Being a night dive enthusiast much to my husband’s dismay, I have come across a variety of crabs. Besides being a gastronomical delight, divers are always amused by the antics of these underwater creatures. While you dream about crab soufflé or curry, let me bore you with a lesson in taxonomy. The largest group of marine anthropods is the class Crustacea and that is where our 10-legged side-walking animal belongs in the subclass Malacostraca in the order of Decapoda. Enough Latin lets get on back to the crab.
Crabs are related to lobsters and shrimps. However their body shape is more compressed, flattened, and/or oval than their close cousins, the shrimp. Found in nearly all marine environments, crabs can range from less than an inch to over 12 inches in diameter, the biggest crab being the Japanese spider crab (Macrocheira kaempferi), which lives on the floor of the north Pacific Ocean; it has a 3.7-meter leg span. The biggest land crab is the coconut crab (Birgus latro), which lives on islands in the Pacific Ocean having a leg span up to 1 meter.
Just when you thought the lessons were over its time for a quick tour of a crab’s anatomy.
The chitinous shell covering the crab’s body is the carapace, which protects the crab from predators and provides support. The abdomen that serves as a brooding pouch for the eggs is tucked under the body. The next time you are in the market and feeling fearless, you might attempt to pick up a crab to see if it’s a male or female: Female crabs have a wide abdomen to hold eggs, while males have a thin, pencil-shaped.
The crab has several pairs of appendages attached to its segmented body. The first two are sensory antennae and the next 5 pairs serve as walking legs. The chelae or pincers are located on the front legs and are used for feeding, courtship displays or fighting. All crustaceans shed their outer shell in a process called molting which allows them to grow and regenerate lost appendages. Crabs are scavengers of any type of edible matter they find, be it animal or plant material. Some will require a deep substrate to burrow in during the day, while others are always on the move.
Now let’s get up close and personal with some of these crabs:
Spiders have always sent shivers down my spine. The larger they are the louder my piercing screams for help which probably shocks the spider as much as it shocks my husband Adi, who dutifully catches them and flings them over the balcony…anything to keep the wife quiet (spiders..hope you packed your parachutes!).
However this fear disappears instantly underwater when I am confronted with spider crabs that share the same long legs and hairy bodies that their land cousins have! Maybe its because they can’t crawl all over me and I am safe in my long wetsuit? Spider crabs, like Naxoides sp. and Hyastenus sp. are delightful little animals that like to dress up in a variety of ways. The main feature of the spider crab is the special hairs on the carapace and legs. These hairs act like minute hooks that play an important role in attaching plant and animal growth like algae, hydroids, and sponges onto the carapace and legs that the crab collects with its chelipeds. This helps the crab to vanish into the environment while it waits for unsuspecting prey. This characteristic has also lead to the fact that some spider crabs are also called decorator crabs.
These Neopetrolisthes sp. are found exclusively in pairs sheltered among the stinging tentacles of anemones where they safely filter tasty morsels brought by the current. They tend to drop their pincers when stressed leading to their name porcelain crab. Their favorite anemone seems to be the carpet anemone that is also home to anemone shrimp (Periclimenes sp.,) and anemonefish (Amphiprion sp.). The Porcelain crab is a bit of a dilemma or taxonomists as it has a tail, which is virtually not found among crabs. Adding to this headache is the existence of long antennae, which is also unusual for crabs. Out of the crab’s mouth protrude feather-like projections which sweep about assisting the crab to feed on the passing nutrients.
Okay you won’t see this crab underwater but if you dive a lot and spend time on beaches in the tropical Indian and Pacific Oceans you will encounter this crab, and it will leave an everlasting impression (still on our minds!). It is the world’s largest hermit crab; an adult weighs 40 pounds and is 3 feet in length with huge pincers that will make you pause while you determine just how close is safe to get that perfect picture. Their favorite meal is obviously the coconut and though they are good climbers, they only eat coconuts that have fallen on the ground, as the fruit has to be ripe. Besides this, ripened figs and pandanus fruits are favored as well.
This is another interesting little crab, which hangs on the outer edges of bubble coral (Plerogyra sinuosa). It’s easy to identify as Achaeus japonicus has long hairs covering its body hence its nicknamed the Orangutan crab. Strangely enough, this rusty red colored crab enjoys publicity as it sits on the edge of the bubble coral highly visible against the white pearly color of the coral. We have enjoyed watching this crab as it bobs up and down the bubble coral with eyes like Christmas tree lights glowing bright.
Besides delighting divers and taste buds all over the world the crab has more noble duties. A lot of them are of substantial economic significance in fisheries, medicine and ecology. There is a need to identify potential food crabs, bioindicators and endangered species. Many species are also habitat specific and thus excellent bio-indicators of habitat health and environmental degradation especially coral reefs and mangroves. So remember to look out for these amusing animals on your next dive.