Hermit Crabs Regulate Growth by Adjusting Nutrient Absorption in Small Shells

For decades, biologists have observed that hermit crabs living in shells too small for their bodies slow their growth, but the mechanism behind this was unclear. A new study published in Invertebrate Biology reveals that rather than eating less, these crabs regulate growth by altering how efficiently they use the food they consume.

Hermit crabs must find and inhabit empty snail shells for protection and survival, but suitable shells are not always available, especially due to human activities like shell collecting. When confined to small shells, crabs grow more slowly and often wait in line to exchange shells in a "vacancy chain" for slightly larger ones. Researchers from Tufts University compared feeding and excretion patterns of crabs in appropriately sized shells versus those in deliberately undersized shells. Surprisingly, crabs in small shells did not reduce their food intake but excreted more frequently, indicating they expelled more nutrients instead of converting them into body tissue.

Dr. Caitlin Bull and Dr. Philip Starks from Tufts explained that this finding suggests hermit crabs regulate growth by adjusting nutrient absorption efficiency rather than food consumption. This points to a broader biological principle that growth is controlled not only by how much an animal eats but also by how effectively it converts food into body mass. Under structural constraints like limited shell size, animals may modify internal energy processing rather than external intake.

While the researchers caution against directly applying these findings to humans, they note that similar principles of growth regulation through nutrient assimilation efficiency exist across animal species. Thus, body mass is influenced by both food consumption and the metabolic processing of nutrients, highlighting that appetite and growth are distinct processes.