Class Cirripedia

ARTHROPODA 

Class Malacostraca Box 2 

Superorder: PERACARIDA 

Isopoda 

Amphipoda 

Mysidacea not discussed 

Cumacea not discussed 

Spelaeogriphacea not discussed 

Tanaidacea not discussed 

Mictacea not discussed 

Class Cirripedia 

Superorder: THECOSTRACA 

Thoracica Barnacles 

Rhizocephala photo 

Ascothoracica not discussed 

Acrothoracica not discussed 

Facetotecta not discussed 

1

Index 

Getting started………………………………..……………………………………….Page 3 

Useful websites……...………………………………………………………………..Page 3 

Background Isopoda…………...…………………………………………………….Page 4 

Morphological structures of Isopoda…………………………….……....….….…..Page 5 

Key to morphological features Isopoda………………………...………………….Page 7 

Morphological features of Idotea ……………….………………………………...Page 11 

Background Amphipoda………………………….…………………………….…..Page 14 

Guide to Amphipod suborder identification……………………………..…….….Page 15 

Background Thoracica – Barnacles…………….…………………………………Page 18 

Morphological features of Barnacles……………………………………………...Page 19 

Photos of barnacles for identification….………………………………………….Page 21 

Glossary……………………………………………………………………………...Page 22 

List of species in the box…………….………………………..……………………Page 24 

2

Getting started 

In this box there are common shore crustaceans. Firstly, get a copy of Hayward & 

Ryland’s Handbook of the Marine Fauna of NorthWest Europe (from now called 

H&R) from shelf C4.3. Go through the identification key to the families (isopoda: pg 

340, amphipoda pg 361 and barnacles pg 305) and try to identify them first to family 

and then to species level. This booklet will help you by showing some of the key 

morphological features of the animals that are used in identification. H&R is a key 

that at each step of the identification process gives two options concerning one 

morphological feature. Therefore it is called a dichotomous key (dichotomous is a 

botanical expression meaning equal branching). Only one of the two descriptions 

can be right and then leads to the next question. This booklet shows both options of 

each question using drawings, photos or the sample animals, the first option is 

illustrated on the lefthand side, the second on the righthand side. Not all features can 

or need to be illustrated and are therefore omitted. 

While going through the key in H&R you will notice that there are two types of 

numbers: the numbers at the end of each option that stand alone are there to guide 

you to the next appropriate questions, while the numbers that are followed by a dot 

and a family name point you towards the family that the specimen belongs to. When 

you decide for an option where a family name is mentioned you then need to 

proceed to the key to that family. At the end of the key to families (not family key!) 

the section commences where each family has a key. These are numbered and in 

order to help orientation you can check the top of the right pages to see which 

families are discussed on this particular page. 

After you find the species name, compare the number on the mould with the species 

list provided at the end of this booklet to see if you identified it correctly. 

While studying taxonomy keep in mind that taxonomy is not a rigid science and 

different workers may use different taxonomic labels, ie Crustacea can make up a 

phylum or be part of the phylum Arthropoda. Also, names change continuously. In 

this booklet Gammarus marinus is affected. In H& R it is called Chaetogammarus m. 

on the web you will also find Echinogammarus marinus. 

Useful websites 

http://www.imv.uit.no/amphipod/links.htm, a website linking to websites related to 

amphipods 

http://www.vims.edu/tcs/, on the left‐hand column (‘Neat society stuff’) click on 

‘Crustacean resources’, then scroll down to ‘General crustacea information’, click 

on ‘Crustacean classification’ for general taxonomic info 

3

http://www.marlin.ac.uk/ for general information on British marine species, find 

decapods there 

http://www.glaucus.org.uk/Decapoda.htm also for general marine life around UK 

waters 

http://crustacea.net/ where you can download an interactive ID key, which will 

illustrate some key features of taxonomy 

http://crustacea.nhm.org/glossary/ a glossary for all the scientific terms used in 

crustacean ID, have a look but it is very extensive 

http://nlbif.eti.uva.nl/bis/crustacea.php a Dutch website introducing crustaceans 

of the North Sea with some very good descriptions, a key and a glossary ‐ worth 

checking out!! 

Background Isopoda 

Isopods are the most diverse in form and the most speciesrich crustaceans of the 

Peracarida. They occur in nearly all environments and marine isopods can be found 

from the intertidal to the deep sea. Approximately 10 000 species have been 

described in 10 suborders. These animals range in length from 0.5 mm to 500 mm 

(Bathynomus giganteus). Phylogenetic analyses and the fossil record (limited though 

it is) suggest that the group dates to at least the Carboniferous Period of the 

Paleozoic, approximately 300 million years ago. 

