More than 65 million years ago, the hermit crab, which is more closely related to certain kinds of lobsters than “true” crabs like the blue crab, basically invented the mobile home. Using its soft, spiral tail, it will quickly back into empty snail shells or even small bottles it finds on the ocean floor.
Why has this way of life been so successful for hermit crabs?
No one is sure, but here’s one guess:
Although many predators, including fish with strong molar-like teeth, can bite into a more conventional crab (Yum! Crunchy on the outside, soft on the inside), a hungry predator needs some serious jaw strength to crush a snail shell. A periwinkle shell, for example, provides an incredibly strong house and home, engineered into a roughly spherical, spiral form that withstands structural stress.
Perhaps the hermit crab saves metabolic energy by avoiding the need to build a shell around its tail-like abdomen.
This video footage, courtesy of the U.S. Fish and Wildlife Service and its Ohio River mussel research team, shows a female freshwater mussel’s lure—an outgrowth of the mussel’s fleshy tissue.
This remarkable adaptation, which mimics a live minnow—complete with false eyespots—attracts hungry or just plain curious fish.
When a fish approaches the “minnow,” the mussel releases a cloud of microscopic larvae, which attach to the fish’s gills and fins. Each larvae resembles a miniature “Pac Man” character.
When the fish moves on, it basically becomes a distribution system for young, developing, hitchhiking mussels, which eventually fall off their swimming host.
Each species of mussel relies on as few as one species of fish as its distributor.
North America holds the world’s greatest diversity of freshwater mussels. Most mussel species are threatened, endangered, or extinct because of freshwater habitat destruction, dam construction, and invasive species such as the zebra mussel.
Biologists at the Ohio River National Wildlife Refuge (West Virginia) and the Genoa National Fish Hatchery (Wisconsin) are working to restore mussel populations and rehabilitate the mollusks’ freshwater river habitats.
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