Emerita is a small genus of decapod crustaceans,[3] known as sand crabs, or mole crabs. These small animals burrow in the sand in the swash zone and use their antennae for filter feeding.[4][5]
Emerita has a barrel-shaped body. It has a tough exoskeleton and can hold its appendages close to the body, allowing it to roll in the tidal currents and waves.[6] It has feathery antennae, which are used to filter plankton and detritus from the swash.[6]
Males are typically smaller than females, and in some species, such as Emerita rathbunae, the minute males live attached to the legs of the female.[7] Females are around 8–37 mm (0.31–1.46 in) in carapace length, depending on the species, while males vary from a similar size to females in E. austroafricana, down to 2.5 mm (0.098 in) carapace length in E. rathbunae and E. talpoida.[8]
The genus as a whole has a broad distribution in tropical and subtropical regions. Most individual species, however, are restricted to smaller areas, and their ranges rarely overlap.[9] The genus is common on both coasts of the United States and along the Atlantic coast of Africa; the related genus Hippa is found across the Indo-Pacific, including Australia.[10]
Twelve species are recognised:[3][Note 1]
The Old World species had been widely thought to form a monophyletic group, as did the New World species. The use of molecular phylogenetics has shown, however, that E. analoga, a species living along the Pacific coast of North America, is more closely related to African species than it is to other New World species.[9]
The genus Emerita was erected by Giovanni Antonio Scopoli in his 1777 work Introductio ad Historiam Naturalem.[11] The type species is Cancer emeritus (now E. emeritus), because at one time, it was the only species in the genus.[2] Other genera with the same name have been rejected for nomenclatural purposes; these were published by Laurens Theodorus Gronovius (1764) and Friedrich Christian Meuschen (1778 and 1781).[1]
Emerita is adept at burrowing, and is capable of burying itself completely in 1.5 seconds.[6] Unlike mud shrimp, Emerita burrows tail-first into the sand, using the pereiopods to scrape the sand from underneath its body.[12] During this action, the carapace is pressed into the sand as anchorage for the digging limbs.[12] The digging requires the sand to be fluidised by wave action, and Emerita must bury itself in the correct orientation before the wave has passed to be safe from predators.[12] (Also see Fluidization.)
As the tide changes, Emerita changes its position on the beach;[6] most individuals stay in the zone of breaking waves.[7] This may be detected by the physical characteristics of the sand. As the tide falls, the sand is allowed to settle; when Emerita detects this, it uses the temporary liquefaction from a breaking wave to emerge from its burrow, and is carried down the beach by the wave action.[7] Longshore drift may also drag Emerita laterally along a beach.[7]
The main predators of Emerita are fish; in the eastern Pacific Ocean, the barred surfperch (Amphistichus argenteus) is particularly important.[7] Seabirds also eat Emerita, but do not appear to target the aggregations of mole crabs.[7] Carcasses of Emerita provide an important food source for the closely related scavenger Blepharipoda.[7]
Due to the Emerita being the predominant diet of the barred surfperch, surf fishermen use sand crabs as bait. Soft-shelled Emerita are kept also as bait by commercial fisheries.
In some cultures, sand crabs are eaten as a popular snack, such as in Thailand. They are often prepared via shallow frying in a pan or deep frying in batter. The taste is often described as falling somewhere between that of shrimp and crab. Eating sand crabs presents a risk of paralytic shellfish poisoning and of infection with Profilicollis parasites.[13] In Malaysia, the sand crab locally known as yat yat, ibu remis, kutu laut or udang pasir can be found along the beaches in Kelantan during the monsoon season. The crab is a local delicacy that is usually fried with eggs or roasted on a skewer like satay.[14]
Emerita has a short lifespan, perhaps no more than two to three years, and can reproduce in its first year of life.[7] The eggs are bright orange, and hatch into larvae, which may live as plankton for more than four months and can be carried long distances by ocean currents.[7] The number of zoeal stages varies between species from six to eleven.[2]
Emerita is a small genus of decapod crustaceans, known as sand crabs, or mole crabs. These small animals burrow in the sand in the swash zone and use their antennae for filter feeding.
