Breath holding marine vertebrates

Question 1

Saltwater crocodiles use breath holding for …

Answer 1

• predator avoidance (particularly for juveniles)
• foraging / hunting
• social interaction (mating)

Question 2

How long can large individuals hold their breath for (crocs)

Answer 2

• Large individuals (>1000 kg) may be able to breath-hold for 2h (at resting metabolic rate at 25 ˚C
• Active-swimming dives are much shorter

Question 3

how do crocodiles show dive response

Answer 3

• brachycardia + peripheral vasoconstriction → lower oxygen demand

Question 4

Crocodile cardio + resp system

Answer 4

• Crocodiles use lungs as O2 store, CO2 sink, and to regulate buoyancy
• the most complex hearts of all vertebrates:
• may redistribute flow to brain & heart in dives (Axelsson et al., 1996) -> BUT whether this actually occurs is uncertain (Eme et al., 2007)

Question 5

Marine iguana (Amblyrhynchus cristatus; Galapagos iguana) feeding

Answer 5

• Feeds primarily on marine algae
• Smaller juveniles forage in intertidal;
  larger adults dive subtidally

Question 6

Marine iguana swimming

Answer 6

• Swims by body undulation- costs are higher to propel small animal through surf and surge

Question 7

Large individuals foraging (iguana)

Answer 7

Large individuals forage offshore for >45 mins
but many short dives
* Max dive depth ~30 m and most dives are much shallower

Question 8

Green sea turtle (Chelonia mydas) diving

Answer 8

• deep active dives typically 10-20 m, 20-30 mins (in migration)
• mostly shorter shallower dives (e.g. feeding on seagrass)
• can submerge for hours at rest (e.g. for sleep)

Question 9

• Leatherback turtle (Dermochelys coriacea):

Answer 9

deepest dives >1200 m, >60 mins (during migration)
most dives (>99%) shallower (<300 m; Houghton et al., 2008)

Question 10

Deep leatherback dives - feeding

Answer 10

• in transit occur around midday
• Foraging for deep patches of gelatinous zooplankton
• If found during deep dive, turtle may remain to feed at night
• No deep dives observed at feeding grounds

Question 11

Problems of deep-diving at ambient pressure

Answer 11

4 physiological challenges from absorbing gas under pressure:
(i) oxygen toxicity
(ii) safe decompression of saturated tissues
(iii) inert gas narcosis
(iv) high-pressure nervous syndrome

Question 12

Oxygen toxicity

Answer 12

Hyperbaric O2 → problems with brain/CNS (seizures, blackout)
* At 90 metres deep in ambient-pressure diving, normal 21% O2
in air → equivalent to 200% O2 at surface pressure
* Problem solved: use lower % O2 for deep phase of dives
(e.g. 1% O2 for very deep dives, e.g. deepest saturation dives)

Question 13

decompression of saturated tissues

Answer 13

• Absorbing gas under pressure → more of them in solution
• Reducing pressure during ascent → gas comes out of solution
• Causes decompression sickness, aka “the bends” (joint pain from bubbles in body fluids → contortions) & can damage lungs in particular (pulmonary embolism)

Question 14

Inert gas narcosis

Answer 14

Main inert gas (78% N2) has narcotic effect at high pressure
* Dissolves in cell membranes, disrupting normal signalling

Question 15

Helium gas narcosis

Answer 15

• Helium has less of a narcotic effect under pressure N2
• helium still has narcotic effects
  at greater depths

Question 16

High pressure nervous syndrome

Answer 16

HPNS results from pressure effects on central nervous system
* Symptoms include tremors, eye twitch, headaches, fatigue
- inert gas narcosis is “opposite”
(overexcitation of nervous responses, rather than narcosis)

Question 17

H2 effects

Answer 17

less dense than N2 or He (easier to breathe under pressure)
* But H2 has greater narcotic effect than He (though less than N2)

Question 18

Aerobic Dive Limit:

Answer 18

amount of time a breath-hold animal can
dive before depletion of oxygen stores (& build-up of lactate)

Question 19

Using lungs as primary oxygen store (like us) causes problems - how?

Answer 19

• variable buoyancy as lungs become compressed during dive
• absorption of pressurised gas into blood during dives

Question 20

Methods for studying animal diving physiology

Answer 20

• Traditional anatomy (identifying potential adaptive structures)
• Immersion experiments on lab subjects
• Tagging individuals in the wild with data-logging sensors- can record depths, durations, surface intervals, water temp etc
• Blood samples in field after behavioural observation
• Comparative genomics (& transcriptomics on blood samples)

Question 21

Pinnipeds: Hooded seals (Cystophora cristata) diving

Answer 21

Exhale before dives to reduce buoyancy
* Alveoli of lungs collapse under pressure during dives
* Anti-adhesive lung surfactants enable reinflation
- brain is cooled during diving by ~3 ˚C which reduces brain oxygen demand by 15-20%

