Green Pages

From Surf to Turf

Connections between marine and terrestrial ecosystems.

Story & Photos By Bill Bigelow, The School for Field Studies, Center for Marine Resource Studies, South Caicos, Turks & Caicos Islands

At The School for Field Studies’ (SFS) Center for Marine Resource Studies, I start my research course by posing a seemingly simple question to students: “What is a land crab?” Typically, a confident hand will shoot up from the front row and proclaim, “A crab that lives on land!” While this is a perfectly accurate response, it turns out that a more precise answer is infinitely more complicated. 

A blue land crab (Cardisoma guanhumi) rests in a pool of water along the rocky shore.

Land crabs are decapod crustaceans and “true crabs,” having the characteristic we generally consider to be, well, crab-like: Two intimidating pincers, four pairs of jointed walking legs, and a fused cephalothorax encased in a hard calcified exoskeleton. Land crabs differ from other terrestrial crustaceans like hermit crabs, which carry the shells of marine snails for protection, and have adapted to live their adult lives almost entirely independent from the sea. 

Land crabs evolved from ocean dwelling crabs some 125 million years ago during the Cretaceous period, when dinosaurs roamed the earth. They retain much of the same anatomy as their marine ancestors (gills, lung-like spaces, and hard exoskeletons) and require moisture and a particular mixture of ions for basic life functions such as respiration and metabolism. Whereas marine species of crab can meet these requirements by simply living in seawater, life on land poses a significant and constant challenge of maintaining hydration and ion balance. 

In the Turks & Caicos Islands, the blue land crab (Cardisoma guanhumi) is the most prevalent land crab species, due largely to the low elevations and abundant mangrove and coastal scrubland habitats of our islands, which this species prizes. Blue land crabs require regular immersion in water to maintain their water balance, be it fresh or brackish. Within these coastal habitats the crabs are prolific burrowers, excavating subterranean tunnels which can reach a depth of four meters.

Active blue land crab burrows were excavated in mucky substrates along the border of saline ponds.

For much of the year, the crabs remain in a state of reduced activity, sheltering within the confines of their burrows which provide protection from temperature extremes and predation. Typically, the burrows of blue land crabs reach the water table and terminate in a water filled chamber which allows them to maintain their body moisture, even during the peak of the dry season.  These crabs will go one step further and seal off their burrows during drought periods to lock in the moisture and humidity of the burrow. Their proclivity to burrow has earned them the title of “ecosystem engineers” since they actively shape their environments, thereby providing further habitat for other organisms.

During the rainy season when moisture is abundant, land crabs emerge from their burrows and engage in an annual spawning migration to their ancestral home, the sea. Egg-bearing females release their clutch into the salt water, where they hatch and are set adrift in the open ocean, progressing through several larval stages for nearly a month before returning to land. Coastal habitats such as mangroves then play a critical role in the successful recruitment of larvae back to land as their soft,  damp substrates and vegetative cover provide protection for the returning baby crabs. As the crabs develop from juvenile to adult, they migrate further inland to higher elevations where they construct their burrows and can live up to 13 years old. 

Understanding this nuanced interplay between terrestrial and marine life phases is important when considering future policy decisions. In an era of heightened urbanization and coastline development, the importance of coastal ecosystems can be easily overlooked compared to the more immediate economic returns from development. In the Turks & Caicos Islands, one must consider terrestrial and marine ecosystems as parts of an interconnected web of biotic and abiotic factors rather than two separate entities. Land crabs are a prime example of the connection of land and sea ecosystems working in concert to provide the necessary conditions for organisms with complex life histories. 

I’d be willing to wager that some of you reading this article thought about classic TCI dishes such as crab ’n’ dough, stuffed crab, or crab ’n’ rice. In the Caribbean, the harvest of land crabs is a practice which dates as far back as the indigenous Taíno and Lucayan peoples of the archipelago. The harvest continues today where the crabs serve as an important source of protein, and act as a large economic resource within small island communities. Donning a bucket and flashlight, crab hunters scour the bush and adjacent roadways listening for rustling leaves as the crabs move about their nighttime world. When you hear them scuttling, there’s no time for hesitation because these critters are fast! In the blink of an eye, they can race back to their burrows where only the most seasoned (and brave) hunter would venture to try and get them out by reaching their hand into the burrow. On a good night, harvesters report catching multiple dozens of crabs which are either kept for personal consumption or sold live to hungry crab connoisseurs.

SFS students and staff work under the light of head torches to record body measurements of the crabs.

Despite their cultural, economic, and ecological importance, there is a critical lack of data surrounding the crabs and their harvest in the TCI. To combat this, students at SFS are leading an initial effort to document the population size of the crabs on South Caicos. Unlike other islands in the archipelago such as Middle and North Caicos, South Caicos is markedly drier with vastly different vegetation and is thus more limited in its capacity for harboring the crabs.

Despite this, the land crabs persist, and in certain areas, at remarkable densities. Along South Caicos’ eastern shoreline runs a pronounced limestone ridge, at the base of which lies some of the most pronounced vegetation on-island. Along this ridge, seasonal rains produce brackish ponds and it’s here that the land crabs thrive. During nighttime surveys, students race to catch the crabs. When successful, they record the physical characteristics of the animals (size, weight, and sex) before returning them to the bush.

These data represent a preliminary effort to understand the habitats required to sustain populations of blue land crabs on South Caicos and to frame future studies investigating the interconnected nature of the crabs’ terrestrial and marine ecosystems. For example, given what we know of the crab’s life cycle, the release of their eggs undoubtably contributes large amounts of land-derived nutrients into the waters surrounding South Caicos. This transfer of energy from terrestrial to marine environments likely plays an important role in the marine food web, nourishing everything from corals to bonefish. However, the exact impact the crabs have is currently unknown, as this ecological relationship has yet to be explored on South Caicos.

On a changing planet, small island states like the TCI are disproportionately affected by the issues of climate change and food security. As we strive towards a more sustainable food future, the answers to our problems may lie in localized small-scale fisheries like that of land crabs. Just as the crabs were harvested by the indigenous peoples of the Caribbean, so too may the sustainable harvest of crabs continue to provide a reliable source of protein throughout the region today.

For this to happen however, we must fully understand their life cycles and how their survival is dependent on the preservation of coastal ecosystems. These crabs will continue to crawl on land as they have for millions of years as long as we preserve the habitats critical for them to survive and thrive.

For detailed article references or more information about The School for Field Studies, contact Director Heidi Hertler on South Caicos at hhertler@fieldstudies.org or visit www.fieldstudies.org.



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South Caicos was once a major exporter of salt harvested from its extensive salinas. Award-winning Master and Craftsman Photographer James Roy of Paradise Photography (myparadisephoto.com) created this vertical composition by assembling a series of six images captured by a high-definition drone which was a half a mile away from his position.

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