"... some of the most remarkable and interesting facts in the distribution

and affinities of organic forms are presented by islands."

    Alfred Russel Wallace (1823-1913)

    Naturalist, geographer, anthropologist

Gecarcinid land crab, Gecarcinus lateralis, male. Santa Cruz de Barahona, south-western Dominican Republic, Hispaniola.

The Armored Divisions

    The arthropod Subphylum Crustacea is mostly marine, though a good number of species have invaded freshwater and even land. Several species of isopods, crabs, and hermit crabs (these last are actually small lobsters whose abdomen has been modified into an asymmetrical shape adapted to occupy empty snail shells) inhabit beaches and forests in the Caribbean islands.

    Most crustaceans have their bodies divided into cephalothorax and abdomen, as is the case with the unrelated arachnids. (One group of crustaceans, the mantid shrimp, have a peculiar, segmented head, as well).  Several groups of the class have the first (sometimes more) pair of legs modified into pincers ("chelipeds" or "chelae"). These serve the purpose of manipulating food items, or as defensive appendages to repel rivals or predators. The mobile "finger" of a crustacean's pincers is actually the last segment of the leg, which can move against the other, immobile "finger", this one being, in fact, a protrusion of the second-to last-segment of the leg. (This is the same arrangement found in the chelipeds of scorpions). The chelae of large species can deliver quite a nasty pinch, indeed, even capable of crushing a human hand. As happens with many other arthropods, legs can be lost in battle or to a predator but, often, the animal will replace them in subsequent molts.

    Several species of West Indian terrestrial and semi-terrestrial crabs return to the water to mate and lay their eggs. Females of others, like those of the family Pseudothelphusidae (shown further below), breed their eggs and young under their short, flat tails, carrying them around for protection. Terrestrial isopods (commonly known as pillbugs or wood lice) have severed their ties to water completely (other species are aquatic in fresh water or in the sea).

    Some Antillean crustaceans have an annual life cycle that requires them to remain dormant, whether as eggs, larvae, or adults, in dry mud or other microhabitats. This happens mostly during dry seasons in xeric areas. When the rains return, eggs laid months or even years before hatch. Larvae develop quickly into adults, which then mate promptly and begin the cycle again.

    I won't even attempt to delve deep into the taxonomy of this group. Ii is all very complex to me, so what comes next is the best I can do.

CLASS BRANCHIPODA: TADPOLE SHRIMP

    Not shrimp at all, these strange little creatures possess flattened carapaces that cover the front part of their bodies, and two compound internal eyes with a naupliar eye between them. This last is derived from their larval ("nauplius") stage.  Their three eyes give one of the only two extant genera their name. "Triops" means "three-eyed". They also have a pair of furcae, long filaments at the end of the abdomen. Most of their thoracic segments bear from 4 to 6 pairs of legs, for a total of up to 70 pairs. This are more than those of any other group of crustaceans.

    Tadpole shrimp are almost Cosmopolitan in distribution, from the Arctic to tropical regions. In spite of that, barely 16 species are so far known, worldwide. Some species are found worldwide. Since they are staple food for many species of wading birds, it is believed that as such birds migrate with the seasons, tadpole shrimp eggs stuck to their feet have managed to colonize wide regions of Earth. Most species live and reproduce in temporary pools even in the middle of deserts. Adults will mature, mate and lay eggs in a matter of days, and then die while their eggs or larvae will remain in a state of diapause, sometimes for years, until the rains fill up their pools once more. They are omnivores which will even cannibalize each other under resource-depleted conditions.

    It is known from the fossil record that one species, Triops cancriformis, arose 220 million years ago, during the Trassic Period. It is the oldest living organic species still alive on Planet Earth.

Tadpole shrimp of the genus Triops inhabit temporary pools in xeric areas. They may spend most of their lives as eggs lying dormant in dry mud.

Related species live in the American continents, where they exhibit similar life cycles.

