Blue Crab on dock, Annapolis, Maryland, 1998. Photo by Elizabeth W. Newell.
Misnamed, Horseshoe crabs are not actual crabs, or even crustaceans. As part of the subphylem Chelicerata, they more closely relate to spiders than "true" crabs. Horseshoe shrimp (Triops cancriformis), though not "true" shrimp, are crustaceans (within class Branchiopoda). While species bearing the title of shrimp can be found in other orders within Malacostraca, and other classes of crustacean, they are not classed as "true" shrimp, which are exclusive to order Decapoda within class Malacostraca.
Crab pots (traps), Chesapeake Beach, Maryland, June 2010. Photo by Diane F. Evartt.
With six extant classes, the most common crustaceans encountered in Maryland are Branchiopoda, Malacostraca, and Maxillopoda. The most visible crustaceans in Maryland are members of order Decapoda (within Malacostraca). Though most crustaceans are aquatic, some are terrestrial. One such species is the woodlouse (Oniscus asellus) within class Malacostraca, which also is known as armadillo bug, pill bug, or potato bug.
Water Fleas are microscopic, between 0.5-3 mm in length. Usually translucent, they have a shell covering most of their kidney-shaped bodies. Water Fleas have a compound eye, wispy limbs, and two antennae. They live in lakes, swamps, and streams, and eat algae, bacteria, and other small particles. The Water Flea gets its name from the way it swims with a hopping motion similar to that of a flea.
Clam Shrimp can be up to 2 cm in length. They have two clear shells covering their brown-tan bodies. They have three eyes, two compound and one naupliar. Clam Shrimp have multiple legs that propel them through the water. Gathering food, primarily plankton, they live in freshwater pools and ponds. Their name comes from their resemblance to the clam from the phylum Mollusca.
Fairy Shrimp are usually between 10-40 mm in length. Depending on the species, they can be translucent white, red, orange, blue, black, or green. They have eleven pairs of legs, compound eyes, and two sets of antennae. Consuming algae, bacteria, and other small particles, they live in vernal pools. Fairy Shrimp swim upside down.
Crab, Atlantic Ghost (Ocypode quadrata)
Adult Atlantic Ghost Crabs have gray- or sand-colored bodies, four pairs of sand-colored walking legs, white claws, and square shells, while the juveniles are darker in color with mottled shells. They have prominent eyestalks that can swivel 360 degrees. They are about 2 inches wide. They live in and around coastal beaches where they build burrows in the sand, but return to the sea to moisten their gills or release larvae. Primarily nocturnal, they eat vegetation, insects, clams, other crabs, and turtle eggs. Atlantic Ghost Crabs get their name from their ability to blend in with their surroundings, making them nearly invisible. Also known as Sand Crab.
Blue Crabs, Lusby (Calvert County), Maryland, July 2014. Photo by Diane F. Evartt.
Crab, Common Spider (Libinia emarginata)
Common Spider Crabs have khaki-colored bodies and triangular shells with nine spines and tubercles. They have tapered snouts and narrow, white-tipped claws. Spider Crabs are Decorator crabs, which means that they use materials around them in their efforts to camouflage themselves from predators. On their shells, they have hook-like hairs that hold algae, debris, and small marine invertebrates in place. Their shells are about 4 inches in length and they have a 12-inch leg span. Slow-moving bottom-dwellers, they eat algae, molluscs, fish, and starfish. Also known as the Portly Spider Crab and the Nine-spined Spider Crab.
Crab, European Green (Carcinus maenas)
European Green Crabs are usually mottled and can be green, brown, yellow, orange, or red with yellow or white patches. They have five teeth behind their eye and three protruding lobes between them. Their shells grow up to 3 inches wide. European Green Crabs eat a wide range of molluscs, fish, and small crustaceans, causing significant damage to the shellfish industry. They live in shallow water of estuaries and bays. The European Green Crab is an invasive species.
Crab, Fiddler (Libinia emarginata)
Fiddler Crabs are brown or tan in color, with stalked eyes, a square shell and four pairs of walking legs. Males have an enlarged claw that can reach up to two inches in length. Red-jointed Fiddler Crabs have a groove behind the eyes and the males have red joints on their large claw. (Male Marsh Fiddler Crabs have a blue spot on the shell, while male Sand Fiddler Crabs have blue- or purple-colored shells.) Fiddler Crab shells range in size from less than 1 inch in width to 1.5 inches. They scrape algae and detritus off sediment, leaving behind small, sifted balls. Burrowing in marshes, mangroves, and beaches, they can only walk sideways. The movement of the male Fiddler Crab's smaller claw along the larger claw during feeding gives the species its name.