Isopods are dorsoventrally flattened crustaceans without a carapace. The body is 

divided into a cephalon (head), a pereon (thorax) with seven segments and a pleon 

(abdomen) with six segments. Figure 1 illustrates the key morphological features of a 

generalised isopod. If you study this drawing you will get familiar with the main 

distinguishing features which will help you later in the identification of isopods. 

Using this booklet and the specimens provided, try to identify the animals to species 

level. Use the glossary at the end of the booklet to look up terms mentioned in the 

identification process, else the glossary website mentioned above is an excellent 

source of information. Beware that sometimes different words can be used for the 

same anatomical feature, for example coxa and coxopodite. Where possible, in this 

booklet the scientific terms were used, making it harder but you will be rewarded for 

sticking with it when you get to identify animals on the seashore using books like 

Hayward & Ryland. 

4

Figure 1: A generalised isopod with the distinguishing features (from Kensley, B. and Schotte, M. (1989) 

Guide to the marine isopod crustaceans of the Caribbean. Smithonian Institution Press, Washington D.C. 

Arrows indicate where the removed parts to the right belong within the isopod. 

5

Figure 2: Drawing of a thoracic leg (From: Holthuis, L.B. 1993. The Recent genera of the caridean and 

stenopodidean shrimps (Crustacea, Decapoda): With an appendix on the order Amphionidacea. 

Nationaal Natuurhistorisch Museum Leiden. 328pp) 

6

ISOPODA 

Pg. 341 in Hayward & Ryland 

Option 1 Question 2 Option 2 

Pereopod 15 

Pereopod 17 

Ventral view of an isopod with 

5 pereopods, see CM3001! 

Ventral view of CM3004 

7

Question 3 

Uropods lateral 

Three distinctly 

different places 

for the uropods, 

See if you can 

identify this 

feature in all 

specimens 

provided and sort 

them 

accordingly! 

Uropods terminal 

Uropods ventral 

8


Check out the drawing at Figure 8.12 in the book, compare the 

flabelliferan isopod with the valviferan isopod because the drawings 

are more accurate than the pictures below. Divide your specimens into 

the two groups and follow the key further! 

Question 5 

Size does not matter, but do the legs 

look alike(ish)? 

Which of the species you have left has 

this particular feature? 

Ambulatory: can be used for walking. 

9

Question 69 are not discussed in this booklet 

Question 10 

If you are left with only one specimen, you did well! This 

specimen is considered terrestrial though it is found around 

HWM of rocky shores. 

Pleon segment 16 

Head of CM3007 

Flagellum with segments 

If you identified the last critter as a member of the Ligiidae family 

you did well and can continue with the right family key! 

10

Idotea 

Pg 349 

As there are several species of Idotea in this box we will continue with 

these to species level, and using the sources given you might want to 

identify the other specimens to species level too. You can check if you 

are right by looking at the species list on the end of the booklet! 


Body of an isopod, dorsal view 

Please refer to Fig. 8.23 

in H &R for the first option 

(Synisoma) 

Pleon with two distinct segments, one 

partially fused, the others completely 

fused with telson, see drawing below! 

11


While in Question 1 we were asked if there is a segmentation at all or 

not, now we are interested in the number of segments of the pleon, or 

pleotelson. 

Body of an isopod, dorsal view 

Pleon with two distinct segments 

and one partial suture 

Question 4 

Pleotelson: 4 possible shapes of the apical border 

Apical border 

Straight Concave Produced Angulate 

Continue with question 7! 

12

Question 5+6 omitted 


Check the drawing of Idotea 

baltica in Figure 8.22 in the 

book 

The sides of the telson are 

rounded as in the drawing 

of Idotea emarginata, Fig. 

8.22 

Question 8 

Pleotelson slightl 

convcave (marked 

in red with an 

acute median 

process(=tip in the 

middle) 

Pleotelson with 

convex apical 

margin (marked 

in red) 

Antennule 

Question 9 

Peduncle 

Flagellum 

Antennule and antenna, the 

peduncle of the antennule has 3 

articles, while the antenna 

peduncle has 5. See also 

Figure 1 on page 5 in this 

booklet.! 

Antenna 

Have you managed to identify all specimens correctly? 

Well done! 

