“One big problem for mangrove trees is that there is no oxygen in the soil because of waterlogging,” says Cannicci, explaining that this is when soil is saturated with water, which decreases the amount of oxygen in it. “If you dig mangrove soil, you will find a few centimetres of normal soil and then pitch-black, smelly soil. That is the layer with no oxygen. These big trees have their roots in that layer.”

Fiddler crabs are something of a natural oddity. First, although it can be hard to determine the gender of some species of crabs, it’s easier with fiddlers as males sport an enormous, often brightly coloured claw, which can account for up to half of their body weight.

Second, the crabs have a peculiar way of seeing. Their eyes are on top of stalks, which they keep perfectly perpendicular to the soil and use to see 360 degrees around, Cannicci says.

“Everything that approaches them from above their eye level is a predator and [upon sight] they will escape; everything that approaches from below this artificial horizon is possibly a female and they will approach.”

Hong Kong has 53 crab species that live in mangroves, according to a recent project by the government’s Environment and Conservation Fund. Six of these species are fiddler crabs that typically inhabit mangrove forests and the adjacent mudflats in intertidal zones – areas above water at low tide and underwater at high tide. Globally, there are about 100 species of fiddler crabs.

The crustacean is not normally eaten by humans, but there is an exception. In southern Portugal, people collect and sell only the male claw of one large fiddler species, Afruca tangeri. “They are considered a delicacy,” Cannicci says.

The claw serves as a signal in mating. To attract mates, males perform an elaborate courtship display by waving their bulky claws in the air. If they have a large one, they will be able to attract females from far away. If fiddlers lose their claws, they can usually grow them back, but it takes time – and a clawless male will not attract females in the meantime.

The females respond to two main cues to make sure they’re targeting a male of the right species, Cannicci says. First, every species has a distinct claw movement. Second, the crabs’ different colours help females to find males. Researchers have studied various aspects of fiddler mating, such as rates of claw wave, and even single and double claw waves.

“The funny thing is that the presence of females activates the waving,” Cannicci says. “You see most of the males are very close to each other, they observe each other and they eat. As soon a female comes into the area, everybody will start waving, and it’s amazing.”

Their bright colours can also make fiddler crabs more visible to predators, which in Hong Kong are mostly birds like gulls, egrets and herons.

An alluring wave isn’t enough for a choosy female fiddler. Most of them select males on the basis of their burrow. The male waves his claw, and once a female arrives she will head into the burrow to check its quality, such as its depth, for example. Burrows extend down to the water level.

“If the burrow is good, they will not come out,” he says. “That is the signal for the male that the female is ready for mating. [As with humans] it’s not just how you dress but also your house, right? I want to check if you have a nice house.” That’s crucial because a female fiddler crab spends about two weeks protecting her eggs in the burrow.

The males’ large claw is also used for fighting, which they engage in to enlarge their territory and defend their burrows.

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All six species are present on Lantau Island, in places such as Tung Chung, near Hong Kong International Airport. They can be spotted from June to late September, but are only active during the day at low tide, where they scoop up mud and feed on microscopic algae and bacteria within.

Fiddler crabs filter pellets of mud to feed, and the investigating team believes this specialised feeding method may prevent microplastics from accumulating in the crab’s gut.

“When the soil is in front of their mouth, they are pumping the water they have in their gill chambers into the soil,” Cannicci says. “The water is flushing the soil. Only the very light particles, the algae, will be ingested. Then, they will release the soil which is why after they eat you see pellets of soil.”

This feeding system, he assumes, allows fiddlers to swallow only their food and not the heavier microplastic particles.

Doctoral student Pedro Julião Jimenez has been studying the physiological effects of warming on four species of fiddler crabs in Hong Kong. He exposed the crabs to incrementally higher temperatures and measured parameters such as their heart rate in lab experiments. He found they are capable of surviving higher temperatures than similar species.

“When we compare this physiological data to intertidal crabs other than fiddler crabs, the fiddlers appear to be much better adapted to high temperatures than other species,” Jimenez says.

He warns, though, that when the mercury soars, fiddlers “have to compensate their temperature with their behaviour or physiological mechanisms”.

“So, instead of staying outside their burrows for a long time, they would have to go inside and thermo-regulate more often,” he explains. “The less time they spend outside, the less they will feed, and the less they will court and mate.”

Similarly, two fiddler crab species in South America spent more time retreating into their burrows in response to warmer temperatures.

With rising temperatures, distributional shifts can also occur, Jimenez says.

If temperatures in temperate and subtropical regions climb, the fiddler crab species that are “better adapted to deal with harsh conditions will expand their distribution” and those that cannot tolerate high temperatures will “lose their grounds and will shift to another distribution”, he says.

Studying crabs was his destiny, says Jimenez, who previously studied fiddler crabs in Brazil.

“I only started liking them after I started working with them, and then I found them pretty interesting.”