Benefits of intertidal development and large egg size in a marine toadfish

The environments in which offspring are reared can profoundly affect their development, growth, and survival. Certain specialized fish species deposit and sometimes care for their young in one of the Earth’s most dynamic ecosystems: the marine intertidal zone. This tendency to deposit eggs in intertidal habitats rather than more stable coastal waters suggests intertidal conditions confer benefits to offspring that outweigh the risks parents face when spawning at the ocean’s edge. Depositing eggs in the warmer intertidal zone might allow fish to produce and hatch larger eggs than would be possible in cooler subtidal waters, where development rates are much slower. We used two laboratory experiments and a field experiment to investigate how the benefits of intertidal development could vary according to egg size in the plainfin midshipman toadfish, Porichthys notatus. These toadfish breed along an intertidal gradient and down into shallow subtidal waters. We found that larger toadfish mothers produced larger eggs that hatched into larger young. Mothers laying these larger eggs also deposited them in warmer nests at higher tidal elevations, where the eggs can hatch into the largest juveniles with the highest survival rates. Juveniles that were raised in warm water in the laboratory (mimicking conditions in the high intertidal zone) experienced the highest survival rates during development and were also bolder and exhibited faster swimming speeds when fleeing a simulated predator. Taken together, these results suggest variation in propagule size can support divergent nest site choices among females, with females producing larger eggs benefiting more from depositing their eggs in warmer areas. Individual variation in propagule size could thus support niche partitioning by expanding the range of suitable conditions for nesting sites.