Rapid embryonic development supports the early onset of gill functions in two coral reef damselfishes

The gill is one of the most important organs for growth and survival of fishes. Early life stages in coral reef fishes often exhibit extreme physiological and demographic characteristics that are linked to well-established respiratory and ionoregulatory processes. However, gill development and function in coral reef fishes is not well-understood. Therefore, we investigated gill morphology, oxygen uptake, and ionoregulatory systems throughout embryogenesis in two coral reef damselfishes, Acanthochromis polyacanthus and Amphiprion melanopus (Pomacentridae). In both species, we found key gill structures to develop rapidly early in the embryonic phase. Ionoregulatory cells appear on gill filaments 3-4 days post fertilization and increase in density, whilst disappearing or shrinking in cutaneous locations. Primary respiratory tissue (lamellae) appears 5-7 days post fertilization, coinciding with a peak in oxygen uptake rates of the developing embryos. Oxygen uptake was unaffected by phenylhydrazine across all ages (pre-hatch), indicating that haemoglobin is not yet required for oxygen uptake. This suggests that gills have limited contribution to respiratory functions during embryonic development, at least until hatching. Rapid gill development in damselfishes, when compared to most of the previously investigated fishes, may reflect preparations for a high-performance, challenging lifestyle on tropical reefs, but may also make reef fishes more vulnerable to anthropogenic stressors.

Diel CO 2 cycles and parental effects have similar benefits to growth of a coral reef fish under ocean acidification

Parental effects have been shown to buffer the negative effects of within-generation exposure to ocean acidification (OA) conditions on the offspring of shallow water marine organisms. However, it remains unknown if parental effects will be impacted by the presence of diel CO 2 cycles that are prevalent in many shallow water marine habitats. Here, we examined the effects that parental exposure to stable elevated (1000 µatm) and diel-cycling elevated (1000 ± 300 µatm) CO 2 had on the survival and growth of juvenile coral reef anemonefish, Amphiprion melanopus . Juvenile survival was unaffected by within-generation exposure to either elevated CO 2 treatment but was significantly increased (8%) by parental exposure to diel-cycling elevated CO 2 . Within-generation exposure to stable elevated CO 2 caused a significant reduction in juvenile growth (10.7–18.5%); however, there was no effect of elevated CO 2 on growth when diel CO 2 cycles were present. Parental exposure to stable elevated CO 2 also ameliorated the negative effects of elevated CO 2 on juvenile growth, and parental exposure to diel CO 2 cycles did not alter the effects of diel CO 2 cycles on juveniles. Our results demonstrate that within-generation exposure to diel-cycling elevated CO 2 and parental exposure to stable elevated CO 2 had similar outcomes on juvenile condition. This study illustrates the importance of considering natural CO 2 cycles when predicting the long-term impacts of OA on marine ecosystems.