A recent study conducted by Adam Downie and his colleagues at James Cook University in Australia, which was published on May 11 in the open access journal PLOS Biology, sheds light on the remarkable physiological changes that coral reef fish larvae undergo during their short development. These larvae experience a significant transformation as they transition from long-distance dispersal in the open ocean to eventually settling on a reef.
Typically, adult coral reef fish reside within the reef, but when they reproduce, their eggs are carried away by ocean currents. Once hatched, the larvae must navigate against these strong currents in order to find a suitable reef to settle on. This journey is not only challenging but also highly demanding in terms of energy expenditure. Moreover, once the larvae finally settle on the reef floor, they face another hurdle – surviving in low-oxygen (hypoxic) environments during the night.
To gain insights into how fish larvae manage these conflicting challenges, the researchers conducted daily assessments of swimming speed, oxygen uptake, and hypoxia tolerance in cinnamon anemonefish (Amphiprion melanopus) larvae within a laboratory setting. These measurements were taken from the time of hatching until day nine, which is typically when the larvae settle.
In this study, the researchers observed remarkable changes in swimming speed, oxygen uptake rates, and hypoxia tolerance in cinnamon anemonefish larvae over their development period. The swimming speed of the larvae increased from three body-lengths per second at hatching to an impressive ten body-lengths per second by day nine. Simultaneously, the larvae exhibited a decrease in oxygen uptake rates while demonstrating an enhanced ability to tolerate hypoxic conditions, particularly around day five.
To investigate the underlying genetic mechanisms driving these physiological changes, the scientists sequenced the mRNA of larvae at different developmental stages. They discovered that gene activity underwent significant alterations during the larvae’s development, with the most substantial shift occurring between days four and nine after hatching. Notably, the production of different hemoglobin subunits correlated with these physiological changes. The larvae exhibited increased expression of myoglobin, cytoglobin, and neuroglobin, which are molecules responsible for oxygen transport.
Interestingly, cinnamon anemonefish larvae possess the highest recorded oxygen uptake rate relative to their body size among all bony fish studied thus far. This high oxygen uptake rate contributes to their exceptional swimming performance. The researchers propose that changes in gene activity enable the larvae to transition from using hemoglobin subunits that deliver large amounts of oxygen for swimming to utilizing different subunits that help them cope with the low oxygen levels characteristic of the reef floor.
Adam Downie, one of the researchers involved in the study, emphasizes the athleticism of anemonefish larvae, who exhibit impressive swimming capabilities to navigate ocean currents and locate a suitable coral reef to settle on. As they approach the reef, their oxygen uptake rates decrease, and their hemoglobin composition adjusts to enable them to tolerate the low oxygen levels prevalent in their new coral reef habitats during the night.
Source: Public Library of Science