MEPS 559:103-116 (2016)  -  DOI: https://doi.org/10.3354/meps11889

ABSTRACT: Many marine animals produce numerous larvae, few of which survive to adulthood. While larval mortality is generally attributed to environmental causes such as predation and transport to unsuitable habitats, mortality deriving from the process of ontogeny has rarely been investigated. This study examines 2 hypotheses (Acquisition of Robustness Hypothesis and Transitional Timing Hypothesis) that relate ontogenescence (high mortality early in life that declines with age) to the biological process of development. We conducted 2 experiments with larvae of the barnacle Amphibalanus improvisus to test these hypotheses. In Expt 1, where the survival and duration-in-stage of hundreds of individual larvae were tracked under low stress conditions, half of the developing individuals died. Deaths were concentrated in the larval stages immediately adjacent to the 2 major transitions (nauplius-to-cyprid and cyprid-to-juvenile). 89% of deaths occurred in individuals that had delayed their transition to the next stage. In almost every stage, delays were associated with increased risk of death before reaching the next stage. In Expt 2, which examined stage-based tolerance of temperature and salinity stress, the cyprid stage was most susceptible to ecological stressors. Results of both experiments closely follow the predictions of the Transitional Timing Hypothesis, while neither support an Acquisition of Robustness across development. Stages adjacent to major transitions have reduced physiological tolerance to stressors. Simultaneously, these individuals must achieve competence for the next transition or remain in the current stage until death. The resulting suppression of adult recruitment likely plays an important and underappreciated role in the population ecology of marine animals.

KEY WORDS: Age-stage mortality · Arrested development · Biodemography · Evolutionary demography · Infant mortality · Juvenile mortality · Larval senescence · Marine invertebrates