13

Background Amphipoda 

Amphipods and isopods have many characters in common such as the lack of a 

carapace, however generally the body of an amphipod is laterally flattened while that 

of an isopod is dorsoventrally flattened. Before you continue reading open H&R to 

page 363 and study the drawing of a generalised amphipod on the top of the page. 

Note the taxonomic terms used and where they occur. This will help you when you 

then turn your attention to the key and the specimens provided in the box. 

The body of an amphipod is divided into three regions: the head, the pereon (thorax) 

and pleon (abdomen) with a telson attached to the last pleon segment. The head 

bears six pairs of appendages: antenna 1 and antenna 2, mandibles, maxillae 1 and 

maxillae 2 as well as maxillipeds., the pleon has seven visible segments each with a 

pair of uniramous (lacking either the exopod or endopod) appendages, the 

eperopods, where pereopods 1 and 2 are modified as gnathopods. Of the seven 

externally visible thorax segments, the first two bear appendages known as 

gnathopods used in feeding and grasping. While the remaining segments of the 

thorax bear pereopods. In many amphipods the gnathopods are subchelate (see 

Figure 8.26 in H&R). 

Body forms vary greatly within the amphipods but the most remark divergence 

occurs in the Caprelllidea. They have long cylindrical bodies and a reduction in 

number and type of appendages. 

14

Examples of key 

morphological features 

Suborders of Amphipodapg 361 

Question 1 Option 1 

In Caprellids the pleon (abdomen) is usually no longer present and no 

pleopods exist,gnathopod 1 is attached to the head which is fused with 

the first segment of the thorax, no uropods either, the gills are paddle 

shaped appendages along the body.. 

15

Question 1 Option 2 

You can see a difference between thorax and 

abdomen, if you look at the coxa and the appendages 

of either part of the body 

Proceed to Question 2 

16


Hyperiidae: notice the large 

eye and rounded body. 

None of these groups will be identified to species level 

as this would be too much for this booklet. Still, a lot of 

research from this University is done on them and for 

this reason it is a good idea to learn to identify them. 

Three species are provided for that. 

17

Background Thoracica – Barnacles 

Barnacles are exclusively marine. They are so ubiquitous, especially on the upper 

reaches of most rocky shores, that they form a distinct greywhite line marking the 

upper limit of the eulittoral. They have three life stages, a planktonic, feeding 

nauplius larva, which metamorphoses into a shrimplike nonfeeding cyprid. The 

cyprid has one pair of antennae, with which it finds suitable habitat on a hard 

substrate to settle. They show a highly selective settlement behavior, and will choose 

a place close to adults of their species. A secretion from the antennae cements the 

cyprid to the rock where they then metamorphose within 24 hours into a juvenile. 

During spring brown cyprids of Semibalanus balanoides settle on the rocky shores 

around Britain, they are just visible to the naked eye. Chthamalus settle in the 

summer and early autumn. Their six thoracic biramous appendages are then used to 

filter particles out of the water. 

An important feature in identification are the calcareous plates that make up the 

sides of their bodies. These will – with time – grow together, making the plates 

difficult to distinguish. Keep that in mind when identifying barnacles on the seashore 

and with time you will get an eye for all the species. 

A: Distinct plates B: Plates grown together 

Before working with the key, turn your attention to Figure 8.6 on page 303 and 

study the two bottom drawings so you know the taxonomic terms for 

barnacles. There are no specimens in the box but you can use the photos in 

this booklet for identification. 

18

Thoracicapg 304 

Start at the key to Families 


One of 6 calcerous plates 

Rhizocephala, no clacereous 

plates, endoparasite on a 

decapod 

Question 2 

Capitulum 

(=head) 

Peduncle 

(=stalk) 

Operculum 

See how the barnacle is 

firmly attached to the 

substrate, lacking a stalk 

19

Question 3 omitted 


Verrucaridae, see also 

Fig 8.8 in H&R 

4 plates clearly 

visible in the 

center, making up 

the operculum 

Question 5 

Rostrum 

Rostrum 

You can now proceed to the respective family keys! 