Specimen born from eggs collected at Morne Rouge, Saint Bartholomew, Lesser Antilles.

CLASS MALACOSTRACA: DECAPODS, ISOPODS, AND AMPHIPODS

    These are the best known and commercially important crustaceans on Earth. Malacostracans possess heads with six segments, with a pair of antennules and another of antennae. Their cephalothorax has 8 segments and is covered by a carapace formed by the fusion of the first three segments. They ussually also have eight pairs of legs. Often, the first two or first few pairs are modified into feeding appendages called maxillipeds, while the pair after those might take the form of pincers (a crab's or lobster's "claws"). The abdomen is divided intro six segments and can be used for swimming, in many species. Most members of the group also have compound eyes, and many of them have very good eyesight.

ORDER ISOPODA: PILLBUGS

    These crustaceans have all their legs approximately of the same size (iso-pous: "same feet"), and look like tiny armored vehicles. Many species are marine, but a large number have invaded land. Usually found in moist microhabitats under logs and rocks, some roll into a tight ball when threatened. Hence their common name: "pill bugs".

Pill bugs, species undetermined. Near Duverge, south-western Dominican Republic, Hispaniola.

Pill bugs, two species, undetermined. Barrenspot, central Saint Croix, United States Virgin Islands.

Pill bug, species undetermined. El Yunque National Forest, north-eastern Puerto Rico.

ORDER DECAPODA: PRAWNS, SHRIMP, CRABS, AND THEIR KIN

    Crabs, shrimp, lobsters, and hermit crabs are the best-known crustaceans. Aside from isopods, they are also the only ones which left the sea and invaded freshwater habitats and, in some cases, land and even trees. Although their bodily structure is the same for all of them, a crab has a particularly short-bodied appearance owed to the fact that it carries its flattened abdomen (what would be a lobster's "tail") tightly pressed against the underside of its cephalothorax. You cannot see a crab's abdomen unless you grab it and look at it from below (if the animal allows you to do that, of course).

Suborder Dendrobranchiata

    Prawns are superficially similar to shrimp but can be distinguished for their branched gill structures (dendron: "tree"; branchia: "gills"). There are both marine and freshwater species.

Family Palaemonidae: Prawns

    These prawns are found in all tropical and subtropical waters on Earth. Several species are very common in Caribbean streams and rivers, and a few of them can reach quite a large size. Not surprisingly, they are readily consumed by some of the local human populations.

Suborder Pleocyemata

    This group contains all the other terrestrials and freshwater decapods of the Antilles. Their distinctive features are their gills, which take the form of lamellae, and the female parental care for their eggs, and sometimes even their offspring.

Family Atyidae: River Shrimp

    These shrimp are abundant in tropical and subtropical freshwater streams and rivers throughout the World. Antillean species like Atya have the ends of the first pair of legs modified each into double fans with which they filter water currents for microscopic nutrients. Others like those of the genus Xyphocaris are transparent, dainty creatures that feed mostly on carrion and dead plant matter.

Family Coenobitidae: Land Hermit Crabs

    These are the terrestrial hermit crabs. One genus, Birgus, includes the largest terrestrial arthropod, the coconut crab of the Indo-Pacific islands. It may attain a legspan of almost two meters. The other genus, Coenobita, is likewise more varied in the Old World. However, one Caribbean species, C. clypeatus, is very common in the islands. Its agility at climbing has earned it the name of "tree crab", while its large purple claw and its penchant for using it readily in defense (sometimes very painfully, too) has made it known as "purple pincher" in the pet trade.

 

One does not usually expect to find a hermit crab climbing trees, but this species does.
This is a Caribbean land hermit crab, Coenobita clypeatus, (family Coenobitidae). The Quill National Park, southern Saint Eustatius, Lesser Antilles.

They are often found far away from the seashore, even high up in some mountains.
During their mating season they will migrate to the sea, sometimes in huge numbers, to leave their eggs in water.
The closest relations of hermit crabs are lobsters and shrimp.

