Swimming capabilities of deep-sea sharks through velocity, anatomical, physiological, and morphological analyses
(2019)
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- Authors
- Supervisors
- Mallefet, Jérôme
- Abstract
- In marine biology, one of the main objectives is to characterize the ecology of organisms: how they live, move, hunt, reproduce, etc. For that purpose, the study of swimming capabilities remains a feature providing several ecological information, especially for sharks which are mainly active predators. Studies regarding shark swimming capabilities have been initiated a long time ago for shallow-water species. They allowed highlighting differences between species explaining shark distribution, home range but also several internal adaptations related to their locomotor capabilities. While swimming capabilities start to be well known for shallow-water species, information regarding deep-sea sharks is still poorly documented. Indeed, only the speeds of the Greenland shark (Somniosus microcephalus) and the bluntnose sixgills shark (Hexancheus griseus) were measured using satellite telemetry and revealed the slowest velocity recorded for a shark. Other studies have focused on indirect measurements (proxies) of the locomotor muscle metabolism to estimate the swimming capabilities. They have shown that anaerobic metabolism of deep-sea species is lower than their shallow-water counterparts. In contrast, the aerobic metabolism does not show significant difference. Therefore, authors have concluded that deep-sea sharks should have lower swimming capabilities than their shallow-water counterparts. This conclusion is mainly based on two hypotheses: (i) the effect of the cold environment decreasing muscle activity and (ii) the visual interaction hypothesis which suggests that the swimming capabilities of an organism decrease when it lives in a darker environment. However, the low number of deep-sea shark species considered in these studies does not allow to draw this general conclusion for all deep-sea sharks. In this work, we studied the swimming capabilities of several deep-sea sharks from the Squaliformes order. Cruise swimming speed values were obtained using stereo-video analyses. Data analyses have revealed that not all deep-sea sharks swim slowly. Indeed, even if Dalatias licha displayed the lowest speed value ever recorded, and other non-luminous deep-sea sharks intermediate slow cruise swimming speeds, luminous sharks from the Etmopteridae family swim at similar velocities than some shallow-water sharks. This higher cruise swimming speed of Etmopteridae species could be an advantage. Indeed, luminous sharks use their ventral light luminescence to vanish from preys and predators underneath them. It is called couterillumination camouflage. However, their luminescence regulation is under hormonal control which is slow and do not fit with the counterillumination hypothesis which need to modulate quickly its light intensity. In this context, a high velocity might be useful to migrate in the water column in order to match their ventral light with the downwelling light of their environment, this hypothesis is called “isolume follower”. In addition to that, studies of the muscular system were done through red and white muscle fibers quantification and specific enzyme assays. Results highlighted more red muscle in Etmopteridae species with higher aerobic metabolism but lower white muscle and anaerobic metabolism than their non-luminous counterparts and D. licha. These results suggest that Etmotperidae display a muscular system design more for an efficient cruise swimming speed. In contrast, the higher white muscle proportion and anaerobic metabolism of other non-luminous deep-sea sharks suggest higher burst capabilities which might explain why Etmopteridae species are found in stomach contents of these other deep-sea sharks. Study regarding the buoyancy of deep-sea sharks have allowed to confirme that the lift provided by the liver is higher for deep-sea shark species than shallow-water ones. It also provided evidence of relation between the hydrostatic lift and the proportion of red muscle fibers as suggested by Bone in 1979 but never proven. Indeed, sharks with a liver providing more buoyancy display less red muscle fibers across their body. This study is thus the first evidence of the Bone’s hypothesis. Finally studies of morphological features have revealed that Squaliformes sharks displayed different shapes while they were grouped in the same morphotype previously. The spine shape might also have an impact on the velocity but future mechanistic studies should be done on the subject. Also, a new shape of caudal fin was described for D. licha. This caudal fin does not display any fork and should be less efficient for the thrust than heterocercal fins but here again, mechanistic studies should be conducted before obtaining any strong conclusion. This thesis sets up as the first multidisciplinary approach regarding the swimming capabilities of deep-sea sharks revealing differences which give information regarding the ecology of these poorly known species. It also brought new questions, future researches pathways and projects are currently in progress in order to still better understand the ecology of these fascinating animals.
- Affiliations
Citations
Pinte, N. (2019). Swimming capabilities of deep-sea sharks through velocity, anatomical, physiological, and morphological analyses. https://hdl.handle.net/2078.5/121945