Emerita analoga
Encyclopedia
Emerita analoga, the Pacific sand crab or Pacific mole crab, is a species
of small, sand-burrowing decapod
crustacean
found living in the sand along the temperate western coasts of America. It is found on exposed sandy beaches in the swash
region of the intertidal zone
.
. The adult is sand-coloured and well camouflaged and has no claws or spines. There are five pairs of legs and three pairs of pleopods. The crab moults periodically so its exoskeleton
may be found washed up on the beach.
The sand crab is well adapted to life in the sand, which presents an unstable substrate, and its shape is an elongated dome-shape designed for fast burrowing. The eyes are on long stalks and the antennules
are also elongated so as to project above the surface of the sand. These form a tube which channels water downwards through the gill
s. The much longer antennae
are retractable. When there is water overhead, they also project above the sand surface in order to collect food particles. The legs and uropods have hairy margins to assist in digging and for use in collecting food and transferring it to the mouth.
to Baja California
and in South America where it is found from Salavery, Peru
southwards to around Cape Horn
and into southern Argentina
. The species is common on the beaches of California but there are greater changes in population levels further north, probably as a result of variations in the coastal currents which in some years passively disperse the planktonic larval stages northwards. In Oregon
the populations seem not to be self-sustaining and recruitment
is largely from larvae originating in California.
On any particular beach, the distribution of sand crabs from one part to another can vary greatly for reasons not properly understood. There is a tendency for females to be found far down the beach whereas males and immature crabs are found at higher levels.
, mostly consisting of dinoflagellate
s.
The sand crab mates in spring and summer. The female lays batches of up to 45,000 eggs each month and carries them about beneath her abdomen tucked under her telson. The eggs hatch in about four weeks. The larvae have five planktonic zoeal stages and a final megalopal stage. The zoeal stages last up to 130 days. The megalopae settle out onto sandy beaches where they moult
and develop into juveniles which mature into adults within a few weeks. The long planktonic stage means that the larvae can become widely dispersed and colonise new areas. The adults reproduce in both their first and second summers and most die in the autumn of their second year.
. These birds actively patrol the part of the beach washed by incoming waves, probing the softened sand with their bills. The sand crab has adopted the strategy of retreating under the sand surface as each wave goes out. By this means it may be out of reach of the bird's beak. In order to maximise the bird's chance of feeding on sand crabs, it needs to scurry around in the edge of the surf.
Other birds that eat sand crabs include sandpipers, willet
s, godwit
s, surf scoters
, blackbellied plovers
and curlew
s. The sea otter
also enjoys them. The crabs are hosts to the intermediate stages of various parasitic worms. These get passed on to the predators when the crabs are eaten by them and the number of worms ingested has been known to kill the predator.
The barred surfperch
(Amphistichus argenteus), found off the coast of California, consumes a large number of sand crabs. Surf fishermen
use the crabs for bait
and there are commercial bait fisheries extracting them from the beach. The sand crabs with soft shells that have just moulted are kept for bait while the hard shelled crabs are thrown back into the sea.
The sand crab has been evaluated as an indicator species for monitoring the level of domoic acid
-synthesizing diatom
s (Pseudo-nitzschia
spp.) which sometimes cause toxic blooms
off the coast of California.
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
of small, sand-burrowing decapod
Decapoda
The decapods or Decapoda are an order of crustaceans within the class Malacostraca, including many familiar groups, such as crayfish, crabs, lobsters, prawns and shrimp. Most decapods are scavengers. It is estimated that the order contains nearly 15,000 species in around 2,700 genera, with...
crustacean
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...
found living in the sand along the temperate western coasts of America. It is found on exposed sandy beaches in the swash
Swash
Swash, in geography, is a turbulent layer of water that washes up on the beach after an incoming wave has broken. The swash action can move beach material up and down on the beach, which results in the cross-shore sediment exchange. The time-scale of swash motion varies from seconds to minutes...
region of the intertidal zone
Intertidal zone
The intertidal zone is the area that is above water at low tide and under water at high tide . This area can include many different types of habitats, with many types of animals like starfish, sea urchins, and some species of coral...
.
Description
The Pacific sand crab is a small crab growing up to 35 millimetres (1.4 in) long and 25 mm (0.984251968503937 in) wide. The female is nearly twice as large as the male and may be identified by the orange egg mass often carried under the telsonTelson
The telson is the last division of the body of a crustacean. It is not considered a true segment because it does not arise in the embryo from teloblast areas as do real segments. It never carries any appendages, but a forked "tail" called the caudal furca is often present. Together with the...
