Take a Deep Breath

The vast majority of sharks and rays are not able to raise their internal body temperature above that of the surrounding water. So, like lizards and snakes, they must change their behaviour in order to warm themselves up or cool themselves down. For many sharks this moving into shallower or warmer water to boost their metabolism. But scientists have discovered scalloped hammerheads have a fascinating and unique way of maintaining their internal temperature during deep dives - they effectively hold their breath! But how on Earth do they do this? And how does it work?

Like the vast majority of sharks, scalloped hammerheads are 'cold-blooded' (Image Credit: Xvic/ WikimediaCommons)

Heat Seeking

Excluding a few oddballs that can control their body temperatures (to learn more, check out Packing Heat), most sharks must rely on "behavioural thermoregulation" to maintain a favourable internal temperature. This means that over varying spatial- and temporal-scales, sharks will move in accordance with the environmental temperature, depending on what their body needs (Speed et al, 2012; Schlaff et al, 2014).

Male and female blacktip reef sharks live in different water temperatures, due to their physiological needs (Image Credit: whatsthatfish.com / WikimediaCommons)

For example, over small spatial scales, female sharks - such as blacktip reef sharks (Carcharhinus melanopterus) - commonly occupy warmer waters than their male counterparts, in order for their unborn young to develop more quickly (Speed et al, 2012). Young sharks - such as bonnetheads (Sphyrna tiburo) - often occupy warmer water for the same reason (Ward-Paige et al, 2014).

Over the long-term, many species of sharks - sandbars (C. plumbeus) for instance - migrate to seek warmer waters during the winter months and then head back polewards, into more temperature regions, during the heat of the summer (Speed et al, 2012; Schlaff et al, 2014).

Sanbar sharks migrate seasonally to seek out favourable water temperatures (Image Credit: National Marine Sanctuaries / WikimediaCommons)

Putting the Hammer Down

However, there are incidents where sharks simply cannot avoid venturing into sub-optimal temperatures. For example, whilst they normally favour warm to temperate surface waters, scalloped hammerheads (Sphyrna lewini) make daily dives down into deep water to hunt for prey. Reaching depths of some 800 metres, they experience extreme cold, with waters as cool as 5°C (Rover et al, 2023).

When they make dives into deep water [TOP], scalloped hammerheads experience very cold temeratures [BLUE] (Rover et al, 2023)

In cold regions, any heat a sharks has generated through metabolism or muscle contractions, or has been absorbed from warm water, will begin to be lost, by "conduction" through their skin. Notably, a significant amount of heat is lost through the gills, as they have a large surface area for gas exchange (see How Do Gills Work? for more detail) (Rover et al, 2023).

You might imagine that 'cold-blooded' (aka "ectothermic") creatures won't be affected too much by the cold, but on the contrary! Any animal can experience serious issues when they are pushed beyond their thermal limits. In scalloped hammerheads, extreme body-cooling can cause heart problems, loss of visual acuity, and reduced muscle power. Because they are "obligate ram ventilators" (that must keep swimming forwards to order to run oxygenated water over their gills), this can also reduce their ability to ventilate (Rover et al, 2023).

Scalloped hammerheads are only able to ventilate if they continually swim forwards; flushing their gills with oxygenated water (Image Credit: Kris Mikael Krister / WikimediaCommons)

Diving In

As their deep dives are essential for scalloped hammerheads to find their food, they can't forgo venturing into the cold, so evolution has provided them a unique strategy for coping... They can suppress their gill function during their dives, in order to reduce the amount of heat loss (Rover et al, 2023).

Footage of a scalloped hammerhead recorded at a depth of 1042 metres appears to show the shark closing it's gills in a breath hold (Moore & Gates, 2015)

Basically, they hold their breath to keep warm! 

Scientists think they either divert blood away from the gills, or close their mouth to stop water flowing over the gills or close the gills completely (Moore & Gates, 2015; Rover et al, 2023).

This means that despite descending through a drop of ~20°C in only five to seven minutes, these sharks are able to actively maintain their body some 20°C above that of the ambient water. It is only on the ascent back to the surface that they start to experience mild cooling, presumably, when they resume gill functioning because they are coming into shallower water, where they will be able to re-warm themselves near the surface. Amazing! (Rover et al, 2023).

Scientists measuring their muscle temperatures during their deep dives have shown that, thanks to their 'breath holding', scalloped hammerheads 'hold their breath' to maintain their internal body temperature [RED LINE] during their descent and bottom time whilst diving [BLACK LINE], and it is only on the ascent that their body starts to cool (Rover et al, 2023).

References

Moore AB & Gates AR (2015). Deep-water observation of scalloped hammerhead Sphyrna lewini in the western Indian Ocean off Tanzania. Marine Biodiversity Records, 8:91. Access online.

Royer M, Meyer C, Royer J, Maloney K, Cardona E, Blandino C, da Silva GF, Whittingham K & Holland KN (2023). “Breath holding” as a thermoregulation strategy in the deep-diving scalloped hammerhead shark. Science, 380:6645. Access online.

Schlaff AM, Heupel MR & Simpfendorfer CA (2014). Influence of environmental factors on shark and ray movement, behaviour and habitat use: a review. Reviews in Fish Biology and Fisheries, 24. Access online.

Speed CW, Meekan MG, Field IC, McMahon CR & Bradshaw CJ (2012). Heat-seeking sharks: support for behavioural thermoregulation in reef sharks. Marine Ecology Progress Series, 463. Access online.

Ward‐Paige CA, Britten GL, Bethea DM & Carlson JK (2015). Characterizing and predicting essential habitat features for juvenile coastal sharks. Marine Ecology, 36:3. Access online.

By Sophie A Maycock for SharkSpeak