12/9/2023 0 Comments Deep water abyssal zone animals2003) and are conspicuous as scavengers at whale carcasses ( Smith & Baco 2003) and baited cameras ( Priede & Bagley 2000) suggesting that the deep sea may harbour a hidden diversity of Chondrichthyes.Īll the major classes of vertebrates, mammals, birds, reptiles, amphibians, bony fish (Osteichthyes) and Chondrichthyes (sharks, rays and chimaeras), are suffering declines in species number and population sizes owing to habitat changes or exploitation ( IUCN 2004 Şekercioğlu et al. They now form an important component of deep-water fisheries down to 2000 m depth ( Gordon et al. Among the Chondrichthyes, including Holocephali (chimaeras) and Elasmobranchii (sharks and rays), many species show anatomical adaptations to deep-sea life including eyes sensitive to low light levels and possession of light organs (e.g. Deep-sea fishes are not a distinct taxonomic group but are derived from a diversity of shallow water types. Deep-sea fishes were first discovered in the 1860s and Günther (1880) reported the deepest bony fish as Gonostoma microdon at 5300 m from the Pacific Ocean whereas the deepest Chondrichthyes were a ray from 1033 m and a shark from 915 m. Owing to stagnation events, final invasion of the eastern basins of the Mediterranean occurred only 6000 years ago ( Rohling 1994). This was enabled and is sustained by oxygenation of deep water by the modern global thermohaline circulation ( Merrett & Haedrich 1997). The abyssal regions of the world's seas and oceans (depths greater than 3000 m) have been colonized by fishes during the last 70 Myr, contemporary with appearance of birds and mammals on land. Sharks may be more vulnerable to over-exploitation than previously thought. All populations are therefore within reach of human fisheries, and there is no hidden reserve of chondrichthyan biomass or biodiversity in the deep sea. Sharks are apparently confined to ca 30% of the total ocean and distribution of many species is fragmented around sea mounts, ocean ridges and ocean margins. We propose that they are excluded from the abyss by high-energy demand, including an oil-rich liver for buoyancy, which cannot be sustained in extreme oligotrophic conditions. Sharks, apparently well adapted to life at high pressures are conspicuous on slopes down to 2000 m including scavenging at food falls such as dead whales. Analysis of a global data set shows a trend of rapid disappearance of chondrichthyan species with depth when compared with bony fishes. Sampling by trawl, baited hooks and cameras we show that the Chondrichthyes (sharks, rays and chimaeras) are absent from, or very rare in this region. The oceanic abyss (depths greater than 3000 m), one of the largest environments on the planet, is characterized by absence of solar light, high pressures and remoteness from surface food supply necessitating special molecular, physiological, behavioural and ecological adaptations of organisms that live there.
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