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 CO2 fluctuations alter the molecular response of coral reef fishes to ocean acidification conditions

Environmental CO2 variation can modify the responses of marine organisms to ocean acidification, yet the underlying mechanisms for this effect remain unclear. On coral reefs, environmental CO2 fluctuates on a regular day-night cycle. Effects of future ocean acidification on coral reef fishes might therefore depend on their response to this diel cycle of CO2. To evaluate the effects on the brain molecular response, we exposed two common reef fishes (Acanthochromis polyacanthus and Amphiprion percula) to two projected future CO2 levels (750 and 1,000 microatmospheres) under both stable and diel fluctuating conditions. We found a common signature to stable elevated pCO2 for both species, which included the downregulation of immediate early genes, indicating lower brain activity. The transcriptional program was more strongly affected by higher average CO2 in a stable treatment than for fluctuating treatments, however, the largest difference in molecular response was between stable and fluctuating CO2 treatments. This indicates that a response to a change in environmental CO2 conditions is different for organisms living in a fluctuating than in stable environments. The differential regulation was related to steroid hormones and circadian rhythm (CR). Both species exhibited a marked difference in the expression of CR genes among CO2 treatments, possibly accommodating a more flexible adaptive approach to acid-base control, which could explain reduced impairment. Our results suggest that environmental CO2 fluctuations might enable reef fishes to phase shift their clocks and anticipate CO2 changes, thereby avoiding impairments and more successfully adjust to ocean acidification conditions.

Coral reef fishes in the coastal waters of Ninh Thuan province

This study was conducted to evaluate the species composition and distribution of coral reef fish communities at 24 sites in three areas Ninh Hai, Phuoc Dinh, and Ca Na in Ninh Thuan province from 2018 to 2019. A total of 301 species belonging to 131 genera and 49 families of coral reef fishes were recorded. Among them, the wrasse (Labridae: 55 species), the damselfish (Pomacentridae: 46 species) and the butterflyfish (Chaetodontidae: 26 species) occupy the three highest proportion. The average density of coral reef fish in coastal waters of Ninh Thuan province is 106.8 ± 23.4 individuals/100 m2, most of them are small sized fish and ornamental fish groups. Ninh Hai has a higher species richness and density than other areas. Meanwhile, Ca Na and Phuoc Dinh have the two highest densities of the large sized fish and food target fish groups. This research also points out the impact of monsoon on the density and distribution of coral reef fish, in which Ninh Hai is most affected.