Emerita (crustacean)

Lore & Background

The genus Emerita was established by Giovanni Antonio Scopoli in his 1777 work Introductio ad Historiam Naturalem. The type species is Cancer emeritus (now E. emeritus), which was at one time the only species in the genus. Other genera with the same name, published by Laurens Theodorus Gronovius (1764) and Friedrich Christian Meuschen (1778 and 1781), have been rejected for nomenclatural purposes.

Emerita has a broad distribution in tropical and subtropical regions, though most individual species are restricted to smaller areas with ranges that rarely overlap. 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. Molecular phylogenetics has shown that E. analoga, a species living along the Pacific coast of North America, is more closely related to African species than to other New World species, contradicting the earlier view that Old World and New World species each formed monophyletic groups.

Males are typically smaller than females. In some species, such as Emerita rathbunae, minute males live attached to the legs of the female. Females range from 8 to 37 mm in carapace length depending on species, while males vary from a similar size to females in E. austroafricana down to 2.5 mm carapace length in E. rathbunae and E. talpoida.

Reader's Guide

Emerita plays a significant role in beach ecosystems as both a filter feeder and a prey item. Its main predators are fish, notably the barred surfperch (Amphistichus argenteus) in the eastern Pacific Ocean. Seabirds also eat Emerita but do not appear to target its aggregations. Carcasses of Emerita provide an important food source for the closely related scavenger Blepharipoda.

Emerita is adept at burrowing, capable of burying itself completely in 1.5 seconds. Unlike mud shrimp, it burrows tail-first into the sand using its pereiopods to scrape sand from underneath its body, pressing its carapace into the sand for anchorage. The digging requires the sand to be fluidised by wave action, and Emerita must bury itself in the correct orientation before the wave passes to be safe from predators. As the tide changes, Emerita changes its position on the beach, staying mostly in the zone of breaking waves, and uses temporary liquefaction from a breaking wave to emerge and be carried down the beach.

Due to Emerita being the predominant diet of the barred surfperch, surf fishermen use sand crabs as bait. Soft-shelled Emerita are also kept as bait by commercial fisheries. In some cultures, sand crabs are eaten as a popular snack, such as in Thailand, where they are shallow fried or deep fried in batter. The taste is described as falling between shrimp and crab. Eating sand crabs presents a risk of paralytic shellfish poisoning or infection with Profilicollis parasites. In Malaysia, the sand crab is a local delicacy found along beaches in Kelantan during the monsoon season, usually fried with eggs or roasted on a skewer like satay.

Did You Know?

• Emerita can bury itself completely in 1.5 seconds.
• Females range from 8 to 37 mm in carapace length, while males can be as small as 2.5 mm.
• The eggs are bright orange and hatch into larvae that may live as plankton for more than four months.