 

Coenobita clypeatus. Gorda Peak National Park, Virgin Gorda, British Virgin Islands.

A hermit crab out if its shell shows how it is but a small, modified lobster.

Unlike typical members of its order, however, its abdomen is soft and vulnerable. Hence the need to use an empty snail's shell for protection.

In order to take this picture, I had to carefully break the shell where the animal was.

After the shot, I placed it near another, empty shell, into which it immediately crawled backwards to resume its normal life.

Caribbean land hermit crabs make extraordinary migrations to sea in order to spawn. Parque Nacional del Este, eastern Dominican Republic, Hispaniola.

Family Gecarcinidae: Land Crabs

    These might be the quintessential land crabs. Although they still need to go down to the sea to breed, adults can sometimes be found many kilometers away from the sea. In many islands they form an important staple of the local human diet. Some species will burrow deeply in soft mud. These include the Caribbean blue land crab, Cardisoma guanhumi, one of the largest terrestrial arthropods in the World.

Family Oxypodidae: Ghost and Fiddler Crabs

    This family contains two of the most picturesque types of crab in the entire Caribbean region. One of them, the ghost crabs, can be seen scurrying at high speed along sandy beaches when a human approaches. They have the peculiar and annoying habit of being unrepentant thieves of colorful or shiny objects left unattended near beach chairs. Everything from watches and earrings to jewels and car keys may disappear down their lairs in an instant. It would not be so aggravating if it weren't for the speed at which they run and their agility in evading you.

Ghost crab  (family Ocypodidae), Ocypode quadrata, female. Castries, north-western Saint Lucia, Lesser Antilles.

    The other - and more virtuous - local members of the family are the fiddler crabs. Fiddlers earn their name from the shape of the hugely enlarged claw of males (sometimes weighing as much as the rest of the animal). With ritualized motions of the larger cheliped males warn competitors to stay away and woo females into their burrows from some crustacean loving.

Fiddler crabs, (family Ocypodidae), Uca vocator, males. San Juan, north-eastern Puerto Rico.
During some times of the year, millions of these crabs congregate on the shores of mangrove swamps and mud-flats to court and mate.

Males wave their chelipeds as a warning to other males, and to entice the females to mate.

Uca pugnax. Mary Point Pond, Virgin Islands National Park, Saint John, United States Virgin Islands.

Fiddler crabs, Una pugnax and Uca vocator. Piñones State Forest, north-eastern Puerto Rico.

Although Uca crabs are small, in some species the male's chelipeds are proportionally

enormous, at times fully as large and heavy as the rest of the animal together.

 

At low tide, Uca crabs leave their burrows and sift the sand and mud with their mouthparts, thus extracting edible microscopic detritus.

Mount Hartman National Park, south-western Grenada, Lesser Antilles.

 

An Uca pugnax falls prey to a Callinectes crab as the water rises over the tidal flat, giving the aquatic predator access to its smaller relatives.

In this particular case, the fiddler was running away from me and took to the water... from the frying pan into the fire.

Aguirre State Forest, Guayama, south-eastern Puerto Rico.

Family Pseudothelphusidae: Neotropical River Crabs

    This group, interestingly, might have originated in the Greater Antilles. From there they spread through the Neotropics and are now more common in South and central America. They are common especially in mountain streams, and females carry their young protected between their cephalothorax and telson, and eventually wash them off in some meander of a creek's current. They can be seen on land, especially at night.

Family Sesarmidae: Mangrove Crabs

    Some of these crabs are semi-terrestrial in that they have severed their breeding ties to open water. Armases, Goniopsis, and Sesarma are frequently seen climbing mangrove trees near sea coasts. Perhaps the most extraordinary Antillean member of the family is the Jamaican bromeliad crab, which lives and breeds in bromeliads that are abundant in the mesic forests of Jamaica.