Class Malacostraca Box 1

Class Malacostraca Box 1 

Superorder: EUCARIDA 

Decapoda – crabs, crayfishes, lobster, shrimp, prawns 

Euphausiacea – Krill and relatives (not discussed) 

Amphionidacea – marine plankton group, (not discussed) 

1

Index 

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

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

Key to morphological features Reptantia……………………………………...Page 6 

Key to morphological features of Portunidae………………………………….Page 14 

Key to morphological features Natantia……………………………………….Page 16 

Table with the name and function of the appendages in Carcinus maenas.Page 18 

Glossary…………………………………………………………………………...Page 19 

List of the moulded species in the box…………………………………………Page 21 

2

Useful websites – these can be accessed via the virtual box (see signs 

around the lab for info on how to access this) 

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

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

‘Crustacean classification’ for general taxonomic info 

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://www.bio.davidson.edu/courses/invertzool/lab/ex8crayfish.pdf is a practical from 

another university which provides good background if you want to study some further 

crustacean anatomy 

3

Background 

The Decapoda is a diverse group within the Malacostraca. Decapoda (tenlegged) 

refers to the 10 thoracic appendages present. 

In order to be able to identify decapods, it is necessary to know the taxonomic terms. 

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 (H&R). 

To get a general idea of what all the legs/appendages are used for by crabs, as an 

example of a decapod, please refer to the page at the end of booklet. 

Frequently the order is divided into two suborders, the swimming Natantia and the 

walking Reptantia, however this distinction is taxonomically difficult as it does omit 

the fact that there are swimming Reptantia and walking Natantia. The Reptantia 

include the lobsters and crayfish, the Anomura (porcelain crabs, squat lobsters and 

hermit crabs) and the Brachyura or true crabs. The Natantia include shrimp. 

Hayward and Ryland lay out their key according to this, which means the user has to 

know into which of these groups a specimen belongs. Specimens with an elongated 

body shape which do not possess a massive chelate are usually prawns. There is no 

taxonomic difference between the terms shrimp and prawn, often the term prawn is 

used for the larger species however this may differ by country and region. 

Getting started 

In this box there are common shore Reptantia. After getting a copy of H&R from 

shelf C4.3, please go through the identification key to the families (pg. 4089 and 

42730) try to identify them first to family then to species level, (only one group can 

be identified to species level with the help of this booklet.) This booklet will help you 

by showing you some of the key morphological features of the animals that are used 

in H&R. 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 drawing, 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 illustrated and are therefore omitted. 

4

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 found the species name, compare the number at the bottom of the mould 

with the species list provided at the end of this booklet to see if you identified 

correctly. 

GOOD LUCK!!!! 

5

Examples of key morphological features 

REPTANTIA (page 427) 

Option 1 Option 2 

Note the well developed 

tailfan in the picture 

Question 1 

Long +curved 

assymetrically as in 

the picture or short, 

look at CM2001 and 

CM2004 

Pereopod 14 Pereopod 5 

Question 2 

Is the last walking leg 

comparable in size to 

the other three? Also 

see CM2007 for 

comparison 

Consult CM2004! 

6

Questions 37 omitted 

Question 8 


Look at CM2002 

This abdomen is modified for 

occupying a gastropod shell, the 

uropod and the last two 

pereopods are modified in such 

a way that they can hold on to 

the shell. 

Proceed to the key for the 

right Family 

Uropod 

Pleura 

Tergum 

Using CM2005 and 

CM2006 see if you can 

identify the structures 

on the abdomen as 

outlined in this image of 

the underside of a 

porcellain crab: 

7

Option 1 Question 9 omitted Option 2 

Question 10 

Compare CM2004 with CM2005 and CM2006, which one(s) will fit 

the description better? 

Proceed to the family key for further identification! 

Question 11 continued from Question 2 

Maxillipeds? Check the glossary in the back of the book to see what that is 

and the table at the end of the book to see what crustaceans use it for. Here 

is a picture showing where they are located – notice their shape! 

