Fig 1.
Conceptual framework of connectivity and life history of reef fishes, along with a priori predictions for how the physiology of reef fishes changes during early life history. (A) Reef fish populations exist in patches on reefs isolated by swift currents and depth (grey, pink, and blue reefs indicate separate, isolated reef fish populations). Connectivity between populations occurs via a dispersive pelagic larval phase, capable of fast swimming speeds and well-developed sensory systems and behaviours. Straight arrows indicate movement of larvae between reefs, and curved arrows indicate self-recruitment back to natal reefs. (B) The general life history of reef fishes. Adult reef fishes living on benthic reef habitats (1), lay their eggs on the reef (2), which are carried off the reef by passive processes like currents and wave energy (3). The eggs hatch, and the larvae undergo a pelagic larval phase (9 days for focal cinnamon anemonefish (Amphiprion melanopus), as indicated by numbers under each larva) in the open ocean (4). The reef fish must quickly develop and grow before swimming back to the reef during this phase, and we predict that mass-specific oxygen uptake rates (ṀO2, a proxy for metabolic rates) are very high for most of the larval duration (indicated by purple gradient). Swimming performance measured as the critical swimming speed (Ucrit) is predicted to be poor during the first few dph but increases as fish grow and develop (indicated by red gradient). Hypoxia tolerance is predicted to increase over larval development during their pelagic phase, which is thought to enable low-oxygen level challenges on the reef at night (indicated by blue gradient). Reef fish larvae settle onto the reef (5) but are already remarkably hypoxia tolerant (low oxygen due to nocturnal coral respiration) and are predicted to lower their oxygen uptake rates to become suitably hypoxia tolerant (indicated by purple gradient). Changes in oxygen uptake rates, swimming performance, and hypoxia tolerance are predicted to be related to gene expression patterns of proteins responsible for oxygen transport and delivery to tissues (e.g., Hb, Mb, Ngb, and Cytgb; indicated by green gradient). The combination of fast swimming speeds during pelagic phases and predicted changing metabolic demands to tolerate hypoxia in new reef habitats (new or natal reef; refer to Fig 1A) may be selected for to support connectivity and recruitment of larvae to adult populations (6). Cytgb, cytoglobin; dph, days post hatch; Hb, haemoglobin; Mb, myoglobin; Ngb, neuroglobin.