[Phylum: Arthropoda] [Class: Malacostraca] [Order: Decapoda] [Subphylum: Crustacean]
There are more than seven thousand (7,000) different species of crabs, and most of them are members of a select group known as decapod crustaceans.
Humans eat tonnes of seafood animals, especially the savoury blue crab. Even so, this section contains information about invertebrate crabs that scuba divers see living in the seas and oceans.
By and large, you can find crabs living in all of the oceans and many of the freshwater ecosystems.
Often, you find them living inside burrows in mud or sand, especially at high tide and during the wintertime.
Scuba divers see most of the decapod crustaceans in the benthic zones of the major tropical regions.
The terrestrial crab species lives exclusively on land, often in areas that are far away from any major body of water.
Yet, some species of crab can actually survive on land and in water. So, they need to keep their gills moist while on terra firma. For this reason, you usually find them residing close to the coastline.
Like most animals of the subphylum Crustacea, the majority of crabs have a thick exoskeleton made of chitin, used for protection. They also have a single pair of hinged, pincer-like claws (called chelae) that they use during feeding and whenever they feel threatened.
In fact, crabs have five (5) pairs of legs (same as its close relatives - lobsters). The large pinching claws located at the front are their defencive appendages. Whereas, the broad and flatter pair of legs at the back serve as paddles.
Yes... crabs can swim.
These stubby sea creatures have a flat, broad belly and a tail that tends to curl under its midsection. They use gills to breathe, even though the gills of land crabs perform more like lungs.
To say that crabs vary in size is an understatement. For example, the tiny pea crab (Pinnotheres pisum) measures no more than 2.5 centimetres across (less than an inch). Whereas, the Japanese spider crab is a giant, measuring thirty (30) centimetres across and four (4) metres (12 feet) from tip to tip when its legs are stretched.
The two large eyes that extend from a crab's head have movable stalks, fixed above two pairs of antennas. But, its mouthparts (e.g. three pairs of appendages) are situated underneath its head.
Fun Fact: The hermit species, anomuran decapod crustaceans, have developed a habit of occupying empty snail or mollusk shells for shelter. It's also common for them to scavenge around for a bigger shell when they have outgrown it.
You can read more in other sections about the most common crab species seen while scuba diving or snorkeling, including:
For the most part, we can separate crabs into two varieties, that being either Brachyura (true crabs) or Anomura (fake crabs).
The main difference is the short abdomen and four pairs of 'walking' legs in true crabs. Whereas, a false crab will have a bigger abdomen and a fewer number of legs used for mobility.
Most crabs are omnivorous scavengers. In other words, besides eating vegetable matter, other crabs, and small fishes (alive, dead, or decaying), they will also feed on.
Most crabs display sexual dimorphism. As a result, it's quite easy to differentiate males (with larger claws) from females. In fact, males use enlarged chela for communication and to attract a mating partner.
In addition, the abdomen of crustaceans (the pleon) tends to be narrower and more of a triangular shape in male crabs. Whereas, female crabs tend to have a broad, somewhat rounded abdomen. This is the area where they will eventually brood fertilised eggs.
Here's the thing:
When female crabs lay their eggs, including land crabs, they need to do it in water. Thus, she will carry the eggs on her body, inside the abdominal sac, until they hatch.
Some baby crabs hatch and look like small adults straight away - but not all. Instead, most newly hatched crabs (zoea) tend to be tiny, transparent creatures absent of any fully formed legs drifting around at the upper column of the water.
They will shed their exterior covering several times, through a process called metamorphosis, before they develop into the adult form.
Fun Fact: A fading moon is the signal for female crabs to enter the ocean and start laying (up to 100,000 eggs at a time). Yet, the blue crab is capable of carrying up to two (2) million eggs in the brooding season.
Note: The short video [4:05 seconds] presented by 'HappyFriday' contains fun and interesting facts about crabs and several other invertebrate crustaceans.