Kategorie: News
Habitat Shapes Hormones and Body Structure
Any diver who has ever observed a school of striped surgeonfish (Acanthurus triostegus) above a reef sees fully formed, finely patterned fish. What many people don’t know is that these animals arrive at the reef far from fully developed—and exactly how they “assemble” themselves there depends largely on where they come ashore.
A Transformation in One Week: The striped surgeonfish, widespread in the Indo-Pacific and called “Manini” by Polynesians, initially spends about 53 days as a transparent larva in the open ocean. It is only in shallow coastal waters that the actual transformation begins: Within a few days, the larva transforms into a juvenile fish adapted to reef life - with new coloration, a restructured jaw and digestive tract, and a complete shift in diet from plankton to algae.
This process is controlled by thyroid hormones—the same signaling molecules that cause tadpoles to lose their tails. A team from the Okinawa Institute of Science and Technology (OIST) and the French marine research station CRIOBE on Moorea (French Polynesia) demonstrated that within the first twelve hours on the reef, over 4,600 genes change their activity.
Three nurseries, three developmental pathways
The researchers studied juvenile fish in three habitats around Moorea: eroded coral rock with abundant fish and little cover; muddy mangrove zones with high temperatures and low oxygen levels; and sandy beaches with little structure but few predators.
Surprisingly, genetically identical fish develop completely differently depending on their habitat. Fish from the coral rock had the highest thyroid hormone levels, while mangrove fish had the lowest—a finding confirmed by a repeat measurement one year later. The pathways leading to these outcomes also differed: in mangrove fish, the overarching hormonal control in the brain remained active, whereas in coral rock fish, the downstream genes were more active. So different pathways lead to the same goal.
Metabolism also adapted: juvenile mangrove fish followed a growth-oriented program with high blood sugar, while juvenile beach fish followed an energy-saving program with lower sugar levels. Even the body’s hunger signals differed depending on the environment.
Study leader Vincent Laudet emphasizes just how challenging the transition is to begin with: On the first day on the reef, around 90 percent of the juvenile fish are eaten, and survivors often lose one-fifth of their body weight in the first week. The fact that the body can also respond flexibly to different environmental conditions acts as a biological buffer—the same genes, different developmental pathways, depending on where the animal ends up.
Reefs adjacent to mangroves or sandy areas serve as transition zones between habitats and are increasingly proving to be crucial for the offspring of many reef fish species. Unfortunately, mangrove ecosystems in particular are already under severe pressure worldwide due to warming and declining oxygen levels (see also: News article - Mangrove forests)
The study explicitly did not test whether this developmental flexibility actually helps the fish cope with degraded habitats—that remains an open question for future research. However, the findings, published in the journal *Science Advances*, provide an important building block for understanding how adaptable reef fish can be in response to a changing ocean.
Sources:
Earth.com: https://www.earth.com/animals/reef-fish-nurseries-shape-bodies/
OIST/EurekAlert: https://www.eurekalert.org/news-releases/1138354
Science Advances (original study): https://www.science.org/doi/10.1126/sciadv.aec5359
Taucher.Net / DiveInside: Mangrove forests - a focus on coastal oases
Video:
https://www.eurekalert.org/multimedia/1144326