Module 11

Question 1

Crustaceans

Answer 1

All have a hard shell that covers all or most of their bodies
- large amounts of minerals are deposited in their cuticle layers in their exoskeleton
Most are free-living and marine
- some are freshwater
-some are parasites

Question 2

Life cycle of crustaceans

Answer 2

Mainly produce through bisexual reproduction
Some groups show parthenogenesis
- males are rare and some are hermaphrodites
Many crustaceans carry their eggs in a sac or on the sides of their abdomen

Most crustaceans have an indirect development with a larval stage called nauplius
Some crustaceans have gradual change to the adult stage with some appendages and segments added over time
Some have a sudden change in morphology

Question 3

Crayfish life cycle

Answer 3

Direct development with no larval stage
- from the egg a tiny crayfish emerges and goes through multiple moultings until the adult stage

Question 4

Crustacean respiration

Answer 4

Crustaceans have to acquire oxygen from their aquatic environments

In small crustaceans:
Diffusion of gases through their cuticle
- Like insects, crustaceans have an open circulatory system where blood is not contained within vessels
- The blood is flowing very close to the integument and gas exchange is possibly in small crustaceans
- Having a large body surface to body mass further facilitates exchange of more gas relative to body size

In large crustaceans:
Respiration is mainly through the gills
- Dissolved oxygen in water is absorbed through the membrane of these feathered surface gills as water pass over them
- The oxygen is then passed onto the haemolymph which can carry oxygen to other parts of the body
- The molecules that create oxygen in crustaceans could be hemoglobin or haemocyanin

Question 5

Characteristics of crustaceans

Answer 5

Two pairs of antennae
Biramous appendages except the 1st pair of antennae (branched)
Most use gills for respiration

Question 6

Body plans of crustaceans

Answer 6

The more primitive body plan comprises of metamerisation of segments in two tagmata → the head and the trunk
- This consists of repeated segments without specialisation of segments or their appendages

In more derived body plans, there is the tendency to evolve towards a reduced number of segments and tagmatisation
- Cephalothorax and abdomen
- There is regionalisation of appendages for specific functions

Question 7

Remipedia body plan

Answer 7

Primitive
- Body is comprised of a series of repeated segments with similar shape and function
- No specialisation of appendages as they all have the same function

Question 8

Diversity of Class Crustacea

Answer 8

Sister group to Hexapoda

Question 9

Classes Remipedia and Cephalocarida

Answer 9

Both are hermaphrodite
Eject sperm and egg from the same duct

Question 10

Class Remipedia

Answer 10

Very small class
Mainly found in caves in connection with the sea
All the trunk segments have paired and biramous swimming appendages

Question 11

Class Cephalocarida

Answer 11

Small class
Live in bottom sediments
Thoracic appendages are very similar to each other → ancestral feature

Question 12

Class Branchiopoda

Answer 12

Primarily live in freshwater
Each thoracic appendage (coxa) is a large, flattened paddle that functions in gas exchange and locomotion
Filter feeders

Question 13

Class Copepoda

Answer 13

Mostly marine with some that live in freshwater
Comprise a huge biomass in marine and freshwaters as zooplanktons
Extremely important as they dominate the primary consumer level in aquatic environments

Free living with some that are parasitic species of other marine invertebrates and freshwater fish
- Some free-living copepods serve as intermediate hosts for parasites of humans as well as animals
Lack gills → respiration is through fusion of gases through the cuticle

Question 14

Class Branchirura

Answer 14

Fish lice
- Mainly fish ectoparasites with sucking mouthparts
- Infestation of fish with fish lice can cause lethargy, itchy skin, restlessness, and abnormal swimming patterns
5-10mm long with a broad shield-like carapace
Compound eyes
Four pairs of legs
Second maxillae have become modified as suction cups enabling the parasites to move on their fish host or even from fish to fish

Question 15

Class Thecostraca

Answer 15

Adult barnacles are usually enclosed in a shell of calcareous plates
- Sessile and attached to a substrate by a stalk or directly
- Their carapace surrounds their body and secretes a shell of calcareous plates
- Head is reduced and no abdomen

Thoracic legs are long many-jointed cirri with hair-like setae
- The cirri are extended through an opening between the calcareous plates to filter small particles on which the animal feeds

Often attach themselves to the bottoms of ships and in heavy infestations they may reduce the speed of the ship
Also seen on whales

Question 16

Class Thecostraca immature stage

Answer 16

In the immature stage, they are free living nauplius that turn into a cyprid larvae
- The cyprid attaches themselves to a substrate and begin to metamorphose, which involves several dramatic changes, including the secretion of calcareous plates, loss of eyes, and transformation of swimming appendages to cirri

Question 17

Class Malacostraca

Answer 17

Comprises about 75% of crustacean species

Order Decapoda → lobsters, crabs, prawn, and shrimp
- Five pairs of legs
- The first pair is usually modified to pincers to grab food

Question 18

Order Isopoda

Answer 18

The only order that contains species that have become completely terrestrial
Pill bugs
To adapt to terrestrial environments, their gills have evolved to lungs
- From dry to humid environments, the size of the lungs and the type of embedding into the body is reduced
- More complex lungs are covered or completely internalised with a larger surface area composed of tubules and wrinkles
- These lungs are highly specialised for extreme arid habitats
- Some species are aquatic as free living organisms or ectoparasites