Marine
Biology
Activity 2
TAXONOMIC
(DICHOTOMOUS)
KEYS
One method of classifying and identifying objects includes using a taxonomic key,
key sometimes called a dichotomous key.
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:
example
Does it have a shell or body armour?
No
Yes
Does it have 5 legs?
No
Anemone
Go to a different key
Yes
Does it have tube feet with suckers?
Yes
Star fish
No
Brittle star
Marine Biodiversity and Ecosystem Functioning EU Network of Excellence
Sustainable development, global change and ecosystems
GOCE-CT-2003-505446
Marine
Biology
Activity 2
r
em b e y
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Do y o e l a s t a c t i a t
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t he cl l s a n d pl an
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anim
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.
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
Marine Biodiversity and Ecosystem Functioning EU Network of Excellence
Sustainable development, global change and ecosystems
GOCE-CT-2003-505446
Marine
Biology
Have a go at being
a Taxonomist
Activity 2
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
Marine Biodiversity and Ecosystem Functioning EU Network of Excellence
Sustainable development, global change and ecosystems
GOCE-CT-2003-505446
Marine
Biology
Activity 2
SPECIES E
Answers
SPECIES A ______________________
SPECIES B _______________________
SPECIES C _______________________
SPECIES D _______________________
SPECIES E _______________________
Marine Biodiversity and Ecosystem Functioning EU Network of Excellence
Sustainable development, global change and ecosystems
GOCE-CT-2003-505446
Marine
Biology
Activity 2
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)?
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
Marine Biodiversity and Ecosystem Functioning EU Network of Excellence
Sustainable development, global change and ecosystems
GOCE-CT-2003-505446
Marine
Biology
FAMILY
Portunidae
Activity 2
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
Marine Biodiversity and Ecosystem Functioning EU Network of Excellence
Sustainable development, global change and ecosystems
GOCE-CT-2003-505446
Marine
Biology
FAMILY
Corystidae
Activity 2
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
Marine Biodiversity and Ecosystem Functioning EU Network of Excellence
Sustainable development, global change and ecosystems
GOCE-CT-2003-505446
Marine
Biology
FAMILY
Majidae
Activity 2
GENERAL DESCRIPTION OF THE FAMILY
The carapace is drawn forward into a pronounced beak (rostrum
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
Marine Biodiversity and Ecosystem Functioning EU Network of Excellence
Sustainable development, global change and ecosystems
GOCE-CT-2003-505446