For the little male sand fiddler crab (Uca pugilator), a big powerful claw is vital for energetic waving displays to attract females and for battling rivals to stake a claim to the best burrows.

Now US scientists have found that in courting males, this "super claw" has greater snapping force around new and full moons.

These moons happen about two weeks apart within the 29.5 day lunar cycle.

The oceans’ tides are the result of gravitational interaction between the Earth and the Moon and Sun. And the researchers postulate male crabs’ enhanced snapping performance could be driven by the abundance of females searching for a mate at new and full moons. Female numbers peak at these times because their babies would emerge at the next new or full moon when greater tidal flux could transport larvae away safely.

 

Males engage in ritualised contests that reveal strength and stamina

“When most females ‘want’ to breed most males will ‘want’ to court to increase their odds of reproducing, causing intense competition among males for the best burrows and resulting in those burrows being controlled by the strongest males,” says Denson McLain from Georgia Southern University, Statesboro, who worked on the study.

“Consequently, we observe stronger average pinching when more females are breeding.”

Details are published in the Journal of Experimental Marine Biology and Ecology.

Female sand fiddler crabs prefer males with a good quality claw and snap.

And these males are more likely to win contests take over burrows in prime locations for breeding preferred by females – on higher-elevated, dry sediments.

 

Once males have established their burrow, they line up and wave at potential mates in high-energy courtship displays, tip-toeing to maximise the height their claw reaches. If a female is interested, the male drums on his burrow until she enters. Before the pair mate, he blocks up the entrance with mud – sealing the deal.

And the claw doubles up as a hefty weapon: male crabs pinch and interlock in battles and can even lift up and flip over their rivals.

Pincer power

“Males engage in ritualised contests that reveal strength and stamina,” says McLain.

At about 55% of the mass of the rest of the body for some sand fiddler crabs, this huge pincer is “one of the most extreme sexually selected organs to have evolved in animals,” he adds.

Researchers collected and studied dozens of male crabs at a salt water marsh habitat on Crescent Beach in north-east Florida, over three lunar cycles. The sand fiddler crabs on Crescent Beach experience a semi-diurnal tidal regime – when there are two high tides and two low tides every day.

The study also shows how males’ condition changes depending on whether they are courting or foraging – the two main behaviours of crabs that are active during the breeding season, from late March to September.

It found males’ claw closing power (an indicator of male quality) diminished as courting periods progressed (when males use a lot of energy but have little access to food) and increased while they spent time foraging (known as “droving”).

Crabs switched to droving to restore power and get ready for courting again.

 

While droving, crabs search for food over areas of moist sand flats and near the waterline of tidal creek banks and at Crescent Beach, droves consist of hundreds to tens of thousands of crabs. Males are not aggressive towards one another when they are foraging, giving them time to recover their strength and start the cycle again.

The researchers measured the pinching force of most of the crabs in the study with a simple test: male crabs were collected in the field and guided to clamp down on bite force plates electrically connected to a force meter that registered the strength of the pinch.

“Most crabs would clamp down once the bite force plates made contact with the interior surfaces of the pincer of the claw," explains Mr McLain.

Individuals were marked with paint and returned to the wild, to be collected and studied again at different points in the lunar cycle.

The latest findings build on a body of research into how the lunar cycle can directly and indirectly affect animals’ behaviour.

For example, some other crab species such as the horseshoe crab are known to come ashore to spawn during evening high tides at full and new moons.

And one study in 2009 found amphibians around the world mate during a full moon, using moonlight to co-ordinate their gatherings.

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