Sand Crab

Sand crabs, also known as mole crabs or sand fleas, burrow backward just beneath the surface of sandy beaches to hide from predators and look for prey.

If you happen to be walking along an untouched virgin beach near the ocean, you may notice these unusual animals hidden directly beneath the sand with only their heads sticking out. Sand crabs are completely harmless to people and even entertaining to observe as they move around with the tide. This article will cover some important facts about this animal’s identification, habitat, and diet, including how to catch them and how to cook them.

Scientific Name

Sand crab is a very loose term for a group of crustaceans that burrow beneath the sand. While there are a few closely related families that are referred to as sand crabs, this article will generally focus on the family of Hippidae and more specifically on the genus of Emerita. Ten different species are recognized within this genus. However, sand crabs are not considered to be a “true” crab like the blue crab or the snow crab, but rather a side group that also includes the hermit crab. There are many subtle differences in body shape and anatomy between true crabs and sand crabs, some of which will be covered below.

Evolution And History

Crabs are such an ancient species that they predate even the dinosaurs and have been on this earth for over 450 million years. Most of the modern species that we see today began to diversify around 250 million years ago after the great mass extinction took place. The crab has not only evolved numerous times in the last 250 million years but they have been created and remade, as evidenced by false crabs.

They have evolved their shape so many times that there is a name for it this process – carcinization – and they still continue to change. Due to this process, the ancestors of some true crabs have been shown to come from the hermit crab, which is not a true crab and is in the same side group of species as the sand crab.

Types Of Sand Crabs

The crab species that are not considered “true” crabs contain ten different species, including the sand crab. A few of those are:

• The Pacific sand crab or Pacific mole crab (Emerita analoga)
• Mole crab (Emerita austroafricana)
• Benedict sand crab (Emerita benedicti)
• Puerto Rican sand crab (Emerita portoricensis)
• Atlantic mole crab or Atlantic sand crab (Emerita talpoida)

Appearance

Most sand crabs are small barrel-shaped animals with a tough domed shell, a sharp tail, pointy legs, and multiple pairs of antennae. The shape of the gray-colored body keeps it balanced and camouflaged beneath the shifting sands. Unlike the so-called “true crabs,” however, it does not have any claws on its first pair of legs with the ability to pinch and grasp. Sand crabs are not considered to be giant animals. Males tend to measure less than an inch long, while females can measure up to 2 inches. In some species, the male will actually attach to the leg of the female.

Behavior

Sand crabs spend a great deal of their time burrowed backward beneath the sand with only their eyes and first antennae visible. If disturbed or accidentally displaced from the sand, the crab will tread water by beating their back legs to move around. Unlike many other types of crustaceans, they cannot move in any direction, only backward. While incredible numbers of these animals can be found along the beach, they exhibit solitary behavior and spend most of their time living and feeding alone except during the reproductive season. One of the most interesting facts is that the sand crab tail has the largest known concentration of sensory neurons in the entire animal kingdom.

Habitat

The sand crab is found on beaches and dunes all over the world, including the Atlantic coast of Africa, both coasts of the United States, the Indo-Pacific region, and Australia. These animals live and feed in an area called the swash zone. This is the small stretch of the beach where the breaking waves lap against the shore. As the swash zone moves along the beach in tandem with the tide, the crabs pick up and move as well.

Predators And Threats

Apart from the giant dangers posed by their natural predators, sand crabs are threatened most of all by human disturbances and the loss of habitat caused by coastal development. This can prevent them from easily burrowing beneath the sand. However, they are very common and abundant all around the world. They are not in any danger of disappearing.

What Eats The Sand Crab?

These crabs are preyed upon by numerous types of fish and birds. Several species such as the barred surfperch rely on sand crabs for up to 90% of their diet. The crabs’ main line of dense is, of course, the hard shell and the ability to burrow in the sand. But they’re so effective at attracting predators that fishers will use the soft-shelled stage of the crab as bait.

What Does The Sand Crab Eat?

These crabs mostly feed upon plankton and dinoflagellates. These are small, almost microscopic marine organisms that move placidly throughout the water and get eaten by other animals. The crabs rapidly wave their long second pair of feather-like antennae to filter the prey directly from the water. This action occurs so quickly that it’s hard to actually see it.

Because these crabs live in the part of the beach most affected by toxins, their bodies are sometimes filled with domoic acid (a natural byproduct of microscopic algae). When the crabs are eaten by other animals, the toxins are then transmitted further up the food chain. This toxin can have a serious effect on larger animals, including humans. In order to understand how much toxin is present in the water, scientists can measure the amount of toxin in these crabs.

Reproduction And Life Cycle

The crab life cycle varies a bit by species, but the reproductive season usually occurs in the spring and summer months. These crabs have the capacity to create a giant number of young every season. After mating, the female will produce as many as 45,000 eggs at a time and carry them directly on her abdomen. It takes about a month for the eggs to hatch, after which they receive no parental care. As the larvae drift along on the current of the ocean, they can end up a long way from their original place of birth.

These crabs will pass through anywhere between six and 11 different immature stages. Each one of these stages is separated by a molt, during which the larva will shed its exoskeleton and then grow a new one again. The larva can look radically different from the adult stage, almost more like a small swimming marine organism. Depending on the water temperature, the crab will settle down and reach sexual maturity after about a year. They do not have long to reproduce, however. The normal lifespan is between two and three years.

Population

These crab populations are large and robust all over the world based on direct observations. However, the IUCN Red List does not currently classify the conservation status of any of the species. There simply aren’t enough facts known about their population numbers.