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FIDDLER crabs on the coast of southern Portugal are under threat because of
local people鈥檚 fondness for a seafood delicacy. But overfishing is not to blame.
Researchers say harvesting of crab claws is depleting populations by turning
male crabs into eunuchs.

Male fiddler crabs have one oversized or 鈥渕ajor鈥� claw, which accounts for up
to 40 per cent of their body weight. Males attract mates by waving the big claw
in a ritual dance and use it to defend their burrows against other males.

Around Ria Formosa in southern Portugal, however, the claws are prized as a
delicacy. 鈥淟ocal people usually eat the claws as a snack with a few beers,鈥� says
Rui Oliveira of the Superior Institute of Applied Psychology in Lisbon. Rather
than killing the crabs, fishermen break off the claws and throw the crabs back,
assuming their claws will regrow.

But the amputee males are at a serious disadvantage. Although they can still
feed using their smaller claws, they can鈥檛 attract the attention of females to
mate. And the de-clawed crabs cannot defend their burrows against rival males,
who treat them as females. 鈥淭hey are courted by other males when they approach
them,鈥� says Oliveira.

With a shortage of intact males available for breeding, the effective sex
ratio of the population becomes biased towards females. To examine the
consequences of this, Oliveira and his colleagues surveyed the population of the
fiddler crabs (Uca tangeri) at Ria Formosa and compared it with the
population at the Mira estuary, where the crabs are not fished.

There were more crab burrows per square metre in the Mira estuary, indicating
that the animals were more numerous. Male crabs here were also larger than their
Ria Formosa counterparts, which have to use up energy regrowing their claws.
鈥淭hese results suggest an effect of claw harvesting at the population level,鈥�
says Oliveira.

The crabs do eventually regenerate their claws, and the crustaceans
periodically shed their hard exoskeletons in favour of roomier ones that allow
space for growth. The new claw gets larger each time a growing crab sheds its
carapace.

鈥淏ut it takes time,鈥� says Oliveira. 鈥淲e still don鈥檛 know how many moults it
takes them to get full-sized claws for their body size.鈥� Meanwhile, the crabs
are missing out on mating. At Ria Formosa, 38 per cent of male crabs had missing
or regenerating claws.

This shortage of males fit for breeding duty has a knock-on effect on
population numbers. 鈥淭he data so far show a decline in the population over the
past two decades,鈥� says Oliveira. He will present his findings to the local
conservation agency and fisheries department, which will consider possible
controls on claw harvesting to protect the crabs.

Jeff Shields of the Virginia Institute of Marine Science in Gloucester Point
even doubts that the fiddler crabs regenerate their missing claws at all, as
they only live for about two years. 鈥淚f these crabs are being harvested at large
size, they just won鈥檛 grow back,鈥� he says. Decline of fiddler crabs could have a
wide impact on other species, he adds.