Amy Deacon, Lecturer in the Department of Life Sciences at The University of the West Indies, St Augustine and Secretary of the Trinidad and Tobago Field Naturalists’ Club, continues to explore the aquatic habitats of the Main Ridge. Meet the manicou crab! This feature was published in the Tobago Newsday on Thursday March 23, 2017
We have already met a few of the incredible fish that make their home in the upper streams of the Main Ridge. However, they are not the only aquatic creatures to have taken on the challenge of living in this extreme habitat – they share the streams with one of the Main Ridge’s most charismatic inhabitants: the mountain, or manicou, crab.
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| Manicou crabs roam throughout the Main Ridge Reserve and can even be found at the highest peaks. Photo courtesy Amy Deacon |
Native to Tobago and Trinidad, Venezuela and the island of Margarita, this ten-legged crustacean can grow to around 10cm in carapace width, and can weigh up to 250g. While most crabs are associated with the marine environment, and will be found scuttling across a sandy beach or rocky shore, the manicou crab can be found exploring the forest floor and mountain streams up to 800 metres above sea level. This means it can happily roam throughout most of Trinidad’s Northern Range and Tobago’s Main Ridge (which reaches around 600m), much of the time several kilometres from the sea. As you might expect, it displays several impressive adaptations that allow it to enjoy this inland existence.
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| Maanicou crabs provide their young a safe pouch in which to grow. Photo courtesy Amy Deacon |
The first relates to its reproduction. Most species of freshwater crab, including our famous hairy and blue crabs, 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 and estuaries. 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 her business.
Eventually these babies will leave the pouch and venture out into the mountain streams alone, having enjoyed a safe head-start in life. Pale yellow on hatching, juveniles become a bright orange-red colour, turning a darker red-brown as they mature, at around three years old. The diet of young crabs consists mainly of insects - primarily mosquito larvae, as well as vegetation and fruit. As adults, they continue to scavenge fruit and seeds, but they also become skilled hunters, adopting a ‘sit and wait’ strategy to pounce on prey such as crayfish, and even each other. A few years ago, scientists working in Tobago were astonished to observe manicou crabs successfully ambushing several different species of snake!
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| Manicou crabs adopt a defensive posture when threatened. Photo courtesy Amy Deacon |
Another adaptation to living in ephemeral mountain streams is a reliance on breathing air (thanks to a lung-like brachial chamber); this has evolved to the point that manicou crabs can no longer survive the sustained submersion that is normal for most crabs. As a result they construct burrows in the river bank, or find crevices under suitable rocks. Being able to breathe air also means that these crabs can travel a long way through the forest when foraging, generally making such trips at night time. They have been recorded as travelling as far as 200m in a single night.
The larger individuals are one of three crab species in T&T that are harvested for use in popular dishes such as crab and dumplings and callaloo. However, catching manicou crabs is a risky business 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, producing a tapping sound. The exact purpose of this display is unclear, but it may be related to territoriality or courtship.
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| They have cannibalistic tendencies. Photo courtesy Amy Deacon |
Their non-human predators include birds of prey. It is not uncommon to happen upon crab remains on riverside rocks which are most likely hawk ‘dining tables’. Mammalian predators include crab-eating racoons and even their namesake - the manicou or opossum. Although not currently endangered, it is possible that declines in other more commonly-harvested crab species such as the hairy crab, have placed increased hunting pressure on the manicou crab. The Main Ridge provides some sanctuary in this regard, as collecting of crabs, or any wildlife, is strictly prohibited within the reserve boundary. As a generalist predator and scavenger, these crabs 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!
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