Crab, Hermit (Libinia emarginata)
Hermit Crabs can be red, brown, and purple and can have tubercles, stripes or rings. They have soft, curved abdomens that are protected inside by shells that the crabs have found. They can range in size from a few millimeters in length to the size of a baseball. As Hermit Crabs grow bigger, they must find new empty shells, usually those of sea snails. They have stalked eyes and ten legs, including two pairs of walking legs and one larger claw. On beaches, reefs, and in shallow waters, they live and eat fish, worms, carrion, and seaweed. The Hermit Crab is so named because of its solitary lifestyle.
Crab, Japanese Shore (Hemigrapsus sanguineus)
Japanese Shore Crabs can be green, red, brown, or purple. They have three spines on each side of their box-like shells. Alternating light and dark bands mark their legs, and red spots are on their claws. Their shells can grow up to 1.65 inches wide. Eating grass, fish, and molluscs, these crabs live in shallow coastal waters, but can be found on oyster reefs and mussel beds, too. Also known as Asian Shore Crab, or Pacific Crab. The Japanese Shore Crab is an invasive species.
Crab, Marsh (Sesarma reticulatum)
Marsh Crabs are brown, olive, or purple with square, notched shells and speckles. Their eyestalks are on the front corners of their bodies, and a tooth lies behind each eye and further back on the shell. Their shells grow to about an 1 inch in length. They live in burrows in salt marshes and eat cordgrass. Also known as Purple Marsh Crab and Heavy Marsh Crab.
Red Swamp Crawfish (Procambarus clarkii), Dept. of Natural Resources exhibit, Maryland State Fair, Timonium, Maryland, September 2015. Photo by Sarah A. Hanks.
Crayfish, Devil (Cambarus diogenes)
Devil Crayfish resemble lobsters. They can be brown, brown-red, green, and blue and they have red-tipped claws. They can grow up to 4.5 inches in length. They eat worms, snails, and dead animal matter, but prefer decaying vegetation. Usually they live in freshwater habitats, digging burrows and pushing the sediment behind them to create a cone-shaped "mud chimney" entrance.
Crayfish, Rusty (Orconectes rusticus)
Rusty Crayfish are dark red or brown with rust-colored spots on the sides of their shells. If they feel threatened, they will put their claws up to scare off potential predators. They can reach up to 4 inches in length. They eat plants, molluscs, crustaceans, insects, and fish eggs. Their life in freshwater takes them to lakes and ponds, often under logs or rocks. They chase other crayfish out of these areas. The Rusty Crayfish is an invasive species.
Crayfish, Virile (Orconectes virilis)
Virile Crayfish are brown, red, or olive. Their blue-green claws have yellow tubercles or bumps, dark specks, and orange tips. They can reach up to 5 inches in length. Plants, fish, and tadpoles are eaten by these crayfish who live in freshwater lakes and wetlands (usually under rocks). Also known as the Northern Crayfish. The Virile Crayfish is an invasive species.
Shrimp, Common Grass (Palaemonetes pugio)
Common Grass Shrimp are transparent gray, with red, yellow, white, or blue spots on their backs. Orange colors their eyestalks. They have a pointed "horn" over their eyes and three pairs of walking legs. They can grow up to 1.5 inches in length. They eat algae, phytoplankton, and small invertebrates. They live among seaweed and bay grasses in marshes and other shallow coastal waters. Also known as Common American Prawn and Marsh Grass Shrimp.
Shrimp, Mantis (Squilla empusa)
Mantis Shrimp have flat, translucent bodies, three pairs of walking legs, and green eyes on stalks. Their segmented bodies can grow up to 10 inches in length. They eat fish, other crustaceans, and worms. They dig "U"-shaped burrows on the seabed or shoreline. Their name comes from their second pair of appendages, claws which resemble those of a Praying Mantis. Using those claws, the Mantis Shrimp takes less than 8 milliseconds to strike its prey, one of the fastest movements of any animal. Also known as Shrimp Snapper, Sea Locust, or Prawn Killer.
Shrimp, Skeleton (Caprellidae)
Skeleton Shrimp may be transparent, brown, or red, but also can change colors to match their surroundings. They have long, slender bodies, two pairs of antennae, folded front raptorial appendages. Their back appendages are used for anchoring themselves to an object. They can grow up to 2 inches in length. They eat detritus, protozoa, worms, and larvae. Among grasses, they live on sponges and hydroids. Their name comes from their ability to disappear, owing to their stick-like body, among seaweed and hydroids. Also known as Ghost Shrimp.
Barnacles start life as free-floating zooplankton. During the larval stage, barnacles swim until they find a suitable spot (which may be rocks, reefs, pilings, and boats) where they attach themselves head-first to the surface with their own glue. Eventually, they form a shell of six gray-white overlapping plates and "doors" that open or close with the tide. They have six pairs of appendages, or cirri, which gather food, including plankton.
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