Born without a shell, hermit crabs have a clever way of protecting their vulnerable soft parts – they pinch a discarded shell from another animal to call home.
But as the crab grows, its shell starts to get too tight and it needs to find another. Like the long chain that can form when people are buying homes, these little critters line up until they can each upgrade into a bigger dwelling.
But until a roomier residence is available, the crabs grow more slowly so they don’t outgrow their current accommodation. Now, scientists think they’ve worked out how – and it all comes down to poop.
Crabs with smaller shells didn’t eat less but pooped more frequently, suggesting they get rid of more nutrients to slow their growth rate. Their findings are published in the journal Invertebrate Biology.
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“Hermit crabs depend on empty snail shells for protection, and the right size shell isn’t always available,” says the study’s lead author Caitlin Ball, a graduate student in the Tufts Department of Biology in a statement. “When they’re stuck in shells that are too small, they grow more slowly. The question was: what’s the mechanism?”
The researchers wanted to get to the bottom of the mystery once and for all. So, they went to Beverly in Massachusetts at low tide and collected some long-wristed hermit crabs to study. They measured how much the crabs grew, ate and defecated when they had perfectly fitting shells compared with those whose shells were too snug.
“For years, people observed the growth slowdown but couldn’t identify the mechanism,” adds the study’s Phil Starks, associate professor of biology at Tufts University. “What we’re seeing is consistent with something subtle – the crabs appear to regulate growth by adjusting nutrient assimilation.”
That’s a fancy way of saying the crustaceans can regulate their own growth by controlling how much of their food is converted into body mass. They don’t need to eat less to limit their growth rate. Instead, they can excrete additional nutrients in their faeces.
“It’s exciting because the study answers a question about hermit crab growth that’s been sitting there for a while,” says co-author Jan Pechenik, professor emeritus of biology at Tufts University.
This principle could extend beyond the marine world, the experts say. “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.”
Top image: Beverly, Massachusetts. Credit: Rachel Rose/Getty Images
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