DELAYED HATCHING AS A RESPONSE OF STREAMSIDE SALAMANDER EGGS TO CHEMICAL CUES FROM PREDATORY SUNFISH

Theory predicts that the timing of life history switch points (e.g., m etamorphosis, onset of reproduction) should respond to variations in m ortality risk and growth rates in the life history stages before and a fter the switch point. Very few studies have considered hatching as an adaptive, plastic, life history switch point. Here, we examined the e ffects of chemical cues from green sunfish (Lepomis cyanellus) on the timing of hatching for streamside salamander (Ambystoma barbouri) eggs . Green sunfish can be voracious predators on hatchlings, but, under n atural conditions, rarely if ever consume streamside salamander eggs. Thus theory predicts that eggs should delay hatching in response to th e presence of sunfish. We compared the date, stage and size of hatchin g for eggs reared in water with sunfish chemicals versus in fishless f reshwater. As predicted, the presence of sunfish chemicals caused eggs to delay hatching to a later date, more advanced stage and larger siz e. While delayed hatching could result in increased hatchling escape s uccess from predatory fish, two other selective pathways might also co ntribute to explaining the biological significance of this hatching re sponse. First, increased size and stage at hatching might result in a decrease in exposure to predatory fish due to an increase in hatchling ability to resist involuntary drift into fish pools. Alternatively, d elayed hatching might not be a direct adaptive response to fish predat ion risk on hatchlings, but might represent a by-product of a correlat ion between larval and embryonic responses to fish. Further studies ar e needed to document the generality of these responses and to better u nderstand their adaptive significance.