July 17: For decades, biologists have known that hermit crabs forced to live in shells that are too small slow their growth. What wasn’t clear was how they did it.
New research suggests the answer isn’t simply that the crabs eat less. Instead, they appear to regulate growth by changing how efficiently they use the food they consume.
“Hermit crabs depend on empty snail shells for protection, and the right size shell isn’t always available,” says Caitlin Ball, who led the study as a graduate student in the Tufts Department of Biology. “When they’re stuck in shells that are too small, they grow more slowly. The question was: what’s the mechanism?”
To investigate, researchers compared feeding behavior and waste production in crabs living in well-fitting shells versus shells that were deliberately too small. Surprisingly, crabs in undersized shells did not eat less. Instead, they defecated more frequently, suggesting they were eliminating a greater proportion of nutrients rather than converting them into new tissue.
“For years, people observed the growth slowdown but couldn’t identify the mechanism,” says Phil Starks, associate professor of biology and senior author on the study, which was recently published in Invertebrate Biology. “What we’re seeing is consistent with something subtle - the crabs appear to regulate growth by adjusting nutrient assimilation.”
The findings point to a broader principle in biology: growth isn’t controlled only by how much an animal eats, but also by how efficiently it turns food into body mass. Under structural constraints - in this case, a shell that limits expansion - animals may alter internal energy processing rather than external consumption.
For humans, how energy is absorbed, partitioned, stored, and expended is also known to vary significantly between individuals, although some people might envy hermit crabs for having so much control over that process.
“We wouldn’t want to imply that hermit crabs are a model for human metabolism, or that this study directly explains human weight variation, but the broader biological principle does extend beyond crabs,” says Starks. “Across animals - including humans - body mass isn’t determined solely by how much food is consumed. Growth regulation can occur at the level of nutrient assimilation, not just appetite. Intake and growth aren’t the same thing.”
“It’s exciting because the study answers a question about hermit crab growth that’s been sitting there for a while,” says Jan Pechenik, professor emeritus of biology, expert in invertebrate biology, and co-author of the paper. “We knew the pattern. Now we have opened the door to explore evidence for how it might work.”
