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For humans, a stroll along the seashore is often a relaxing pleasure. Whether you're wading in the ocean or simply walking in the sand, it’s hard to imagine all that's living beneath your feet. Certain creatures in the sand can make your beach walk unpleasant with their bites or stings. Others, though, quietly help save our lives. Among them are some crab groups commonly called sand fleas.
Sand fleas are commonly referred to as beach flies or sand flies. But by no means are they insects; they’re tiny crustaceans from taxonomic family Talitridae in the Amphipoda order. About 60 genera of them exist. They’re common on North American beaches, and are usually less than an inch long. They jump around in the sand, giving an impression of fleas -- but they look more like shrimps. According the University of Florida, they’re also known as lawn shrimp. They feed on organic and rotting plants when they can’t make a meal out of their favorite organic substance, human blood.
Sand fleas often cluster around seaweed that has washed ashore, and a beach goer’s legs make an opportune meal. Female sand fleas use the protein from the blood as nutrition for egg-laying. Their saliva has an anticoagulant that makes it easier for them to drink, and their saliva is what causes the human body to react with welts and itching.
Mole crabs, indeed crabs, are often called sand crabs, and they're sometimes thought to be sand fleas. But they're members of the Emerita genus of Decapoda, a different order of crustaceans than the sand fleas. They’re slightly larger, averaging 1 to 2 inches long. Like regular crabs, they have hard outer shells. However, like sand fleas, they do not have claws. They cannot pinch, nor do they feed on humans. These crabs are filter feeders: They bury themselves in the sand in the area of breaking waves, leaving their eyes and antennae above the ground. When a wave recedes, they uncoil their antennae feeding on the plankton as they flow across.
Mole crabs are a popular food for seabirds, shorebirds and fish. For certain fish, these crabs make up the majority of their diet. Because of this, fishermen and fisheries often use them as bait. Since they are filter feeders, they play an important ecological role, and scientists use them to indicate levels of toxins in the water, such as Domoic acid. Domoic acid is neurotoxin caused by algae. It is a source of shellfish poisoning and is dangerous to humans.
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.[3][4] Linnaeus, 1767 [2] 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.[5] It has feathery antennae, which are used to filter plankton and detritus from the swash.[5] 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.[6] 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.[7]
Scopoli, 1777 [1]
Type species
Cancer emeritus
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.[8] 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.[9]
Ten species are recognised as of 2019[update]:[Note 1][2]
- Emerita analoga (Stimpson, 1857) – western North America and western South America
- Emerita austroafricana Schmitt, 1937 – southeastern Africa and Madagascar
- Emerita benedicti Schmitt, 1935 – Gulf of Mexico
- Emerita brasiliensis Schmitt, 1935 – southeastern Brazil and northeastern Brazil
- Emerita emeritus (Linnaeus, 1767) – South Asia and Southeast Asia
- Emerita holthuisi Sankolli, 1965 – western India, Persian Gulf, and Red Sea
- Emerita karachiensis Niazi & Haque, 1974 – Pakistan
- Emerita portoricensis Schmitt, 1935 – Caribbean Sea
- Emerita rathbunae Schmitt, 1935 – western Central America
- Emerita talpoida (Say, 1817) – eastern North America
- Emerita asiatica (H. Milne Edwards, 1837) – southern India
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.[8]
The genus Emerita was erected by Giovanni Antonio Scopoli in his 1777 work Introductio ad Historiam Naturalem.[10] 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]
E. analoga digging in the sand
Emerita is adept at burrowing, and is capable of burying itself completely in 1.5 seconds.[5] Unlike mud shrimp, Emerita burrows tail-first into the sand, using the pereiopods to scrape the sand from underneath its body.[11] During this action, the carapace is pressed into the sand as anchorage for the digging limbs.[11] 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.[11] (Also see Fluidization.)
