Zonation of Deep Biota on Continental Margins
Pioneering deep-sea surveys established that the fauna of the continental margins is zoned in the sense that individual species and assemblages occupy restricted depth bands. It has been speculated that the causes of this wide-spread pattern might involve cold temperatures, high pressures and limited food availability. Increased sampling over the past two decades has confirmed the global presence of depth zonation. Well-defined zonation in the cold polar oceans and the warm Mediterranean indicate that temperature
may be of less importance on ecological timescales than originally proposed. Strong alternatives are range restriction by pressure and food availability. Understanding of pressure physiology has advanced greatly, and it is to be expected that all deep organisms possess some form of genetic adaptation for pressure tolerance. Since high pressure and low temperatures affect membrane and enzyme systems similarly, combined piezo-thermal thresholds may limit depth ranges. There is a negative, exponential gradient of food availability caused by the decrease in labile carbon influx to bottom. The TROX model linking carbon influx with interstitial oxygen levels has been successful in explaining deep distributions of benthic Foraminifera and may be more broadly applicable. Current efforts to relate metazoan ranges to food availability are, however, hindered by limited understanding of how organisms recognise and utilise the nutritious content of detritus. Thus, the exact controls of depth zonation remain conjectural. Zonation studies are gaining in importance due to the increasing availability of deep fauna databases and the need to establish regulatory boundaries. Future studies may benefit from a growing body of biogeographic theory, especially the understanding of bounded domains. It is proposed that continental slope fauna may be more effectively studied if viewed as the overlapping of three components: species extending down from the shelf, species extending up from the abyss and species truly restricted to the slope.