(Edited 2025-03-28 to add a note about bull sharks; Edited 2021-02-04 to explain what obligate ram ventilators do.)

A- and I have been curious about which shark species are obligate ram ventilators and need to keep swimming with their mouths open in order to get oxygen from the water flowing through their gills. Here is an incomplete list. (Disclaimer: We're not marine biologists and have no idea what we're doing.)

20210128_121612 Obligate ram ventilators #nature #sketch.jpg

1. Great white shark (Carcharodon carcharias) Source
2. Bull shark (Carcharhinus leucas) ? Source - see section on bull sharks below
3. Sandbar shark (Carcharhinus plumbeus) Source
4. Porbeagle (Lamna nasus) Source
5. Salmon shark (Lamna ditropis) Source
6. Thresher shark (Alopiidae) Source
   • Pelagic thresher (Alopias pelagicus)
   • Bigeye thresher (Alopias superciliosus)
   • Alopias vulpinus
7. Mako shark (Isurus oxyrinchus, Isurus paucus) Source
8. Hammerhead (Sphyrnidae) Source
   • Winghead shark (Eusphyra blochii)
   • Scalloped bonnethead (Sphyrna corona)
   • Carolina hammerhead (Sphyrna gilberti)
   • Scalloped hammerhead (Sphyrna lewini)
   • Scoophead (Sphyrna media)
   • Great hammerhead (Sphyrna mokarran)
   • Bonnethead (Sphyrna tiburo)
   • Smalleye hammerhead (Sphyrna tudes)
   • Smooth hammerhead (Sphyrna zygaena)
9. Whale shark (Rhincodon typus) Source

Some fishes are also obligate ram ventilators. Here is an incomplete list:

• Tuna Source
• Billfish Source – Structural Adaptations for Ram Ventilation: Gill Fusions in Scombrids and Billfishes
• Paddlefish (Polyodon) Source
• Swordfish Source
• Bonito Source – Structural Adaptations for Ram Ventilation: Gill Fusions in Scombrids and Billfishes
• Bluefish Source

So… how do they keep swimming? What about sleep?

Apparently, part of their brain stays active and keeps them swimming. Also, sometimes they rest in currents with their mouth open. Source

Further notes on bull sharks

2025-03-28: Thanks to Wonderment and Oliiive's comments, I got inspired to dig further into bull sharks, which are usually mentioned as obligate ram ventilators but apparently might also tolerate other ways of breathing, which could explain their relatively lower mortality when caught (compared to other obligate ram ventilators). I've added some emphasis to the quotes.

Wonderment mentioned D.R. Dapp et al. 2016 Respiratory mode and gear type are important determinants of elasmobranch immediate and post-release mortality. Fish and Fisheries 17: 507-524, which had this:

We assumed that all species with large spiracles are capable of respiring while stationary and that species which lacked large spiracles could not do so. These assumptions were made because enlarged spiracles have been identified as key respiratory structures for species which respire while stationary (Hughes 1960; Grigg 1970) and spiracles of a reduced size have been shown to have little to no ability to aid respiration (Grigg 1970). Some exceptions were noted because a few species that had reduced or absent spiracles could still perform buccal pumping or ventilation through the first gill slit while stationary. Examples of such species are presented in Table S1 and include the bull shark (Carcharhinus leucas, Carcharhinidae), nurse shark (Ginglymostoma cirratum, Ginglymostomatidae), Port Jackson shark (Heterodontus portusjacksoni, Heterodontidae), lemon shark (Negaprion brevirostris, Carcharhinidae) and rusty carpetshark (Parascyllium ferrugineum, Parascylliidae) (Grigg 1970; Manire et al. 2001; Morgan et al. 2009; Brooks et al. 2011; Goto et al. 2013). Although we performed extensive literature searches, contacted experts in the field for advice on specific species and used the best available knowledge to confirm the respiratory mode of each species within this study, we acknowledge that the respiration mode of some species could not be verified. Throughout the rest of this study, we collectively refer to buccal pumping exception species and species with large spiracles as being capable of stationary respiration. We refer to species which are not capable of stationary respiration as obligate ram ventilators. Obligate ram ventilators must continuously move forward to respire because they possess reduced brachiostegal systems which are incapable of forcing water over their gills once movement stops (Roberts 1975; Carlson et al. 2004b).

The sources were:

• Grigg, G.C. (1970) Use of the first gill slits for water intake in a shark. Journal of Experimental Biology 52, 569–574. - this focuses on Port Jackson sharks (Heterodontus portusjacksoni)
• Manire, C., Hueter, R., Hull, E. and Spieler, R. (2001) Serological changes associated with gill-net capture and restraint in three species of sharks. Transactions of the American Fisheries Society 130, 1038–1048
  • This analyed bonnethead sharks Sphyrna tiburo, blacktip sharks Carcharhinus limbatus, and bull sharks C. leucas.
  • Relevant:

    We are unaware of any reports of bonnethead or blacktip sharks that were observed in the wild resting motionless on the bottom while healthy, and both species are known to swim continuously in captivity. In contrast, bull sharks will routinely cease activity and rest on the bottom in captivity (C. Manire, personal observation).

