TAXONOMIC (DICHOTOMOUS) KEYS

(1)

Marine

Biology

Activity 2

One method of classifying and identifying objects includes using a taxonomic keytaxonomic keytaxonomic key, some-taxonomic key times called a dichotomous keydichotomous keydichotomous key. dichotomous key

A taxonomic key looks at the similarities similarities similarities and differencessimilarities differencesdifferencesdifferences between objects using a series of paired statements statements statements statements or a or a or a or a question. question. question. 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 exampleexampleexample: example

Does it have a shell or body armour?

No

Yes

Does it have 5 legs?

Go to a different key

No

Yes

Anemone

Does it have tube feet with suckers?

Yes

No

Star fish

Brittle star

TAXONOMIC

(DICHOTOMOUS)

KEYS

(2)

Marine

Biology

Activity 2

Here we will have a closer look at a very large collection of animals called ArthropodsArthropodsArthropodsArthropods (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 crustaceanscrustaceanscrustaceanscrustaceans which means 'shelled ones' and they belong to the Phylum CrustaceaPhylum CrustaceaPhylum Crustacea. 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!

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

N

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IT

’’’’

S

YOUR

TURN

!!!!

Do you remem ber Do you remem ber Do you remem ber Do you remem ber from the la st activity from the la st activity from the la st activity from the la st activity where you looked at where you looked at where you looked at where you looked at the classific ation of the classific ation of the classific ation of the classific ation of animals an d plants? animals an d plants? animals an d plants? animals an d plants? pereopod

(3)

Marine

Biology

Activity 2

Look at the key physical characteristics which describe each species and then use the Look at the key physical characteristics which describe each species and then use the Look at the key physical characteristics which describe each species and then use the Look at the key physical characteristics which describe each species and then use the

keys provided to identify the species. keys provided to identify the species. keys provided to identify the species. keys provided to identify the species.

SPECIES A

SPECIES B

SPECIES C

SPECIES D

Have a go at being

a Taxonomist

(4)

Marine

Biology

Activity 2

SPECIES E

Answers

SPECIES A

SPECIES A ______________________

______________________

SPECIES B _______________________

SPECIES C _______________________

SPECIES D _______________________

SPECIES E _______________________

(5)

Marine

Biology

Activity 2

FFFFIRSTIRSTIRSTIRST WEWEWEWE NEEDNEEDNEEDNEED TOTOTOTO FINDFINDFINDFIND OUTOUTOUTOUT WHICHWHICHWHICHWHICH FAMILYFAMILYFAMILYFAMILY THETHETHETHE CRABSCRABSCRABSCRABS BELONGBELONGBELONGBELONG TOTOTOTO. . . .

AAAANSWERNSWERNSWERNSWER THETHETHETHE QUESTIONSQUESTIONSQUESTIONSQUESTIONS ANDANDANDAND GOGOGOGO TOTOTOTO THETHETHETHE KEYSKEYSKEYSKEYS FORFORFORFOR EACHEACHEACHEACH FAMILYFAMILYFAMILYFAMILY

1) Is the last pereopodpereopodpereopodpereopod (leg), especially the dactylusdactylusdactylusdactylus (last segment), flattened to allow the crab to swim?

Yes - Go to key for the family PortunidaePortunidaePortunidae Portunidae No - Go to number 2222

2) Is the carapacecarapacecarapace (main body) slightly longer than wide and the antennaecarapace antennaeantennaeantennae is longer than the carapace?

Yes - Go to key for the family CorystidaeCorystidaeCorystidaeCorystidae No - Go to number 3333

3) Is the carapace drawn forward into a pronounced beak (rostrumrostrumrostrum)? rostrum Yes - Go to the key for the family MajidaeMajidaeMajidaeMajidae No - go to number 4444

4) Is the carapace nearly square with very conspicuous eyes (on stalks)? Yes - Go to the key for the family GrapsidaeGrapsidaeGrapsidaeGrapsidae

Taxonomic

Family Key

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Marine

Biology

Activity 2

G GG

GENERALENERALENERALENERAL D D D DESCRIPTIONESCRIPTIONESCRIPTIONESCRIPTION OFOFOFOF THETHETHETHE FAMILYFAMILYFAMILYFAMILY

The carapace is depressed and usually broader than long. The orbits broad and the eyes prominent. The last pereopodpereopodpereopodpereopod (leg), especially the dactylusdactylusdactylusdactylus (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 denticulatePirimela denticulatePirimela denticulate 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 maenusCarcinus maenusCarcinus maenus Carcinus maenus

3) Carapace and parts of the limbs covered by transverse rows of hairs (looks like wrinkled lines across the body)

Yes - Liocarcinus corrugatusLiocarcinus corrugatusLiocarcinus corrugatus Liocarcinus corrugatus

No - Liocarcinus puscillusLiocarcinus puscillusLiocarcinus puscillusLiocarcinus puscillus

FAMILY

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Marine

Biology

Activity 2

G

GG

Burrowing crabs with the carapacecarapacecarapacecarapace (main body) slightly longer than wide and the antennaeantennaeantennaeantennae is longer than the carapace. Antennae are long and stiff and hairy.

This family contains a single genus - CorystesCorystesCorystesCorystes

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 cassivelaunusCorystes cassivelaunusCorystes cassivelaunus Corystes cassivelaunus

FAMILY

Corystidae

G G G

The carapace is nearly square with very conspicuous eyes (on stalks). 1) Mat of hairs on the chela

Yes - Eriocheir sinensisEriocheir sinensisEriocheir sinensis Eriocheir sinensis

No - Go to number 2 2) Carapace narrowing posteriorly

Yes - Brachynotus sexdentatusBrachynotus sexdentatusBrachynotus sexdentatus Brachynotus sexdentatus

FAMILY

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Marine

Biology

Activity 2

G

GG

The carapace is drawn forward into a pronounced beak (rostrumrostrumrostrumrostrum).

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 cranchiiAchaeus cranchiiAchaeus cranchii 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 tenuirostrisMacropodia tenuirostrisMacropodia tenuirostrisMacropodia tenuirostris

No - Go to number 3

3) Carapace covered with large, irregular warty tubercles Yes - Go to number 4

No - Pisa armataPisa armataPisa armata Pisa armata

4) Tubercles at posterior (end) margin of the carapace usually fused into an oval-shaped cluster