Video: The wild dance of male blue crabs propels a jet of pheromones to attract would-be lovers, say biologists
Blue crabs are the Fred Astaires of the crustacean world. The males of the species dance wildly to attract far-away females, directing a jet of pheromones to bring would-be lovers out of hiding.
In the dense submarine thickets of seagrass that blue crabs (Callinectes sapidus) call home, males and females can鈥檛 easily rendezvous. Sending out a pheromone love letter helps the pair find one another, says , a marine biologist at Georgia State University in Atlanta, who with colleague Michiya Kamio has now investigated the behaviour.
Derby and other researchers had seen the male crabs dance in the wild. But to determine why the crustaceans perform the ritual, his team examined some of them in a small aquarium.
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When a male and female crab shared a tank, males wasted little time. Within minutes, males scuttled over to the female and grasped her in preparation for mating. But when a divider separated the sexes, the males got creative.
Wafting water
After realising the female couldn鈥檛 be reached, ten out of 12 males reared on their hind legs and flailed their flattened swimming arms for several seconds 鈥� a move the researchers dub 鈥渟tationary paddling鈥� (see movie, top right).
To gauge the effect of the dance, Derby鈥檚 team measured the water currents created by the crab dance using a method called particle imaging velocimetry.
The technique illuminates tiny particles with a laser to detect the speed of a current. The crustaceans managed to create a current that averaged 3 cm per second and peaked at 17 cm per second 鈥� enough to send pheromones a distance of up to 1 meter, Derby estimates.
In the wild, crabs might waft their pheromones even further. 鈥淭hese were not the most vigorous dancing crabs,鈥� Derby says. 鈥淚鈥檓 sure they can generate even faster currents.鈥�
Hidden females
This behaviour probably developed as an adaptation to the blue crab鈥檚 habitat, full of sea grass and rocky hideaways, Derby says, where males have a tough time seeing their mates.
Derby鈥檚 team didn鈥檛 identify the pheromone spread by the dance, but other researchers have found evidence for such a scent, which probably comes from their urine.
鈥淧erhaps if she鈥檚 able to detect this male odour, that would entice her to come out of hiding,鈥� says , a marine biologist at the Whitney Laboratory for Marine Science in St. Augustine, Florida.
But it still takes two to tango 鈥� Gleeson notes that males only boogie when they catch a whiff of a fertile female.
Journal reference:
Evolution 鈥� Learn more about the struggle to survive in our comprehensive special report.