. The adult is sand-coloured and well camouflaged and has no claws or spines. There are five pairs of legs and three pairs of pleopods. The crab moults periodically so its exoskeleton
Exoskeleton
An exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton of, for example, a human. In popular usage, some of the larger kinds of exoskeletons are known as "shells". Examples of exoskeleton animals include insects such as grasshoppers...
may be found washed up on the beach.
The sand crab is well adapted to life in the sand, which presents an unstable substrate, and its shape is an elongated dome-shape designed for fast burrowing. The eyes are on long stalks and the antennules
Antenna (biology)
Antennae in biology have historically been paired appendages used for sensing in arthropods. More recently, the term has also been applied to cilium structures present in most cell types of eukaryotes....
are also elongated so as to project above the surface of the sand. These form a tube which channels water downwards through the gill
Gill
A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...
s. The much longer antennae
Antenna (biology)
Antennae in biology have historically been paired appendages used for sensing in arthropods. More recently, the term has also been applied to cilium structures present in most cell types of eukaryotes....
are retractable. When there is water overhead, they also project above the sand surface in order to collect food particles. The legs and uropods have hairy margins to assist in digging and for use in collecting food and transferring it to the mouth.
Distribution
The sand crab occurs in North America from AlaskaAlaska
Alaska is the largest state in the United States by area. It is situated in the northwest extremity of the North American continent, with Canada to the east, the Arctic Ocean to the north, and the Pacific Ocean to the west and south, with Russia further west across the Bering Strait...
to Baja California
Baja California
Baja California officially Estado Libre y Soberano de Baja California is one of the 31 states which, with the Federal District, comprise the 32 Federal Entities of Mexico. It is both the northernmost and westernmost state of Mexico. Before becoming a state in 1953, the area was known as the North...
and in South America where it is found from Salavery, Peru
Peru
Peru , officially the Republic of Peru , is a country in western South America. It is bordered on the north by Ecuador and Colombia, on the east by Brazil, on the southeast by Bolivia, on the south by Chile, and on the west by the Pacific Ocean....
southwards to around Cape Horn
Cape Horn
Cape Horn is the southernmost headland of the Tierra del Fuego archipelago of southern Chile, and is located on the small Hornos Island...
and into southern Argentina
Argentina
Argentina , officially the Argentine Republic , is the second largest country in South America by land area, after Brazil. It is constituted as a federation of 23 provinces and an autonomous city, Buenos Aires...
. The species is common on the beaches of California but there are greater changes in population levels further north, probably as a result of variations in the coastal currents which in some years passively disperse the planktonic larval stages northwards. In Oregon
Oregon
Oregon is a state in the Pacific Northwest region of the United States. It is located on the Pacific coast, with Washington to the north, California to the south, Nevada on the southeast and Idaho to the east. The Columbia and Snake rivers delineate much of Oregon's northern and eastern...
the populations seem not to be self-sustaining and recruitment
Recruitment (biology)
In biology, recruitment occurs when juvenile organisms survive to be added to a population. The term is generally used to refer to a stage whereby the organisms are settled and able to be detected by an observer....
is largely from larvae originating in California.
On any particular beach, the distribution of sand crabs from one part to another can vary greatly for reasons not properly understood. There is a tendency for females to be found far down the beach whereas males and immature crabs are found at higher levels.
Biology
The sand crab always moves backwards when burrowing or crawling. It can also swim (backwards) and tread water using its back legs. It is a suspension feeder. It burrows backwards into the sand and faces the sea. As each wave retreats, it extends its antennae and catches floating organisms. It then retracts the antennae and scrapes the particles into its mouth. It can do this several times per wave. When it becomes uncovered by water, it coils its antennae and burrows backwards deeper into the sand. The diet is planktonPlankton
Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...
, mostly consisting of dinoflagellate
Dinoflagellate
The dinoflagellates are a large group of flagellate protists. Most are marine plankton, but they are common in fresh water habitats as well. Their populations are distributed depending on temperature, salinity, or depth...
s.
The sand crab mates in spring and summer. The female lays batches of up to 45,000 eggs each month and carries them about beneath her abdomen tucked under her telson. The eggs hatch in about four weeks. The larvae have five planktonic zoeal stages and a final megalopal stage. The zoeal stages last up to 130 days. The megalopae settle out onto sandy beaches where they moult
Ecdysis
Ecdysis is the moulting of the cuticula in many invertebrates. This process of moulting is the defining feature of the clade Ecdysozoa, comprising the arthropods, nematodes, velvet worms, horsehair worms, rotifers, tardigrades and Cephalorhyncha...
and develop into juveniles which mature into adults within a few weeks. The long planktonic stage means that the larvae can become widely dispersed and colonise new areas. The adults reproduce in both their first and second summers and most die in the autumn of their second year.
