Identifying Animals in Seaweed
To identify your animal, start at number 1 in the following questions and choose the characteristics
which it possesses at each stage. Follow on to the next numbered step according to what you have
found, until you have named the animal.
1. Does the animal have legs? (NB: Arms and tentacles are not legs)
 If yes, go to 2
 If no, go to 7
2. Does the animal have only eight legs?
 If yes, go to 3
 If more than or less than 8 legs, go to 4
3. Animals with eight legs:
 The legs are very thin and spider-like, and are usually much longer than the
body. The head has a long cylindrical snout (proboscis). The body is very long
and narrow. This is a sea spider.
 The body is very rounded, or oval. The back (dorsal area) is flattened. The
body is not very obviously segmented. The legs are not much longer than the
body. This is a mite.
Sea spider
Mite
4. Animals with more than or less than 8 legs: Are the legs all the same?
 If yes, go to 5
 If no, go to 6
5. Animals with all legs alike:
 The body is very long and tapering (goes into a point). The head has one pair
of short and one pair of long antennae. The legs are very small and are
present on the front end of the body. There is a slender ‘forked’ tail. This is a
copepod.
 The body is flattened from the top to the bottom. The middle region of the
body is segmented, and bears the legs. There is a head and a tail region. The
first pair of antennae on the head is short, but the second may be long. These
are isopods, related to the wood louse in your garden.
6. Animals with legs that differ from each other:
 The animal lies on its side, as the body is flattened from side to side. There is
a head, a segmented middle region that bears the legs, and a tail. Two pairs
of antennae can be seen (different lengths). The legs are of different lengths,
and the first two pairs have claws or pincers. These are amphipods (sandhoppers).
 The body is long and thin and consists of a head and 7 segments. There are
two pairs of pincers at the front end, and 3 pairs of short legs at the rear,
often with shorter limbs in between. This is a skeleton shrimp.
7. Animals with no legs:
 The animal has a shell, go to 8
 The animal has no shell, go to 11
Copepod
Isopod
Amphipod
Skeleton shrimp
8. Animals with shells:
 The shell is in two halves, go to 9
 The shell is not in two halves, go to 10
 The shell is really a white, hard calcareous tube, go to 11
9. Animals with a shell in two halves (= hinged valves):
 Bivalve molluscs include the common mussel you can eat. You may find
juvenile mussels in your seaweed. There is no head. The body is flattened
from side to side and is enclosed within the two hard valves or shell halves.
 Ostracods look a bit like mussels but they have bean-shaped shells, and the
animal can swim freely. There are two small antennae at the front and tips of
two small legs at the back. Ostracods are related to crabs, barnacles,
shrimps and lobsters.
Mussel
10. Animals with shells that are not in two halves:
 The shell is coiled in a spire (but not a white calcareous tube). The head is
well developed and can be seen when the animal peeps out from under the
shell. The head has tentacles and eyes. The animal crawls on a welldeveloped foot. These are gastropod molluscs, similar to the snails you find
in your garden.
 Molluscs with an oval body covered by a shell of 8 plates. The lower surface
has a powerful foot to hold the animal to the substratum. These molluscs are
chitons.
 The shell is made up of 4-8 small plates, and is closed by 2-4 hinged flaps.
You may not be able to see anything of the animal’s body inside. These are
barnacles, and are more commonly found clinging to open rock than in
seaweed. Barnacles are crustaceans and so are related to crabs, shrimps,
amphipods and lobsters.
Ostracod
Gastropod
Chiton
Barnacle
11. Animals with no legs and no real shell:
 If the animal is a worm, go to 12
 If the animal has no features resembling a worm shape, go to 13
12. Worms:
 The worm is inside a coiled calcareous tube generally cannot be seen. The
body of this worm is segmented and each segment has bristles on it, so it is a
type of bristle worm. The head has tentacles, which stick out from the tube
opening when the worm needs to feed. These are spirorbid bristle worms.
 The worm’s body is segmented and each segment has bristles, but there is no
hard tube. The head has tentacles, eyes and jaws to catch its prey. These are
free-living bristle worms.
 The body of the worm is not segmented, and is very stiff. There are no
bristles, and no white tube. This is a roundworm.
 The body of the worm is not segmented, and is not stiff. It is very flexible,
and there is no tube. These are very soft, slimy and sometimes very long
worms. There is no obvious head, but there may be eyes at the front. This is a
ribbon worm.
13. Animals that are not worm-like have no legs, and no shells:
 If solitary (single animals on their own), go to 14
 If colonial (animals that occur in colonies, sheets and mats), go to 15
Spirorbid
worm
Free-living
bristle worm
Round worm
Ribbon
worm
14. Solitary animals without shells.
Sea slug
 The animal has a soft body and a well-developed head with eyes and
tentacles. There may be finger-like gills on the back of the animal.
These are sea slug molluscs.
 The animal has a star-shaped body with 5 arms. These are brittle stars, and
Brittle
like starfish belong to the group Echinoderms.
star
15. Colonial animals: Colonial animals very often don’t look like animals at all. They
may resemble plants at first glance. Your demonstrator will be able to help you
identify them.
 If you find grey rough patches on the seaweed, you may have discovered
bryozoans. These ‘moss animals’ are formed of sheets that may be very
calcareous or more gelatinous. Under the microscope you should be able to
see numerous small ‘boxes’ within the sheet. Each box is an individual zooid.
 These animals are also easily passed over, mistaken for plants. They jut out
from the seaweed that they live on with branching, stalked growths.
Hydroids are otherwise known as ‘sea-firs’, and belong to the Cnidaria
group and so are related to sea anemones and jellyfish. Like anemones and
jellyfish, they have stinging cells to catch their prey, although they can’t
harm you.
 Sea squirts live in clumps of oval or rounded bodies, or in flat sheets with
individual ‘zooids’ arranged in lines or stars.
 Sponges occur in smooth sheets. The one you are likely to find is green or
yellow in colour.
Bryozoan
Hydroids
Sponge
Sea Squirt