• Morgan, A., Cooper, P.W., Curtis, T.H. and Burgess, G.H. (2009) Overview of the U.S. east coast bottom longline shark fishery, 1994–2003. Marine Fisheries Review 71, 23–38.
  • The relevant part of this paper just cites Manire et al. 2001:

    Species such as nurse, tiger, and bull sharks possibly suffer lower at-vessel mortality rates because they are not obligate ram ventilators and may not become greatly stressed when hooked (Manire and Hueter, 2001).

• Goto, T., Shiba, Y., Shibagaki, K. and Nakaya, K. (2013) Morphology and ventilatory function of gills in the carpet shark family Parascylliidae (Elasmobranchii, Orectolobiformes). Zoological Science 30, 461–468. - can't easily find free PDF of this

I found a simlar mention in S. Kottillil, T. Gupta, M. Manoharakrishnan, C. Rao, and K. Shanker (2022) Post capture survival rate of bamboo sharks, Chiloscyllium arabicum and Chiloscyllium griseum, in Malvan, Maharashtra J. Mar. Biol. Ass. India, 64 (1), January-June 2022 doi:10.6024/jmbai.2022.64.1.2216-13:

Sharks with buccal pump respiration (with spiracles) respire while stationary (Dapp et al., 2016) while others, with obligate ram ventilation, have to keep swimming (reduced branchiostegal systems). Thus, when captured, those with stationary respiration survive better (Manire et al., 2001). Exceptions exist as species such as Carcharhinus leucas (bull shark) and Parascyllium ferrugineum (rusty carpet shark) show buccal pump with absent/reduced spiracles while Galeocerdo cuvier (tiger shark) switches between the two modes (Heupel and Simpfendorfer, 2011).

... but when I looked up Heupel, M. R. and C. A. Simpfendorfer. 2011. Estuarine nursery areas provide a low- mortality environment for young bull sharks Carcharhinus leucas. Mar. Ecol. Prog. Ser., 433: 237-244, I didn't find any mention of buccal pumping for bull sharks.

On the other hand, Alyssa M. Andres's 2022 dissertation The Effects of Temperature and Oxygen Availability on Aerobic Performance in Three Coastal Shark Species; Squalus acanthias, Carcharhinus limbatus, and Carcharhinus leucas says:

In addition, C. limbatus and C. leucas are obligate ram ventilators, thought to require forward motion to maintain oxygen supply, whereas S. acanthias have the ability to decrease activity and forcibly pump water over their gills while at rest (buccal pumping).

Bull sharks by contrast demonstrated continuous swimming as well as some resting behavior under normoxia and hypoxia, allowing for opportunistic measurement of demands and oxygen supply capacity across various modes of activity. Metabolic rate at rest for bull sharks was ~57% of minimum routine metabolic rate across temperature, and though resting reduced oxygen demands significantly, oxygen supply capacity was significantly diminished when bull shark activity was reduced. The oxygen supply capacity at rest was not enough to support energetic demands at rest, and for this reason, resting metabolism ultimately declined unsustainably after brief periods of rest in bull sharks. The use of buccal pumping (active ventilation) was not a sustainable or likely hypoxic coping strategy for either species, as a higher oxygen partial pressure was required to meet metabolic needs at rest, relative to swimming under hypoxia.

while Dylann Turffs's 2020 masters thesis "The Effect Of Caudal Stimulation On Hematological Indicators Of Stress In Blacknose Sharks, Carcharhinus Acronotus" says:

A search of the IUCN Red list database, filtering for sharks present along the US's Atlantic coast, suggests challenges worth exploring to understand how different respiratory strategies might impact a species conservation. When filtering the results to only show species in this geographic area that have been designated as “Least Concern” or “Near Threatened,” the search returned twenty sharks, with a mix of species known to use buccal pumping, including the tiger shark (Galeocerdo cuvier) and bull shark (Carcharhinus leucas) and obligate ram ventilators, including the Bonnethead (Sphyrna tiburon) and the Atlantic sharpnose (Rhizoprionodon terraenovae) (IUCN, 2020).

(... but I don't see a source for the buccal pumping in the paper or in the IUCN entry.) Kelly ML, Collin SP, Hemmi JM, Lesku JA. Evidence for Sleep in Sharks and Rays: Behavioural, Physiological, and Evolutionary Considerations (Brain Behav Evol. 2019;94(1-4):37-50. doi: 10.1159/000504123. Epub 2019 Nov 27. PMID: 31775150.) has a table that lists immobility as evidence for sleep in bull sharks, based on Clark, E. (1973) Sleeping Sharks in Mexico. Underwater Naturalist, 8, 4-7., which I can't find a copy of. The paper also says:

Similar suggestive evidence of increased arousal thresholds has also been reported in four species of wild requiem sharks (C. leucas, C. perezii, Prionace glauca, N. brevirostris), two of which (the bull and blue shark) are thought to be obligate ram ventilators, found in an underwater cave off the coast of Mexico.

de Carvalho, Marcelo R., Bernard Séret, and John D. McEachran. "Genre Carcharhinus Blainville, 1816." says bull sharks don't have spiracles.

So I think this means bull sharks can either they can tolerate hypoxia well enough to occasionally rest on the floor and to improve their survival rates when caught in nets, or they can pump using their first gill slits despite the lack of spiracles but it's not their favourite thing to do, so they might not be obligate ram ventilators after all, but I'm not sure. Anyway, I've put a question mark in the list, and maybe someone with more experience can share their thoughts!