Antenna 

Antennule 

Eye 

Third maxilliped 

8

Option 1 

Question 11 continued 

Option 2 

CM2001 is a good 

example for a square 

maxilliped, look at the 

flap over their mouth 

An example of a triangular 3 rd maxilliped 

Question 12 omitted 

Question 13 

See also 

CM2008 and 

pg 4489 in 

Hayward for 

drawings 

CM2001 and CM2007 are 

good examples of this option 

Carapace 

outline 

9


Question 14 

Corystes 

cassivelaunus 

CM2001 is an 

example of 

Option 2 

Carapace outline 

Question 15 

First some drawings to brush up your anatomical 

knowledge: the appendages are sectioned into these 

parts: 

10


Outer 

dorsal 

margin 

Center 

dorsal 

margin 

Inner 

dorsal 

margin 

Carpus 

Merus 

Ischium 

Basis 

3. Right maxililiped of 

Pachygrapsus marmoratus 

3. Right maxililiped of Necora puber 

11


Question 16 

Compare the last leg of CM2007 with CM2001 

then proceed to the respective key/question! 

To find out which part of the leg is the dactylus, 

look at the top drawing for Question 15. 

Question 17 

Circular 

Hexagonal 

Square 

Hairy 

or not? 

Oval 

12


Question 18 

The median lobe is located 

between the eyes 

The median lobe is the middle lobe in 

the frontal region, no matter how 

many others are there beside it 

Take CM2001 as an 

example for a single 

median lobe! 

Carapace 

Pair of median lobes 

Eyes 

8 

Question 19 

Compare CM2007 with CM2001! 

Now turn your attention to the suitable family key! 

You have come very far by now, you will manage the 

rest in no time! 

13

Portunidae: key morphological features 

Use with Hayward and Ryland 


Question 1 

Dactyl? Pereopod? Come again? Check the below 

drawing or the glossary! 

Pereopod 24 

The crab in this picture is 

a good example for 

Option 2 

14

Question 2 omitted 


Question 3 

Look at CM 2007 as an 

example of Option 2. This 

paddlelike dactyl helps in 

swimmingContinue to 

Question 6 

Questions 4+5 omitted 

Question 6 

Teeth at front of carapace 

Front of Necora puber, check also the specimen CM2007. 

How many teeth does it have? 

15

NATANTIA‐pg 408 

Option 1 Step 1 Option 2 

Left Pereopod 1‐ chelate 

Right Pereopod 1‐ subchelate 

Step 2 

In this specimen of Option 1 

you can see how slim the 

‘fingers’ of the chelae are 

Left Pereopod 1 of a 

Palaemonidae, fingers are 

hairy but not serrated 

16

Step 3 

Chela 

Check Fig. 8.41 center Left drawing 

on pg 407 in H&R for this feature 

Animal 

Carpus of left Pereopod 2 

Check also CM2009 for this feature! 

To study each family in‐depth it is better to use freshly 

caught animals and a means of magnification! 

Well done so far! 

17

This table is taken from Crothers, J. and Crothers, M. (1987): A key to the crabs and 

crablike animals of British inshore waters, Field Studies Council, Taunton 

18

Glossary 

Anterior Front end of organism or 

head end 

Bifid a structure with two distal tips 

Biramous Having two branches or 

rami (exopodite and endopodite). 

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 

Cheliped the walking leg that bears 

the chela 

Dorsal Top of an animal 

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, see 

the table on Page 19 

Maxilliped, 3 rd the outermost mouth 

appendage in crustaceans and used in 

identification in some crustaceans 

Median lobe The middle lobe of the 

frontal region of the carapace 

Pereopod, periopod a thoraric 

appendage used for movement, 

feeding, and defense 

Pleura lateral plates forming a ring 

with the sternum and the tergum 

around decapods’ abdomen 

Quadrangular shaped like a square or 

diamond 

Sclerite Articulated plates around the 

abdomen of decapods 

Seta A cuticular hair arising from the 

outside of the exoskeleton 

Somite Body segment, especially of 

the abdomen 

Tergum Dorsal plate of the abdomen 

of crustaceans 

Uropod Part of tail fan; a paired 

biramous appendage borne on the 

sixth abdominal somite 

Ventral underside 

19

Phylum Crustacea Class Malacostraca Box 1: CM200110 

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

CM2001 Cancridae Cancer pagurus 

CM2002 Paguridae Pagurus prideauxi 

CM2003 Palaemonidae Paleamon longirostris 

CM2004 Galatheidae Galathea squamifera 

CM2005 Porcellanidae Pisidia longicornis 

CM2006 Porcellanidae Porcellana platycheles 

CM2007 Portunidae Necora puber 

CM2008 Majidae Macropodia sp 

CM2009 Palaemonidae Palaemon serratus 

CM2010 Crangonidae Crangon crangon 

20

Class Malacostraca Box 1

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