TAXONOMIC (DICHOTOMOUS) KEYS

Transcription

1 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

2 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! 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 pereopod

3 Have a go at being a Taxonomist Look at the key physical characteristics which describe each species and then use the keys provided to identify the species. SPECIES A SPECIES B SPECIES C SPECIES D

4 SPECIES E Answers SPECIES A SPECIES B SPECIES C SPECIES D SPECIES E

5 Taxonomic Family Key FIRST WE NEED TO FIND OUT WHICH FAMILY THE CRABS BELONG TO. ANSWER THE QUESTIONS AND GO TO THE KEYS FOR EACH FAMILY 1) Is the last pereopod (leg), especially the dactylus (last segment), flattened to allow the crab to swim? Yes - Go to key for the family Portunidae No - Go to number 2 2) Is the carapace (main body) slightly longer than wide and the antennae is longer than the carapace? Yes - Go to key for the family Corystidae No - Go to number 3 3) Is the carapace drawn forward into a pronounced beak (rostrum rostrum)? Yes - Go to the key for the family Majidae No - go to number 4 4) Is the carapace nearly square with very conspicuous eyes (on stalks)? Yes - Go to the key for the family Grapsidae

6 FAMILY Portunidae GENERAL DESCRIPTION OF THE FAMILY The carapace is depressed and usually broader than long. The orbits broad and the eyes prominent. The last pereopod (leg), especially the dactylus (last segment), flattened to allow the crab to swim. 1) Dactyl (dactylus plural) of all the visible perepods (legs) are long and conical Yes - Pirimela denticulate No - Go to number 3 2) Dactyl of the most posterior perepod is more broad and flat (like a paddle) than the other visible perepods Yes - Go to number 4 No - Carcinus maenus 3) Carapace and parts of the limbs covered by transverse rows of hairs (looks like wrinkled lines across the body) Yes - Liocarcinus corrugatus No - Liocarcinus puscillus

7 FAMILY Corystidae GENERAL DESCRIPTION OF THE FAMILY Burrowing crabs with the carapace (main body) slightly longer than wide and the antennae is longer than the carapace. Antennae are long and stiff and hairy. This family contains a single genus - Corystes And the genus contains a single species - to make sure you have the right crab species go tp question 1 to see if the final statement is true 1) Chelipeds (front arms/claws) twice the length of the carapace Yes - Corystes cassivelaunus FAMILY Grapsidae GENERAL DESCRIPTION OF THE FAMILY The carapace is nearly square with very conspicuous eyes (on stalks). 1) Mat of hairs on the chela Yes - Eriocheir sinensis No - Go to number 2 2) Carapace narrowing posteriorly Yes - Brachynotus sexdentatus

8 FAMILY Majidae GENERAL DESCRIPTION OF THE FAMILY The carapace is drawn forward into a pronounced beak (rostrum rostrum). Chelipeds are very mobile and used for attaching pieces of weed, sponge and other sedentary organisms the hooked setae. A large family with 17 species recorded around Britain. 1) Rostrum (nose/beak) short, extending to the end of the first free segment of antennae Yes - Achaeus cranchii No - Go to number 2 2) Rostrum straight (split into two but not visibly separated into a V) and the rostrum is half the length of the antennae Yes - Macropodia tenuirostris No - Go to number 3 3) Carapace covered with large, irregular warty tubercles Yes - Go to number 4 No - Pisa armata 4) Tubercles at posterior (end) margin of the carapace usually fused into an oval-shaped cluster Yes - Eurynome aspera No - Eurynome spinosa