Feast or Famine: How Do Animals Adapt to Extreme Fluctuations in Their Environment?
It’s hard to imagine a life more awful than a barnacle’s.
For half the day you’re battered by salty, cold water. For the other half, you bake in the hot sun. All the while, you still have to gather food, keep yourself safe from predators, and find a mate. Somehow, these strange crustaceans thrive in this environment. Nicolas Rohner, a Whitman Center scientist at the Marine Biological Laboratory (MBL), wants to understand how they do it.
Rohner, a developmental biologist at the Stowers Institute for Medical Research, has spent much of his career studying animals that make themselves at home in extreme environments. He visited the MBL this summer with the intention of returning next year to expand his research towards marine animals, specifically barnacles.
The animals that Rohner studies are characterized by a “feast or famine” lifestyle. They oscillate between periods of intense starvation and having a profound wealth of food, and they must capitalize on the latter to survive the former. Being able to switch between modes requires enormously flexible physiology, which fascinates Rohner.
The first species that Rohner studied is a fish named Astyanax mexicanus, or the Mexican tetra. This fish species consists of two distinct populations. One group lives in creeks and pools throughout southern Texas and northern Mexico and looks like any other small, silvery fish. The second group, meanwhile, is blind, colorless, and lives deep in caves. These caves are so dark that no plants grow within them; the fishes’ only food comes in the summer months, when flooding pushes debris into their subterranean pools. During this time, the fish must eat fast to consume enough food to get through the rest of the year.
“A lot of food comes in, they eat a lot, get really fat, and then starve for a very long time,” Rohner said. It is one of the most extreme feast-or-famine lifestyles of any animal on the planet.
Rohner has identified a whole suite of adaptations that help cave populations of Astyanax mexicanus survive in this dramatically fluctuating environment. Their melanocortin 4 cell-surface receptors, for instance, are mutated both to increase appetite and resist starvation. They are also resistant to insulin so their blood sugar can rise more than would otherwise be possible, and they retain more fat than their river-dwelling populations. All of these physiological adaptations allow them to survive in the extreme environments of the Mexican caves.
Marine extremes
Rohner now wants to expand his research towards marine environments. Compared to the relative placidity and consistency of freshwater ecosystems, the intertidal zone of the ocean is a much more hostile habitat. For organisms such as barnacles and mussels, the transition from high to low tide is as extreme as the transition between flooding and dry seasons for Astyanax mexicanus.
“Every day, the water comes, [and] everything is great. And then the water retreats and then it gets hot and there’s nothing to eat,” said Rohner. “It's like feast or famine on a microscale.” He hopes that investigating how these intertidal species adapt to these conditions will reveal new ways that animals can change their physiology.
The first step of Rohner’s work on barnacles will be to simply sequence the genome of the more heat-tolerant barnacle Chthamalus fragilis and compare it with Semibalanus balanoides, one of the most common species of barnacles around Woods Hole, Massachusetts. Then, he will sequence the RNA that the crustaceans express at different time points to understand how they change their physiology throughout the day. He will also compare how the metabolic rates of the barnacle species adjust to these daily cycles.
Rohner chose to do this work at the MBL because of the experience of other scientists on campus. “There’s a lot of expertise in marine animals,” he said. “There’s a lot of developmental biology happening here.” He also hopes to collaborate with other Whitman Center researchers such as Valentina di Santo from Stockholm University, who is starting a year-long sabbatical at the MBL in October and is an expert on the effect of climate change stressors on marine organisms.