Every year in Cuba, millions of crabs emerge from the forest at the beginning of the spring rains and head for the waters of the Bay of Pigs, crossing streets and highways on a perilous journey to mate and reproduce.

Now under way, the migration causes concern to drivers who try to swerve in an often futile attempt not to kill the crustaceans.

The crabs are a nuisance to residents but the sight of them crossing the road is a wonder for tourists and other first-time onlookers.

“They got here before us,” said Amaury Urra, a 50-year-old hiking guide from the Cienega de Zapata, the largest wetland in the Caribbean, known for its picturesque backdrop of turquoise sea waters and coastal cliffs.

“We’re used to this.

“Where I live, which is in the centre of the town of Giron, the crabs don’t get there as much,” he said – though there are plenty on the outskirts.

Located about 110 miles south-east of Havana, the area was the scene of a 1961 failed invasion by Cuban exiles who signed up for a covertly CIA-funded operation to overthrow Fidel Castro.

This year, the crabs started their journey early. At the end of March, local l authorities issued a warning to drivers to avoid travelling in the morning and evening hours – the favourite crossing times for the crabs.

Environmentalists usually demand the closure of the main road, especially at key migration times.

The passage of the red crustaceans – gecarcinus ruricola – could last until July.

The largest amount of traffic occurs between April and May. And residents have to be careful – when the crabs feel threatened, they can puncture car tyres with their pincers.

Official figures estimate that some 3.5 million crabs die each season on the road, many crushed by passing vehicles. They take a minute and a half to cross.

This type of crab lives and migrates in the Bahamas, Nicaragua, Jamaica and Dominica.

But only in this part of Cuba, and perhaps in another sector of the coast towards the neighbouring province of Cienfuegos, does its path collide so dramatically with human traffic.