THE WORLD OF LIFE with Douglas Drenkow

Most crustaceans live in the sea. Hermit crabs live in abandoned seashells, barnacles live not only on rocks but sometimes on the hulls of ships or the bodies of whales, and parasitic fishlice live attached to the bodies of their "hosts". Other marine crustaceans, such as lobsters or crabs, actively swim. However, most crustaceans float free in the sea, forming the bulk of the "zooplankton", near the base of the oceans' food chains.

Because they breathe by gills (like all other crustaceans), sowbugs, pillbugs, and woodlice thrive only in moist areas and usually come out only at night (During dry spells sowbugs burrow deeply into the soil and pillbugs just roll-up).

DESCRIPTION

The body of crustaceans is minute to large, and often brightly -- sometimes changeably -- colored.

Sometimes covered or enclosed by a one- or two-part "carapace" (shell), the "cephalothorax" (front part of the body) of a crustacean bears image-resolving "compound" eyes or a light-sensing "simple" eye, two pairs of sometimes branched antennae, one pair of typically jaw-like "mandibles", and two pairs of typically food-manipulating "maxillae". The "abdomen" (rear body region), sometimes also enclosed by the carapace, typically bears paired, specialized appendages. Crustaceans typically use their "swimmerets", legs, or even antennae to swim.

Branchiopod crustaceans (fairy shrimp, brine shrimp, tadpole shrimp, clam shrimp, and waterfleas) are often partially encased by a carapace and typically have a trunk bearing leafy appendages. Branchiopods are typically minute.

Copepod crustaceans have a long pair of antennae on the head of their tapering, typically minute body.

Ostracod crustaceans (mussel, or seed shrimp, typically minute) and barnacles (up to an inch or more) are each completely encased within a two-part carapace -- ostracods typically crawl and swim with their long antennae; and barnacles (additionally covered by stony plates) are attached upside-down to rocks (or even the body of a whale or the hull of a ship), often by means of a fleshy stalk.

Amphipod crustaceans (including shrimp-like species and beachfleas) -- flattened from side-to-side -- and isopods (including pillbugs) -- flattened from top-to-bottom -- both have an approximately half-inch-long trunk (behind the head) composed of an eight-segmented thorax (bearing one pair of "maxillipeds", seven pairs of true legs, and a "brood pouch" on the underside of the body) and a six-segmented abdomen.

The cephalothorax of decapod crustaceans (shrimp, crayfish, lobsters, and crabs) is composed of the head plus an eight-segmented thorax. Covered by a carapace, the thorax bears three pairs of "maxillipeds" and five pairs of true legs -- the first pair often modified as claws. The six-segmented abdomen of a decapod typically bears five pairs of "swimmerets" and one pair of paddle-like tail-appendages. The abdomen of an adult crab is simplified and curled under the body -- it only appears to have no tail. The largest arthropods are marine crabs, buoyed by the saltwater, with a legspan as wide as a human armspan!

FEEDING HABITS

Many crustaceans (such as barnacles and various shrimps) are "filter feeders", gracefully waving feathery appendages in the water to catch passing zooplankton and "phytoplankton" (microscopic plantlife in the sea). Some crustaceans are herbivorous; some are "omnivorous" (eating almost anything); other, typically larger crustaceans (such as lobsters, with their formidable claws) are predators or scavengers; and various small crustaceans (such as fishlice) are parasites on other, larger animals.

Sowbugs and pillbugs usually feed on tender, moist plant materials, such as mulches, grass clippings, leaf litter, and fallen overripe fruit; but they sometimes feed on the young roots and tender shoots of seedlings and older plants, as in gardens, flower beds, greenhouses, and indoor pots.

MOTION

Crustaceans typically use their segmented swimmerets, legs, or even antennae to swim.

DIGESTION

Please see the general discussion for Arthropods.

RESPIRATION

In aquatic arthropods, gases are usually "exchanged" (between the body and the environment) through typically external, feather- or comb-like gills (as in a "ventilating" current of water generated under the carapace of lobsters and crabs).

Because they breathe by gills, pillbugs and sowbugs are found only in moist environments.

CIRCULATION

Please see the general discussion for Arthropods.

Such as many copepods and ostracods and all barnacles have no developed circulatory system at all -- dissolved materials simply diffuse throughout the tiny body.

EXCRETION

Please see the general discussion for Arthropods. Nitrogen-rich and other soluble wastes are excreted near the base of the antennae in many crustaceans.

An exoskeleton is invaluable in preventing the evaporation of bodily water from terrestrial species.

COORDINATION

Please see the general discussion for Arthropods.

In addition to the light-sensing "simple" eyes typical of arthropods, most crustaceans (like insects) have a pair of fixed-focus "compound" eyes, composed of many "hexagonal facets" (6-sided parts), each contributing a small portion to the overall (fairly crude, wide-angled) image, like pixels on a computer screen.

REPRODUCTION

Crustaceans reproduce sexually and usually have separate sexes, although barnacles are "hermaphroditic" (each individual with both male and female "gonads", producing either sperms or eggs) -- this reproductive strategy is typically an advantage for an animal that cannot move about in search of a mate.

Crustaceans (like terrestrial arthropods) typically "copulate" (mate with physical contact). However, some crustaceans may actually fertilize their eggs "externally" (in environmental water); and some branchiopods in inland waters (prone to drying up) can reproduce "parthenogentically" (without sexual fertilization of the eggs) -- a quick way to build-up large populations.

Crustaceans typically brood their eggs on their body, as on specialized appendages or in the "marsupium" (a kangaroo-like pouch) on the underside of amphipods and isopods (from which the young eventually emerge).

The often bizarre-looking larvae of crustaceans are typically "planktonic" (microscopic and drifting). Even land crabs, found on Tropical islands, typically return to the sea to lay their fertilized eggs.

Arthropods (Arthropoda)

Doug@DouglasDrenkow.com

Photo of Cells: H.D.A. Lindquist, US EPA

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