Gaseous exchange

Question 1

Gaseous exchange vs respiration

Answer 1

Gas exchange is the uptake of oxygen (O2) from the surrounding environment and the release of carbon dioxide (CO2) which is a metabolic waste product of respiration. Gas exchange of O2 and CO2 in invertebrates typically occurs across the integument of the animal or via specialized structures.
Respiration is the energy-producing metabolic activities within cells (also referred to “cellular (internal) respiration”).

One example of a biochemical reaction involved in metabolism is when large molecules are broken down into small ones (also called a catabolic reaction):
The substrate (e.g. glucose) is broken down, which requires 6 molecules of oxygen O2 and several enzymes. The reaction releases 6 molecules of carbon dioxide (CO2) and 6 molecules of water (H2O), and generates energy in the process.Metabolism is often estimated by respiratory rates (i.e., rates of oxygen uptake and/or carbon dioxide release)

Question 2

What is metabolism?

Answer 2

Metabolism consists of “all of the biochemical reactions involved in the uptake & use of energy & materials by organisms”

Question 3

What are the gaseous exchange surfaces?

Answer 3

Skin / integument
Gills
Lungs
Trachea

Question 4

How does GE occur over the skin?

Answer 4

In this case, the invertebrate will use the skin or integument as the structure for gas exchange and will typically lack other specialized structures.

Some examples are seaspiders (Order: Pantopoda), micro-scorpions (Order: Palpigradi) and cnidarians (Phylum: Cnidaria).

Gas exchange occurs across most of the animal’s skin or integument which means that it will have a propensity to lose water.

Question 5

How are invertebrates adapted to prevent water loss when GE occurs over skin?

Answer 5

• small body size - which means that the organism will require less oxygen and therefore, less gas exchange demand and therefore less water loss.
• large body size, especially in soft-bodied forms such as cnidarians and flatworms. The large body size means that they have
• low surface-to-volume ratios and therefore, they lose water at a lower rate per unit volume compared to smaller forms.
  Most inhabit aquatic habitats or damp terrestrial environments.

In some cases, the skin/integument gas exchange route supplements other modes of gas exchange (this is the case for some insects and mites)

Question 6

What is the tracheal system?

Answer 6

open respiratory system composing of spiracles, trachea and tracheoles.

Question 7

What are spiracles?

Answer 7

Openings of the tracheae onto the body surface, there are closable and non-closable spiracles - depending on the organism.

Question 8

What is a trachea?

Answer 8

tube-like structure that cinveys air from external environment to tissues and vice-versa. They are invaginations of the cuticular exoskeleton and extend towards multiple tracheoles.

Question 9

What are tracheoles?

Answer 9

the smallest branches of the tracheal system where the transfer and diffusion of oxygen occurs into and out of the tissue and cells.

Question 10

Evolution and natural history of Onychophora.

Answer 10

Cambrian fossils (~500Ma) named Xenusians are thought to be the ancestors of Onychophorans - most likely have a marine origin

The earliest terrestrial ancestor of Onychophorans originated in the upper carboniferous (~300Ma)
Fossil comparison shows huge similarity. Thus, we suspect that at least some of their morphology has been conserved through evolutionary history.

Terrestrial in moist habitats (leaf litter & rotten logs), typically in forests
1 pair of antennae, 2 eyes with spherical lenses
13 to 43 pairs of legs (also called ‘oncopods’) depending on the species
Their thin cuticle has numerous sensory papillae (hence the ‘velvet’ in their popular name: velvet worm)
They have a malleable body and use a hydrostatic muscle skeleton using pressurized fluid in the body to extend and contract their body and legs.

Question 11

Onychophora nad gaseous exchange.

Answer 11

• Onychophorans use multiple non-closable spiracles to exchange O2 and CO2 with the environment.
• Known as “continuous” gas exchange which is ancestral to arthropods.

Animals showed behavioural changes across time which altered gas exchange.

Question 12

How can we measure this gaseous exchange?

Answer 12

The insert picture in A shows the individual in curling position, and in B, pictures show several postures: curled and uncurled body positions. When the animal is uncurled, the traces transitioned into continuous gas exchange (you can see that CO2 (black line) and H2O (blue line) were more random during these periods).

lecture 5 slide 16

Question 13

Did these velvet worms use curling behavior to save water?

Answer 13

This figure shows the proportion of individuals with each gas exchange pattern type across temperature trials (5, 10, 15, 20 and 25°C) and 0% relative humidity:
- Downregulated traces (= curling behaviour) were more frequent at 5°C while
- continuous traces were more frequent at 25°C (5x3
- contingency test, G = 22.6, P<0.05)
- The opposite to our prediction!
- Therefore, P. capensis does not use curling behaviour to conserve water.

slide 17

Question 14

Insects: ventilatory systems

Answer 14

open and closed

Question 15

What is an open ventilatory system?

Answer 15

Spiracles are found dorsally and/or laterally, e.g. grasshopper (picture below).
Some insects just have two posterior spiracles, e.g. mosquito larvaW

Question 16

What is a closed ventilatory system?

Answer 16

• The insect has trachea but no spiracles as in aquatic larvae; e.g Mayfly larvae

Question 17

What is passive ventilation?

Answer 17

Works with simple diffusion and is driven by gas concentration gradients
Requires open spiracles
Follows the Fick’s law of diffusion (the spiracle is represented as a small cylindrical tube in the formula below)
J = A x D x ΔC/L where,
J (mol.s-1) = the diffusion flux of the gas,
A = area (mm2) of the cross section of the tube
D = diffusion constant (m2.s-1)
ΔC = concentration gradient between inside and outside of the tube or spiracle
L= length of the tube or spiracle

Question 18

What is passive ventilation?

