Ecdysozoa Nematodes & Pan-arthropods

Question 1

Ecdysozoa – Key Features

Answer 1

● Distinctive exoskeleton
● All have an external cuticle that requires shedding in order to grow (moult, called ecdysis)
● Embryos undergo radial cleavage (unlike the sister group, lophotrochozoa

Question 2

Caenorhabditis elegans – The world’s most famous nematode

Answer 2

● Model organism in developmental biology (and beyond)
● Simple, transparent, easily manipulated
● Development of every single cell has been mapped out
● To date > 4.5 million scientific papers published on this worm C. elegans – are ecdysozoans, and go
through repeated moults/ecdysis

Question 3

All Ecdysozoa

Answer 3

Nematoda>18,500 species
Tardigrades~1,300 species
Onychophora 218 described species
Tardigrades and Onychophora form the pan-arthropods

Question 4

Arthropod Cuticle

Answer 4

• Thin outer layer called epicuticle
• Underneath this is the procuticle, made up of the exo-and endocuticle
  The cuticle + epidermis together are the ‘integument’

Question 5

Epicuticle basics

Answer 5

Covered by hydrophobic liquid or wax layer (cuticular hydrocarbons) in terrestrial groups
It reduces dehydration in terrestrial groups and is also used for signalling/recognition

Question 6

Procuticle basics

Answer 6

Mostly chitin (a polysaccharide).
Sclerotisation strengthens chitin in the exocuticle with cross-linking proteins. Scerotised cuticle is often darker in appearance.
Crustacea use biomineralisation (depositing calcium carbonate into the chitin) to provide strength and weight.

Question 7

How can arthropods with a hard cuticle grow?

Answer 7

● Moulting or Ecdysis
The epidermis starts mitotic division and produces enzymes at the junction
of the epidermis and endocuticle.
Disolved endocuticle is reabsorbed (up to
90% of chitin and protein recycled)
Old exocuticle is shed (splits along fault lines)
New exocuticle is scerotised, and new
endocuticle grown

Question 8

Arthropod musculature basics:

Answer 8

● Striated muscle bundles
● These are attached to the epidermis which is attached to hard cuticle by microtubules.
● Inward folding parts of the cuticle called
apodemes provide more attachment area for
muscles

Question 9

Arthropod exoskeleton basics

Answer 9

● The cuticle is divided into
separate plates
● The plates are connected by articular membranes (thin & flexible)
● Many arthropods also have articular condyles (=joints) & sockets
↳ Shows convergence with
vertebrate skeletal structures

Question 10

Arthropods hollow exoskeletal structure

Answer 10

● Large surface area for muscle attachment
● Rigid, strong, tubular shape
● Exoskeleton investment does not scale isometrically
● But after moulting, the soft skeleton would buckle if the animal is too large.
↳ Limits size of arthropods,
terrestrial ones in particular

Question 11

Arthropod movement (exoskeleton)

Answer 11

● Locomotion uses muscle contraction with exoskeleton providing attachment, levers &
joints (see earlier lectures on biomechanics; trade off between force, speed etc…)
● Muscles typically work in pairs: flexors & extensors (antagonistic action)
● Increase in blood pressure can extend plates in thorax / abdomen
● Muscle fibre arrangements can maximise cross-sectional area

Question 12

The arthropod exoskeleton encloses a liquid-filled body cavity, the haemocoel:

Answer 12

● Acts as a non-compressible hydrostatic skeleton in soft-bodied inverts, often used for locomotion
● Arthropod hard, articulated exoskeletons likely reduce the need for a hydrostatic skeleton
↳ Reduced haemocoel in
arthropods

Question 13

Arthropod terrestrialisation

Answer 13

Arthropods have repeatedly undergone terrestrialisation;
● Molecular and fossil evidence dates this to around the Cambrian, potentially early Cambrian for myriapods (potentially before fungi!)
● Arthropod exoskeletons likely important in terrestrialisation (slowing dessication, supporting movement and structural support in air)

Question 14

Arthropod sensilla (sense organs)

Answer 14

● The exoskeleton prevents the detection of stimuli
● Sensilla have chemo- or mechano- receptors that
detect external stimuli
↳ The most common forms are hairs, bristles and seta but there are also pegs, pits & slits

Question 15

3 main types of arthropod eyes

Answer 15

Camera eye, Superposition compound eye, Apposition compound eye.
Apposition - most common, performs well, though has limited spatial resolution.
Superposition are amazing for low light e.g, hawkmoths can see colour under starlight.
Camera are required for high spatial resolution.