation in reproductive timing of a fish

Many marine organisms have a specific reproductive timing, but its adaptive nature has not been directly tested because of the difficulty of manipulat ing reproductive timing in the field. Egg survival in relation to the timin g of egg release in the puffer Takifugu niphobles, an intertidal spawner, w as assessed by experimentally transplanting, eggs to several tidal zones, a nd the observed pattern of variation in spawning time across environments ( a pebble vs. a sandy beach) was examined to see if it was consistent with t he presumed selection regime. The vertical distribution of eggs and the obs ervation of spawning zones showed that substrate characteristics affected t he fate of released eggs: at the pebble beach eggs were stranded in the hig h intertidal zone, while they were washed out toward the lower zone at the sandy beach. The ego-transplant experiment showed that egg survival was low er in the high zone than in the middle and low zones at both the pebble and sandy beaches, indicating that individual fishes should spawn earlier rela tive to the tidal cycle. However, the selection pressure for the fish to sp awn earlier was more intense at the pebble beach than at the sandy beach, w hich was attributed to the difference between the beaches in the degree to which eggs were transported seaward. At the pebble beach (1) the timing of spawning was more strictly synchronized with the tidal cycle, (2) a day's s pawning started, peaked, and ended earlier relative to the tidal cycle, and (3) a day's spawning was concentrated in a shorter time, than at the sandy beach. These temporal patterns of spawning are allexpected from the presum ed selection regime across the beaches, suggesting that there is an evoluti onary response of the trait to this selective force. This study explains in terpopulation variation in reproductive timing of a marine fish by combinin g experimental manipulations of the reproductive timing with classic compar ative methods. This experimental approach performed in different ecological settings is essential to further understanding of the ecology and evolutio n of temporal patterns of reproduction.