The first relates to its reproduction. Most species of freshwater crab, including our blue crab, need to migrate or remain close to the coast in order to release their eggs into the water and out to sea, where the larvae develop as part of the zooplankton before migrating back into the rivers.
However, the manicou crab is one of only a few species worldwide that no longer produce free-swimming larvae. Instead, astonishingly, its 200-300 eggs hatch and develop within a pouch formed on the female’s abdomen.
This incredible adaptation is the origin of its common name; ‘manicou’ is an Amerindian word for the opossum which, as a marsupial, also raises its young in a pouch. It is quite surreal to stumble upon a female with a pouch full of miniature crabs clinging onto her underside, as she goes about business as usual.
The diet of young crabs consists mainly of insects - primarily mosquito larvae, as well as vegetation and fruit. As adults, they are also known to eat meat, and will adopt a ‘sit and wait’ strategy to ambush prey such as crayfish (Macrobrachium spp.) – and even each other; individuals have also been observed predating on several different species of snake.
Their cannibalistic tendencies may be the reason for their largely solitary existence, coming together only for mating.
Another adaptation to living in ephemeral mountain streams is a reliance on breathing air (thanks to a lung-like brachial chamber); this has gone so far that manicou crabs can no longer survive the sustained submersion that is normal for most crabs. As a result they construct burrows on the river bank, or find crevices under suitable rocks.
Being able to breathe air also means that crabs can travel a long way through the forest when foraging, generally making such trips at night time. They have been recorded as travelling up to 200m in one night.
Crab hunters can be seen on mountain roadsides at the onset of the rainy season, as this is when the crabs are most active and easily caught. This task has to be undertaken with care, as their claws (or ‘chelipeds’) are powerful and capable of inflicting a painful wound. When threatened, they will spread their chelipeds in a wide defensive pose, at which point it is wise not to let your fingers get within striking range.
They also use their chelipeds for communication by striking the inside of their burrows which produces a tapping sound. The exact purpose of this display is unclear, but it may be related to territoriality or courtship.
Their non-human predators include birds of prey, such as the crab-eating hawk – it is not uncommon to happen upon crab remains on riverside rocks up in the Northern Range streams, which are most likely hawk ‘dining tables’.
They are even eaten by their namesake - the manicou or opossum.
Although not currently endangered, it is possible that declines in other crab species such as the blue crab, are starting to place increased pressure from hunting on the manicou crab.
As generalist predators and scavengers, they play a central role in the ecosystem of our forest streams and therefore should be valued for more than just their contribution to crab and dumplings. We should all feel lucky to share this island with such an amazing and unusual crustacean!