20

Now, please identify the barnacles below: 

CM3009 

CM3012 

CM3010 

CM3013 

CM3011 

21

Glossary 

Aesthetasc Sensory part of the 

antenna and antennule 

Ambulatory Used for walking 

Antenna Mobile sensory appendages 

arising below the eyes at the front of 

the head, located outside the 

antennules 

Antennule Inner of two paired 

flagellate appendages projecting from 

anterior end of body 

Anterior Front end of organism or 

head end 

Appendage A general term for a body 

part attached to a body segment such 

as a leg or antenna 

Bifid A structure with two distal tips 

Biramous Having two branches or 

rami (exopod and endopod), some 

arthropod legs have two parts an inner 

and an outer branch 

Carapace A shield that covers the 

head and thorax of crustaceans where 

the thoracic segments have fused 

Chelate, chela An appendage in 

which the movable dactylus is 

opposed to an equally welldeveloped 

distal prolongation of the propodus, 

basically the ‘claw’ (Figure 2 depicts 

the parts of an arthropod leg) 

Cheliped The walking leg that bears 

the chela 

Concave Curving in or having a dent 

22 

Convex Curving or bulging outward 

Coxa First article of a limb as 

Figure 2 

Coxal plate The coxa is a part of the 

leg (see Fig. 2), in some isopods and 

amphipods this is expanded to form a 

lateral plate 

Cuticle Forms the exoskeleton 

Distal Further from the body, ie in the 

human leg the toe is distal while the 

knee is more proximal 

Dorsal Top of an animal 

Flagellum A long, multisegmented 

sensory appendage found attached to 

the peduncle of the antennae and 

antennules 

Flagellate Resembling a flagellum, 

whiplike extension of the antenna 

Lateral The sides of an animal 

Maxilla One of two paired appendages 

in mouth region, posterior to (above in 

ventral view) mandible 

Maxilliped Mouth appendage, there 

are three pairs of maxillipeds plus 3 

pairs of other mouth appendages 

Maxilliped, 3 rd the outermost mouth 

appendage in crustaceans and used in 

identification in some crustacean 

groups 

Median lobe The middle lobe of the 

frontal region of the carapace (mainly 

in decapods) 

Peduncle 1. Proximal (close to the 

body) division of antennule or antenna.

Typically consists of three segments in 

antennule, five segments in antenna 

2. Stalk supporting the head 

(capitulum) of lepadomorph barnacles 

3. Sometimes used for the eye stalk in 

shrimp 

Pereopod, periopod an appendage of 

the thorax used for movement, 

feeding, and defense 

Pereon Thorax 

Pleon Abdominal region of the body, 

primitively consisting of six segments 

or pleonites, and bearing paired 

pleopod and uropod appendages 

Pleopod Appendages attached to the 

abdominal segments 

Pleotelson Structure formed by fusion 

of one or more abdominal somites with 

telson, as in most isopods 

Pleura Lateral plates forming a ring 

with the sternum and the tergum 

around decapods’ abdomen 

Proximal Closer to the body, eg in the 

human leg the knee is proximal while 

the toe is distal 

Quadrangular Shaped like a square 

or diamond 

Sclerite Articulated plates around the 

abdomen of decapods 

Seta A hair arising from the outside of 

the exoskeleton 

Somite Single body segment, 

especially of the abdomen 

Subchelate The last segment of an 

appendage closes against the 

preceding segment (see Fig. 8.26 in 

H&R) 

Suture a junction between two hard 

parts of the exoskeleton 

Tergum Dorsal (top) plate of the 

abdomen of crustaceans 

Tridentate Possessing 3 teeth or 

prongs 

Uniramous An appendage having 

only one branch, so either lacking the 

endopod or the exopod (see biramous) 

Uropod Part of tail fan; a paired 

biramous (having two parts) 

appendage borne on the sixth 

abdominal somite (body segment) 

Ventral Underside of an animal 

Vestigial A body part that is small, 

degenerated or not fully developed 

23

Phylum Crustacea Class Malacostraca Box 2: CM300113 

Mould number……………………….Family………………………………………Species 

CM3001 Gnathiidae Gnathia sp. 

(May only be present in one box) 

CM3002 Idoteidae Idotea pelagica 

CM3003 Idoteidae Idotea baltica 

CM3004 Idoteidae Idotea granulosa 

CM3005 Sphaeromatidae Dynamene bidentata 

CM3006 Corophiidae Corophium volutator 

CM3007 Ligiidae Ligia oceanica 

CM3008 Gammaridae Gammarus marinus 

CM3009 Chthamalidae Chthamalus montagui 

CM3010 Balanidae Elminius modestus 

CM3011 Balanidae Semibalanus balanoides 

CM3012 Balanidae Balanus balanus 

CM3013 Chthamalidae Chthamalus stellatus 

24

Class Cirripedia

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