As the tide changes, Emerita changes its position on the beach;[5] most individuals stay in the zone of breaking waves.[6] 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.[6] Longshore drift may also drag Emerita laterally along a beach.[6]
The main predators of Emerita are fish; in the eastern Pacific Ocean, the barred surfperch (Amphistichus argenteus) is particularly important.[6] Seabirds also eat Emerita, but do not appear to target the aggregations of mole crabs.[6] Carcasses of Emerita provide an important food source for the closely related scavenger Blepharipoda.[6]
Due to the Emerita being the predominant diet of the barred surfperch, surf fisherman 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 and parts of India. 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.
In some locations, such as on the Pacific coast of the United States, digging up Emerita is a recreational activity, like digging holes or surfing.
Eggs on the underside of a female E. analoga
Emerita has a short lifespan, perhaps no more than two to three years, and can reproduce in its first year of life.[6] 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.[6] The number of zoeal stages varies between species from six to eleven.[2]
- Crustaceans portal
- Arthropods portal
- ^ Distributions follow Haye et al. (2003).[8]
- ^ a b International Commission on Zoological Nomenclature (1963). "Opinion 643. Idotea Fabricius, 1798, and Mesidotea Richardson, 1905 (Crustacea, Decapoda); validation under the Plenary Powers". Bulletin of Zoological Nomenclature. 20 (1): 18–25.
- ^ a b c d Christopher B. Boyko; Patsy A. McLaughlin (2010). Martyn E. Y. Low; S. H. Tan (eds.). "Annotated checklist of anomuran decapod crustaceans of the world (exclusive of the Kiwaoidea and families Chirostylidae and Galatheidae of the Galatheoidea): Part IV – Hippoidea" (PDF). Zootaxa. Suppl. 23: 109–129. Archived from the original (PDF) on 2012-03-16.
- ^ Sand Fleas (Mole Crabs or Sand Crabs) Prime surf fishing bait
- ^ //www.baymoon.com/~ilga/crabs/ All About Mole Crabs
- ^ a b c d Kenneth Henry Mann (2000). "Sandy beaches". Ecology of Coastal Waters, with Implications for Management. Volume 8 of Studies in Ecology (2nd ed.). Wiley-Blackwell. pp. 218–236. ISBN 978-0-86542-550-7.
- ^ a b c d e f g h i Edward F. Ricketts; Jack Calvin; David W. Phillips & Joel W. Hedgpeth (1992). "Open-coast sandy beaches". Between Pacific Tides (5th ed.). Stanford University Press. pp. 249–265. ISBN 978-0-8047-2068-7.
- ^ T. Subramoniam (1981). "Protandric hermaphroditism in a mole crab, Emerita asiatica (Decapoda:Anomura)". Biological Bulletin. 160 (1): 161–174. doi:10.2307/1540910. JSTOR 1540910. BIOSTOR 9638.
- ^ a b c Pilar A. Haye; Yan K. Tam; Irv Kornfield (2002). "Molecular phylogenetics of mole crabs (Hippidae: Emerita)" (PDF). Journal of Crustacean Biology. 22 (4): 903–915. doi:10.1651/0278-0372(2002)022[0903:MPOMHE]2.0.CO;2. JSTOR 1549850.
- ^ Colin Little (2000). "The coarse extreme: life on sandy beaches". The Biology of Soft Shores and Estuaries. Oxford University Press. pp. 35–57. ISBN 978-0-19-850426-9.
- ^ Ioannis Ant. Scopoli (1777). Introductio ad historiam naturalem sistens genera lapidum, plantarum, et animalium hactenus detecta, caracteribus essentialibus donata, in tribus divisa, subinde ad leges naturae (in Latin). Prague: Wolfgang Gerle.
- ^ a b c Adolf Seilacher (2007). "Plate 21. Burrowing techniques". Trace Fossil Analysis. Springer. p. 64. ISBN 978-3-540-47225-4.
- Media related to Emerita at Wikimedia Commons
- Data related to Emerita at Wikispecies
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