Question 22

Pinniped blood and vascular system

Answer 22

Large blood volume for body size & high haematocrit
(60% in hooded seal cf. 45% in humans)
* Large spleen also releases store of high haematocrit blood

Hooded seal myoglobin (higher o2 affinity than hb) can store 37 ml O2 kg-1
(~6x that of human muscle)
* Total O2 store of hooded seal: 90 ml O2 kg-1

Question 23

Pinnibed cardio system

Answer 23

Peripheral vasoconstriction prioritises blood to brain & heart
(muscles run on myoglobin stores, then anaerobic respiration)
* Increased blood pressure from arterial constriction is
compensatedby brachycardia (heat slows to 4-6 beats per min)

Question 24

Northern Elephant Seals)
Dive Profiles:

Answer 24

• Dive repeatedly to depths >500 meters for 20-25 minutes.
• Short surface intervals of 1-3 minutes.
  EEG studies show they sleep briefly during dives

Question 25

Northern Elephant Seals - High Blood Volume and Hemoglobin Concentration

Answer 25

• Blood volume: 216 ml/kg.
• Hemoglobin concentration allows 70% of total O2 store to be in the blood.

Question 26

Seal Peripheral Circulation Shutdown:

Answer 26

• Circulation to non-essential organs shuts off during dives, conserving oxygen.
• This can cause issues like inflammation during re-perfusion after dives.

Question 27

Elevated Carbon Monoxide Levels within seals:

Answer 27

• Maintains higher levels of carbon monoxide bound to hemoglobin, potentially protecting tissues during re-perfusion (Tift et al., 2014).

Question 28

Extended Sea Periods - seals

Answer 28

Spend ~8 months per year at sea.
* Engage in continuous diving for foraging and predator avoidance.

Question 29

Bone Structure - penguins:

Answer 29

Bones lack air spaces, reducing buoyancy and conserving energy.
* Strengthened bones help in propelling through the water.
* Potential carbonate reserve for buffering lactate after long dives (Ksepka et al., 2015).

Question 30

Penguin diving

Answer 30

Lung Collapse During Dives:
* Reduces buoyancy and prevents decompression sickness.
* Rely on myoglobin in muscles for oxygen storage.

Air Bubble Release:
* Release air bubbles from plumage before jumping ashore, reducing drag and enabling acceleration

Question 31

Chelonians (e.g., Turtles)
Leatherback Turtles body temp:

Answer 31

• Maintain body temperature through large size, insulating heat-exchange circulation, and fat layers.

Question 32

Trachemys scripta (Freshwater Turtle) diving:

Answer 32

• Can hold breath for months during winter hibernation.
• Reduces brain metabolism to 10-15%, relying on anaerobic metabolism (Milton et al., 2006).
• Upregulates genes for neuroglobin to manage reactive oxygen species post-dive.
• Uses Heat-Shock Proteins (HSP) and SuperOxide Dismutase (SOD) to protect brain during re-oxygenation (Milton et al., 2007).

Question 33

Cetaceans (e.g., Whales and Dolphins) - Genomic Adaptations:

Answer 33

• Comparative genomics reveal whale-specific mutations in genes for antioxidant enzymes and expanded gene families for managing hypoxia-induced reactive oxygen species

Question 34

Vascular Plexus (Retia Mirabilia) - cetaceans:

Answer 34

Network of blood vessels that dampen pressure spikes, protecting the brain from high blood pressure during swimming (Lillie et al., 2022).

Question 35

Blue Whales diving:

Answer 35

• Largest animal (~120 tonnes) with the largest bone (jaw mandible).
• Dive for 10-15 minutes, feeding by lunge-feeding which increases resistance (Goldbogen & Madsen, 2021).
• Reduce heart rate to ~4 bpm during descent, increase to 20 bpm during lunge-feeding, and 10-30 bpm during ascent and surface recovery.

Question 36

Sound Production:

Answer 36

• Mysticetes: Produce vocalizations for social communication during dives, using modified larynx structures
• Odontocetes: Use nasal cavities for echolocation clicks, employing different vocal registers, including “vocal fry” for loud sound generation with minimal air volume

Question 37

Buoyancy Control - cetaceans:

Answer 37

• Sperm whales use the spermaceti organ to adjust buoyancy: solidifying wax for descent, melting wax for ascent.

Question 38

Beaked whales diving

Answer 38

to >1000 m, possibly to ~3000 m, feeding on deeper-living mature squid for higher calorie intake (Visser et al., 2021).

Question 39

Breath-hold Diving in Hammerhead Sharks (Sphyrna lewini)

Answer 39

Not Air-breathers:
* Reduce blood flow to gills or close gill slits during deep dives to conserve heat.

Thermal Regulation:
* Maintain muscle temperature in cold deep waters, managing a gradient from warm surface (~26 °C) to cold depths (~5 °C) (Royer et al., 2023).