TAXONOMIC (DICHOTOMOUS) KEYS One method of classifying and identifying objects includes using a taxonomic key, sometimes called a dichotomous key. A taxonomic key looks at the similarities and differences between objects using a series of paired statements or a question. The paired statements describe contrasting characteristics (it is best to use observable, physical characteristics). You choose one statement out of the pair that happens to be true of the object you are trying to identify. The statement you choose may ask you to go on to another pair of statements or it may give you the name of the object. See below for a simple generic example: Does it have a shell or body armour? No Does it have 5 legs? Yes Go to a different key No Anemone Yes Does it have tube feet with suckers? Yes Star fish No Brittle star
Do you remember from the last activity where you looked at the classification of animals and plants? Here we will have a closer look at a very large collection of animals called Arthropods (beetles and other creatures with several legs). All arthropods have a jointed body, a shell and three or more pairs of legs. There are many different types of arthropods living in the sea. Shrimp, crabs, lobsters, and barnacles are seagoing arthropods that have a special name called crustaceans which means 'shelled ones' and they belong to the Phylum Crustacea. All crustaceans have a number of pairs of legs. Most have feelers, or antennae for touching and feeling. Some can also smell with their antennae. All crustaceans breathe with gills and most shed their shells from time to time. For a while the crustacean's body is soft, and can grow. Then its outer skin becomes hard and is its new shell. Some crustaceans attach themselves head down onto a rock and never move again - except to wiggle their legs! NOW IT S YOUR TURN! (Rostrum) Can you name/identify different crab species using a taxonomic key? First let us get familiar with the different parts of a crab so you will be able to use the key Main parts to identify/highlight here for students pereopod
Have a go at being a Taxonomist SPECIES A - Family Corystidae SPECIES B - Family Grapsidae Corystes cassivelaunus SPECIES C - Family Majidae Eriocheir sinesis SPECIES D - Family Portunidae Eurynome aspera Liocarcinus corrugatus
SPECIES E - Family Majidae Macropodia tenuirostris Answers SPECIES A Corystes cassivelaunus SPECIES B Eriocheir sinesis SPECIES C Eurynome aspera SPECIES D Liocarcinus corrugatus SPECIES E Macropodia tenuirostris
General information on the species SPECIES A - Family Corystidae Corystes cassivelaunus Common names - Long-clawed crab, sand crab, helmet crab, masked crab An intertidal crab found in sandy soft bottoms with a pale mauve or reddish brown body. The legs are yellowish-white with brown tips. Dorsal (top) surface of carapace is minutely granulated and the markings sometimes give the impression of a face :-) SPECIES B - Family Grapsidae Eriocheir sinesis Common name - Chinese Mitten crab, big binding crab and Shanghai hairy crab A medium-sized burrowing crab that is native in the coastal estuaries of eastern Asia from Korea in the north to the Fujian province of China in the south, but migrated to Europe and North-America. The crab is greyish-green to dark brown and legs are lighter. The carapace is almost square with four conspicuous teeth separated by three curved notches. Strictly speaking this is a freshwater river crab however it returns to estuaries to breed. SPECIES C - Family Majidae Eurynome aspera Common names - Strawberry crab, rough crab An intertidal crab found on the lower shore or sublittoral on hard (stony sand and shell substrate) and muddy sand. The body and legs are pale red to brown, sometimes blue-grey, and the tubercles/warts are white or brownish.
General information on the species SPECIES D - Family Portunidae Liocarcinus corrugatus Common name - Wrinkled swimming crab An intertidal reddish brown swimming crab found on coarse sand and gravel beaches, extending to water depths of 100m on marl. Carapace (body) with numerous strong transverse hairy ridges. SPECIES E - Family Majidae Macropodia tenuirostris Common name - Slender spider crab A reddish brown crab found on mud to hard substrates (stones and gravel) and in estuaries and deep water to 300m.