WILD LIFE | 07.31.18
Grabbing ahold with both claws, a crab defends itself—and steers the course of another species’ evolution.
With a massive clutch of bright orange eggs spilling out from under its shell, this boxer crab (Lybia tessellata
) looks like an easy—and calorie-rich—target, but don’t be fooled. She’s got two not-so-secret weapons. In the grasp of each of the crab’s two main claws is a tiny live anemone, equipped with stinging tentacles. And true to the crustacean’s other common name—the pom-pom crab—she’s not shy about displaying them.
Exactly how much of a deterrent to would-be predators or competitors these stinging accessories are isn’t clear. But their role is important enough that the crab’s claws have become adapted specifically for little more than carting them around; the claws no longer function in food-gathering or other tasks that these main appendages perform in most other crustaceans. And boxer crabs are seldom, if ever, seen in the wild without their anemones, suggesting that the relationship may be key to their survival.
Studying a closely related species in the Red Sea, researchers couldn’t help wondering how the crabs acquire their weapons. While every wild crab they observed carried its own pair of stinging pom-poms, free-living anemones were nowhere to be seen. So where, or how, were the crabs acquiring their anemones? To answer this question, the team conducted laboratory experiments and genetic analyses and found that the crabs obtain their weapons through theft—from other boxer crabs—and that they have an ingenious way of multiplying what they steal.
In lab experiments, have-not boxer crabs consistently sought out individuals in possession of anemones and battled for them. When fights resulted in each crab walking away with a single anemone, researchers found that both crustaceans divided what they had into two halves. Not an ideal outcome for the anemones, but one that they were able to survive—as two identical clones.
In a study published in 2017, the researchers concluded not only that this is the primary mechanism through which boxer crabs acquire their anemones, but also that it’s the primary means of reproduction in the anemones. According to the scientists, such induced asexual reproduction by one species in another is unique in the animal kingdom—proof that there’s no such thing as simple accessorizing in nature.
Bunaken National Marine Park,
ABOUT THE Photographer
A native of Bologna, Italy, Alex Varani’s interest in nature—and marine environments in particular—stems from his father's passion for freshwater fishing. Varani began scuba diving 15 years ago to more closely observe what interested him: corals, crustaceans, fish, and other fantastic creatures that populate the seas. Photography followed as a means of capturing and sharing the wonders and biodiversity he observed. Varani now dedicates most of his travels to underwater photography, especially in warm, tropical waters.
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