A Closer Look at Arthropod Features

Question 1

What is the exoskeleton of arthropods made up of?

Answer 1

Chitin

Question 2

How do increases in body size occur in arthropods?

Answer 2

Through ecdysis (moulting), occurs periodically when the exoskeleton is shed in a stepwise pattern

Question 3

What are periods between moults of exoskeleton called?

Answer 3

Instars

Question 4

What are the two parts of the procuticle of the exoskeleton?

Answer 4

Endocuticle and exocuticle

Question 5

What part of the exoskeleton is re-absorbed at moulting?

Answer 5

Endocuticle

Question 6

What is the exocuticle strengthened by?

Answer 6

Biomineralisation and cross-linking of proteins

Question 7

Where do arthropods not have exoskeleton and why?

Answer 7

At the joints as these need to be flexible

Question 8

What is the outer section of the exoskeleton called?

Answer 8

The epicuticle

Question 9

What is a key characteristic of the outer epicuticle in terrestrial athropods?

Answer 9

Waxy epicuticle

Question 10

Explain the moulting process - ecdysis

Answer 10

1. Intermoult stage - the epidermis starts to separate from the old cuticle
2. Apolysis - epidermis separates from the old cuticle
3. A new epicuticle is secreted from the epidermis to protect it from digestive enzymes
4. Fluid is secreted from the epidermis to fill the space between the two layers
5. Epidermal cells divide to prepare for increase in size
6. New cuticle is secreted by the epidermal cells
7. Enzymes in the moulting fluid are activated and reabsorb the endocuticle
8. The old exocuticle splits due to fluid pressure
9. Animal sheds old cuticle and new starts to harden

Question 11

Are crustaceans all free-living?

Answer 11

Majority are free-living, but many sessile, commensal and parasitic species

Question 12

What is a paraphyletic group?

Answer 12

A group that includes a single common ancestor, and only some of its descendent lineages

Question 13

What is a monophyletic group?

Answer 13

A group that consists of a single common ancestor and all of its descendent lineages.

Question 14

What is a polyphyletic group?

Answer 14

A group that has shared characteristics that have evolved in different lineages and do not have a common ancestor.

Question 15

What is included in Pancrustacea?

Answer 15

Crustacea and Hexapoda

Question 16

Is Crustacea a monophyletic, paraphyletic or polyphyletic group?

Answer 16

Paraphyletic

Question 17

Is Pancrustacea a monophyletic, paraphyletic or polyphyletic group?

Answer 17

Monophyletic

Question 18

What are the definitive features of the external morphology of crustaceans?

Answer 18

• Fused head and thorax
• Elongated abdomen (referred to as carapace)
• Long antenna to sense differences in chemicals in vast body of water
• Range of walking limbs
• Biramous limbs (2 parts) - mouthparts are also modified limbs
• Mouth situated underneath body

Question 19

What is special about the leg morphology of crustaceans?

Answer 19

Show remarkable specialisation for a range of tasks.

Question 20

How is there a high diversity of crustaceans successful in different environments?

Answer 20

Due to modified, biramous limbs

Question 21

What kind of legs did ancestral arthropods likely have?

Answer 21

Biramous legs with limited specialisation, although uniramous legs thought to gave evolved multiple times

Question 22

What is leg development controlled by?

Answer 22

Hox-genes

Question 23

Describe the internal morphology of crustaceans.

Answer 23

• Range of glands at front of mouth to detect chemicals in water, such as for prey or mating
• Antennal gland linked closely with the bladder
• Large stomach behing head
• Malphigian tubules allow waste to be excreted while retaining water and salt in haemolymph
• Have internal gills - water taken up between jointer regions of gills and pushed over mouth using modified appendanges
• Similar to book lungs
• Store water within their exoskeleton, meaning can breathe on land

Question 24

What are the evolutionary trends within crustaceans?

Answer 24

• Specialisation of biramous limbs
• Shortening of body, linking with development of the carapace - means more robust and less susceptible to damage (hardened carapace beneficial for water current)
• Development of the carapace
• Calcification and pigmentation - for hiding in marine environment or to display certain characteristics (e.g. defence)

Question 25

What is the carapace?

Answer 25

The carapace is part of the exoskeleton of crustaceans, covering the fused head and thorax

Question 26

What are the advantages of the carapace?

Answer 26

Hydrodynamics - streamlining and reduces drag. Allows for association with the substrate without being pulled off and increases swimming speed and/or makes it less costly.
Feeding - sets up a feeding current for species such as branchipods as limbs create a vacuum in the internal cavity to allow water and particles to be drawn in.
Respiratory chamber - location of the gills within the carapace allows water to be retained meaning they can be on land
- Air based gas exchange also takes place above the gills in air-filled cavities lined with vascular epithelium
- Brooding - protection of offspring, retains eggs. For examples in branchipods such as clam shrimps and daphnia eggs held in place with biramous female appendages