Hartmann, A.C.

When juveniles must tolerate harsh environments early in life, the disproportionate success of certain phenotypes across multiple early life stages will dramatically influence adult community composition and dynamics. In many species, large offspring have a higher tolerance for stressful environments than do smaller conspecifics (parental effects). However, we have a poor understanding of whether the benefits of increased parental investment carry over after juveniles escape harsh environments or progress to later life stages (latent effects). To investigate whether parental effects and latent effects interactively influence offspring success, we determined the degree to which latent effects of harsh abiotic conditions are mediated by offspring size in two stony coral species. Larvae of both species were sorted by size class and exposed to relatively high-temperature or low-salinity conditions. Survivorship was quantified for six days in these stressful environments, after which surviving larvae were placed in ambient conditions and evaluated for their ability to settle and metamorphose. We subsequently assessed long-term post-settlement survival of one species in its natural environment. Following existing theory, we expected that, within and between species, larger offspring would have a higher tolerance for harsh environmental conditions than smaller offspring. We found that large size did enhance offspring performance in each species. However, large offspring size within a species did not reduce the proportional, negative latent effects of harsh larval environments. Furthermore, the coral species that produces larger offspring was more, not less, prone to negative latent effects. We conclude that, within species, large offspring size does not increase resistance to latent effects. Comparing between species, we conclude that larger offspring size does not inherently confer greater robustness, and we instead propose that other life history characteristics such as larval duration better predict the tolerance of offspring to harsh and variable abiotic conditions. Additionally, when considering how stressful environments influence offspring performance, studies that only evaluate direct effects may miss crucial downstream (latent) effects on juveniles that have significant consequences for long-term population dynamics.

Many marine invertebrates provide their offspring with symbionts. Yet the consequences of maternally inherited symbionts on larval fitness remain largely unexplored. In the stony coral Favia fragum (Esper 1797), mothers produce larvae with highly variable amounts of endosymbiotic algae, and we examined the implications of this variation in symbiont density on the performance of F. fragum larvae under different environmental scenarios. High symbiont densities prolonged the period that larvae actively swam and searched for suitable settlement habitats. Thermal stress reduced survival and settlement success in F. fragum larvae, whereby larvae with high symbiont densities suffered more from non-lethal stress and were five times more likely to die compared with larvae with low symbiont densities. These results show that maternally inherited algal symbionts can be either beneficial or harmful to coral larvae depending on the environmental conditions at hand, and suggest that F. fragum mothers use a bet-hedging strategy to minimize risks associated with spatio-temporal variability in their offspring's environment.

In coral reef conservation and management, the prevailing metric of reef health is percent coral cover, a measurement commonly used with the assumption that each unit of live coral tissue has equivalent ecological value. Here we show that the reproductive output of a coral population is not proportional to the cover of coral present. Instead, when compared to declining populations nearby, high cover coral populations produced up to four times more larvae per square centimeter of tissue, resulting in up to 200 times higher larval production per square meter of reef. Importantly, corals that produced more larvae did not produce smaller larvae, as predicted by resource allocation theory. Instead, higher fecundity corresponded to higher energetic lipid reserves in higher cover coral populations. In the wake of unprecedented global coral bleaching, our findings suggest that the largest reductions in coral reproduction may occur when corals are lost from previously healthy populations.

Oil spills cause damage to marine wildlife that lasts well past their immediate aftermath. Marine offspring that must settle and metamorphose to reach adulthood may be particularly prone to harm if the legacy of oil exposure interrupts later transitions across life stages. Following an oil spill on Curaçao, we found that oil-contaminated seawater reduced settlement of 2 coral species by 85% and 40% after exposure had ended. The effect of contamination on settlement was more severe than any direct or latent effects on survival. Therefore, oil exposure reduces the ability of corals to transition to their adult life stage, even after they move away from oil contamination. This interruption of the life cycle likely has severe consequences for recruitment success in these foundational and threatened organisms. Latent, sublethal, and behavioral effects on marine organisms—as shown in this study—are not commonly considered during oil-spill impact assessments, increasing the likelihood that harm to marine species goes underestimated or unmeasured.