Answer 18

Requires rhythmic pumping actions, facilitated by muscles
Needs elasticity of the insect cuticle

Question 19

What is continous GE?

Answer 19

primarily driven by diffusion and convection and spiracles are open (fig. 3.6)

Question 20

What next after discontinous GE?

Answer 20

Perhaps DGC evolved to save energy and then came with bonus of water saving? To resolve this question, researchers need to understand trade-offs between e.g. costs of living, saving energy and water and design experiments that tests for these.

Perhaps different insect taxa have DGC for different purposes? To resolve this question, we need to measure many more species and test the functional role of DGC for each one. For example, to test the hygric hypothesis, we would examine if species with DGC inhabit arid areas.

Question 21

What is cyclic GE?

Answer 21

continuous trace of CO2 production with some cyclic variation (and random noise) (Fig. 3.8)

Question 22

What is discontinuous GE?

Answer 22

During DGE, oxygen (O2) uptake and carbon dioxide (CO2) release from the whole insect follow a cyclical pattern characterized by periods of little to no release of CO2 to the external environment (Fig. 3.7).

Perhaps an explanation of the function or role of discontinuous gas exchange (DGC) in insects: closing spiracles would mean a lower water loss.

Question 23

What are 4 adaptions of aquatic insects?

Answer 23

a. Cuticular gas exchange
b. Tracheal and cuticular gills
c. Breathing tubes and siphons
d. Plastrons and air bubbles

Question 24

What is Cuticular GE?

Answer 24

Some aquatic insects have very thin integument that allows diffusion of gasses through the body wall
This mechanism is only found in small, inactive insects or those living in highly oxygenated environments
An example of a highly oxygenated environment is a cold, fast moving stream. In this habitat, the animal’s O2 demand is less than the O2 availability in water.

Question 25

What is tracheal and cuticular gills?

Answer 25

Gills consist of an organ that allows dissolved oxygen from the water to diffuse into an organism’s body
In insects, gills are usually extensions of the tracheal system and are covered by a thin cuticle that is permeable to O2 and CO2

Question 26

What is breathing tubes and siphons?

Answer 26

Some aquatic insects intake air straight from the water surface through a hollow breathing tube (also calledsiphons)
Examples: A: mosquito larvae, B: water scorpions (Hemiptera: Nepidae), C: rat-tailed maggots (Diptera).

Question 27

What is plastron and air bubbles?

Answer 27

A plastron is special array of rigid, closely spaced hydrophobic hairs that creates an airspace next to the body.
The air gets trapped between these hairs and operates as a physical gill (the insect obtains oxygen from the trapped air – just like having your own diving O2 tank!).
The volume of air trapped in the plastron does not change during gas exchange: the O2 is consumed which lowers the partial pressure inside the plastron but it is quickly compensated by dissolved O2 from water that enters the plastron.
Nitrogen (N) tends to diffuse out of the plastron and there is little N in water (low solubility). This pressure deficit is also “corrected” by O2 uptake from the water.
Plastrons are found in aquatic insects that are permanently submerged (for e.g. in A, riffle beetles, family Elmidae) or those that lack the ability to reach the surface (eggs of floodwater mosquitoes). Fig. B provides several other examples.
Some aquatic insects can take an air bubble with them when they dive
The bubble is typically trapped under the elytra (wing covers) or against the body with specialized hairs
The air bubble covers one or more spiracles and the insect can “breathe” from the air in the bubble
When O2 in the bubble is consumed, its volume changes: this is the major distinction from a plastron.
An air bubble is a limited ‘physical gill’: there is passive diffusion of O2 from the water into the bubble but in the long run, the oxygen uptake cannot match the O2 demand and the bubble’s volume decreases as O2 is depleted.
The picture shows diving beetles with their respective air bubbles when diving in pond water.

Question 28

What are gills?

Answer 28

Some small species such as copepods, exchange respiratory gas through their thin integument but most crustaceans have gills (within thoracic cavity or appendages)
Gills are feathered surfaces that contain membranes that bind dissolved oxygen in the water when water passes over these structures.

The circulatory system of crustaceans contains hemolymph (fluid) that transports oxygen as follows: from the heart, the hemolymph is transported to the dorsal vessel towards organs that need oxygen and then to the sternal sinus and gills where it picks up oxygen, it is then transported to the pericardial sinus and back to the ostium in the heart.
In the hemolymph of larger species, oxygen is transported by respiratory pigments (e.g. hemocyanin)

Can be internal or external

Question 29

What are book lungs?

Answer 29

Book lungs are made of slits that are open in the ventral abdomen and a chamber (atrium) expands to many leaf-like air pockets (Fig. A). A very thin cellular layer allows gas exchange by simple diffusion as hemolymph circulates through the book lungs (Fig. B).
The folds of the ‘book’ increase the surface area exposed to air and therefore it optimizes oxygen uptake and gas exchange

Question 30

What is special about the diving bell spider?

Answer 30

Argyroneta aquatica is an air-breathing spider that lives permanently under freshwater. It is able to do so by creating a bell with its web attached across aquatic plants. The spider fills the bell with air from the surface which is transported and held with the abdomen and fine hairs on the rear legs. The diving bell takes a role of a ‘physical gill’ by maintaining and providing oxygen to the spiders for long periods. As it gets depleted, spiders to return to the surface and carry large air bubbles to replenish the diving bell. need

Question 31

What are book gills?

Answer 31

book lungs could have been derived from book gills on going research about this.