Bathymetric limits of chondrichthyans in the deep sea: A re-evaluation

Chondrichthyans are largely absent in abyssal (>3000 m) habitats in most regions of the world ocean and are uncommon below 2000 m. The deeper-living chondrichthyans include certain rajids, squaliforms and holocephalans. Several hypotheses have been erected to explain the absence of chondrichthyans from the abyss. These are mostly based on energetics: deep-sea food webs are impoverished due to their distance from primary production, and chondrichthyans, occupying the highest trophic levels, cannot be supported due to entropy among trophic levels. We examined this hypothesis by comparing trophic levels, calculated from dietary data, of deep-sea chondrichthyans with those of deep-sea teleosts. Chondrichthyans were mostly above trophic level 4, whereas all the teleosts examined were below that level. Both small and medium squaloids, as well as sharks and skates of large size, feed on fishes, cephalopods and scavenged prey, and thus occupy the highest trophic levels in bathydemersal fish communities. In addition, whereas teleosts and chondrichthyans both store lipids in their livers to support long periods of fasting, chondrichthyans must devote much of their liver lipids to maintain neutral buoyancy. Consequently teleosts with swim bladders are better adapted to survive in the abyss where food sources are sparse and unpredictable. The potential prey field for both chondrichthyans and teleosts declines in biomass and diversity with depth, but teleosts have more flexibility in their feeding mechanisms and food habits, and occupy abyssal trophic guilds for which chondrichthyans are ill adapted.