When a shark dies, its body embarks on a journey within the marine ecosystem, playing a role in nutrient cycling and providing sustenance for various organisms. This natural process is essential for ocean health, though human activities can significantly alter it.
The Immediate Fate
Upon death, a shark’s body typically sinks. Unlike many bony fish, sharks do not possess a swim bladder, an air-filled organ that helps with buoyancy control. Their cartilaginous skeletons, while lighter than bone, and dense muscle tissue contribute to their negative buoyancy. While a shark’s large, oil-rich liver provides some lift, it is generally not enough to keep the body afloat once swimming ceases.
The sinking process can take hours or even days, depending on the shark’s size, condition, and water currents. During this descent, the carcass may attract immediate scavengers in the water column or near the surface, such as smaller sharks, bony fish, and seabirds, beginning consumption even before reaching the seabed.
Decomposition and Ecosystem Role
Once a shark’s body settles, decomposition begins, driven by biological agents. Bacteria and other microorganisms initiate the breakdown of soft tissues, consuming organic matter. This microbial activity can be rapid, with soft tissues decomposing in weeks to months, influenced by factors like water temperature and the size of the shark.
Various marine scavengers, including crabs, hagfish, and smaller fish, are drawn to the carcass, rapidly consuming flesh and organs. These detritivores and decomposers recycle nutrients back into the ecosystem. Even after larger scavengers consume the soft parts, the cartilaginous skeleton persists longer, eventually breaking down and releasing minerals back into the environment. This process contributes to the overall health of marine ecosystems by returning essential elements, such as carbon and nitrogen, into the food web.
The Phenomenon of Deep-Sea Falls
When a large shark dies and sinks to the deep sea, it creates a unique, temporary ecosystem known as a “shark fall,” similar to well-studied whale falls. These events provide a substantial pulse of food to the typically food-scarce abyssal plains. The decomposition process at these depths unfolds in distinct stages.
The initial phase is the “mobile-scavenger” stage, where large, mobile organisms like hagfish, grenadiers, and various crustaceans quickly arrive to consume the soft tissues. Following this, the “enrichment opportunist” stage commences, characterized by a diverse community of smaller invertebrates, such as polychaete worms and mollusks, that feed on remaining scraps and the enriched sediments around the carcass. The final, “sulfophilic” stage involves specialized bacteria and organisms that thrive on the sulfide produced as the bones decay. Osedax worms, also known as bone-eating worms, bore into the bones, using symbiotic bacteria to extract nutrients. These shark falls support unique biodiversity and demonstrate how marine life adapts to exploit rare, concentrated food sources in the deep ocean.
Human Influence on Shark Mortality
Human activities significantly interfere with the natural processes of shark mortality and decomposition, disrupting their ecological role. Shark finning, for instance, involves removing a shark’s fins and often discarding the body back into the ocean, sometimes while the shark is still alive. Without fins, the shark is unable to swim or breathe effectively and often drowns or is consumed by other predators, preventing its body from undergoing a natural decomposition process.
Bycatch, the unintentional capture of sharks in fishing gear targeting other species, also leads to shark mortality. These sharks may be discarded, or their bodies might not reach the seabed in a manner that supports deep-sea fall ecosystems. Pollution, including plastic debris and chemical runoff, further impacts marine decomposition. Plastic pollution has been shown to reduce decomposition rates of organic matter, and contaminants can affect the health of scavengers and microorganisms vital to the breakdown process. These human-induced disruptions reduce the ecological benefits that a shark’s body provides to the marine environment after death.