Ecology
The sand crab lives under the surface of the sand, moving up and down the beach according to the state of the tide. As each wave advances and retreats, the crab comes to the surface and extends its antennae to feed. This makes it vulnerable to predatory birds such as the sanderlingSanderling
The Sanderling is a small wader. It is a circumpolar Arctic breeder, and is a long-distance migrant, wintering south to South America, South Europe, Africa, and Australia...
. These birds actively patrol the part of the beach washed by incoming waves, probing the softened sand with their bills. The sand crab has adopted the strategy of retreating under the sand surface as each wave goes out. By this means it may be out of reach of the bird's beak. In order to maximise the bird's chance of feeding on sand crabs, it needs to scurry around in the edge of the surf.
Other birds that eat sand crabs include sandpipers, willet
Willet
The Willet, Tringa semipalmata , is a large shorebird in the sandpiper family. It is a good-sized and stout scolopacid, the largest of the shanks...
s, godwit
Godwit
The godwits are a group of large, long-billed, long-legged and strongly migratory wading birds of the genus Limosa. They form large flocks on coasts and estuaries in winter....
s, surf scoters
Surf Scoter
The Surf Scoter is a large sea duck, which breeds in Canada and Alaska. It is placed in the subgenus Melanitta, along with the Velvet and White-winged Scoters, distinct from the subgenus Oidemia, Black and Common Scoters.It winters further south in temperate zones, on the coasts of the northern USA...
, blackbellied plovers
Grey Plover
The Grey Plover , known as the Black-bellied Plover in North America, is a medium-sized plover breeding in arctic regions. It is a long-distance migrant, with a nearly worldwide coastal distribution when not breeding....
and curlew
Curlew
The curlews , genus Numenius, are a group of eight species of birds, characterised by long, slender, downcurved bills and mottled brown plumage. They are one of the most ancient lineages of scolopacid waders, together with the godwits which look similar but have straight bills...
s. The sea otter
Sea Otter
The sea otter is a marine mammal native to the coasts of the northern and eastern North Pacific Ocean. Adult sea otters typically weigh between 14 and 45 kg , making them the heaviest members of the weasel family, but among the smallest marine mammals...
also enjoys them. The crabs are hosts to the intermediate stages of various parasitic worms. These get passed on to the predators when the crabs are eaten by them and the number of worms ingested has been known to kill the predator.
The barred surfperch
Barred surfperch
The barred surfperch occurs from northern California to southern Baja California. Like other surfperches, it gives birth to live young. The diet of the barred surfperch is made up predominantly of sand crabs .- Fishery :...
(Amphistichus argenteus), found off the coast of California, consumes a large number of sand crabs. Surf fishermen
Surf fishing
Surf fishing is the sport of catching fish standing on the shoreline or wading in the surf. A general term, surf fishing may or may not include casting a lure or bait, and refers to all types of shore fishing - from sandy and rocky beaches, rock jetties, or even fishing piers...
use the crabs for bait
Fishing bait
Fishing bait is any substance used to attract and catch fish, e.g. on the end of a fishing hook, or inside a fish trap. Traditionally, nightcrawlers, insects, and smaller bait fish have been used for this purpose...
and there are commercial bait fisheries extracting them from the beach. The sand crabs with soft shells that have just moulted are kept for bait while the hard shelled crabs are thrown back into the sea.
The sand crab has been evaluated as an indicator species for monitoring the level of domoic acid
Domoic acid
Domoic acid , the neurotoxin that causes amnesic shellfish poisoning , also known as zombie acid, is a kainic acid analog, heterocyclic amino acid associated with certain harmful algal blooms.-Occurrence:...
-synthesizing diatom
Diatom
Diatoms are a major group of algae, and are one of the most common types of phytoplankton. Most diatoms are unicellular, although they can exist as colonies in the shape of filaments or ribbons , fans , zigzags , or stellate colonies . Diatoms are producers within the food chain...
s (Pseudo-nitzschia
Pseudo-nitzschia
The genus Pseudo-nitzschia includes several species of diatoms known to produce the neurotoxin known as domoic acid, a toxin which is responsible for the human illness called amnesic shellfish poisoning...
spp.) which sometimes cause toxic blooms
Algal bloom
An algal bloom is a rapid increase or accumulation in the population of algae in an aquatic system. Algal blooms may occur in freshwater as well as marine environments. Typically, only one or a small number of phytoplankton species are involved, and some blooms may be recognized by discoloration...
off the coast of California.