Magazine article Geographical

Creatures of the Night

Magazine article Geographical

Creatures of the Night

Article excerpt

The manicou crab is an amphibious freshwater land crab with an appetite for snakes. David Maitland braved the heat and the rain of the tropical forests of Tobago for a glimpse of this extraordinary crustacean, and saw much more besides.

Even the old men on the island kept saying "This is the hottest we can remember." It was August 1997 and well into the six-month rainy season when high temperatures and humidity are the norm. But this year, globally the warmest on record, the heat was unbearable in Tobago. Just sitting out in it made the skin glisten with perspiration. We knew our research in the dense rainforest was going to be clothes-drenching work.

Tobago is a small Caribbean island only 60 kilometres long situated northeast off the main island of Trinidad, which is itself less than 40 kilometres from the coast of Venezuela. The highest point on Tobago is 550 metres and because it is only about 10 kilometres wide, rainforests cut deeply by fast-running rivers rise in steep ridges from the coastline.

In and around the crystal clear waters of the rivers in the Main Ridge Forest Reserve (established by the British in 1776 to "preserve the rains"), you find an extraordinary creature -- the manicou crab (Pseudothelphusa garmani), which was the subject of our research.

The manicou crab is unusual in that it is amphibious, living both in freshwater and on land. Manicou is an Amerindian name meaning opossum, and female manicou crabs have a "pouch" (formed by the abdomen which is tucked underneath the body) to protect the eggs and the young after they have hatched. Because it is terrestrial and can grow to be bigger than a person's hand, the manicou crab is an easy target for humans. In Trinidad, where it is a popular food, it has been overhunted. It survives relatively untouched in Tobago, however, where the meat of the more common ironback crab (Cardisoma guanhumi) is preferred.

I came to Tobago a year ago on a grant from the Royal Society to collect some of the manicou crabs for study back in Britain. My particular interest in crustacean respiratory physiology made me curious about the crab's lung and how it carried out its amphibious existence, breathing either water or air. While collecting, I noticed a remarkable thing. So different in colour were the bright red young crabs from the older brown ones that I thought for a moment that I had found two different species, not one.

In addition it became evident that the crabs were an important component of rainforest freshwater ecosystems. Voracious predators of aquatic habitats when young, they were just as aggressive on land when large and mature. It appeared that the manicou crab only reached sexual maturity when very large, and therefore old, indicating that the population was extremely vulnerable.

To assess whether or not the manicou crab was threatened as a species, a study on its population dynamics was required including growth rates, age structure, size and weight, population sex ratios, ages at sexual maturity, mortality rates and mating behaviour. Working with Dr Paul Ward, a zoologist at Leeds University, a project to conduct field sampling during the wet and dry seasons in Tobago began to take shape.

Our project won a substantial grant from the world conservation organisation Earthwatch and the first expedition to Tobago during the 1997 wet season was mounted. We stayed for four weeks with two groups of Fellows from the conservation and education sectors in the UK, each of whom had won Earthwatch Millennium Fellowship Awards. With their enthusiastic help, we were able to carry out the first stages of field research on the life history of the manicou crab and on its ecology which, like most land crabs, is almost unknown.

We had been advised to carry out the research on the smaller island of Tobago rather than Trinidad, which is the centre of the country's petroleum-based economy and has a much larger population. …

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