Hydrostatic

Respiratory Response of the Deep-Sea Amphipod
Stephonyx biscayensis Indicates Bathymetric Range
Limitation by Temperature and Hydrostatic Pressure

Abstract
Depth zonation of fauna on continental margins is well documented. Whilst increasing hydrostatic pressure with depth has long been considered a factor contributing significantly to this pattern, discussion of the relative significance ofdecreasing temperature with depth has continued. This study investigates the physiological tolerances of fed and starved specimens of the bathyal lysianassoid amphipod Stephonyx biscayensis at varying temperature to acute pressure exposure by measuring the rate of oxygen consumption. Acclimation to atmospheric pressure is shown to have no significant interaction with temperature and/or pressureeffects. Similarly, starvation is shown to have no significant effect on the interaction of temperature and pressure. Subsequently, the effect of pressure on respiration rate is revealed to be dependent on temperature: pressure equivalent to 2000 m depth was tolerated at 1 and 3uC; pressure equivalent to 2500 m depth was tolerated at 5.5uC; at 10uC pressure equivalent to 3000 m depth was tolerated.The variation in tolerance is consistent with the natural distribution range reported for this species. There are clear implications for hypotheses relating to the observed phenomenon of a biodiversity bottleneck between 2000 and 3000 metres, and for the potential for bathymetric range shifts
in response to global climate change.

Introduction
The deep sea is one of the largest habitats onEarth. Phenotypic and genetic clines with depth are evident in deep-sea organisms, which appear biochemically adapted to specific depth regimes. A compilation of 34 regional case histories and additional studies by Carney indicates that deep-sea zonation patterns are widespread, with continental slope fauna clearly distinct from shelf fauna above and abyssal plain fauna below. High species turnoverconsistently indicates biodiversity bottlenecks at a shelfslope transition between the shelf break and 1000 m, and a slopeabyss transition between 2000 and 3000 m. Subsequently, considerable research has focused on the physiological constraints to species bathymetric distributions (reviewed by.
Hydrostatic pressure (0.1 MPa =10 m depth) effects on living systems initially result fromthermodynamic shifts in chemical reaction rates. Significant effects of hydrostatic pressure have been shown by pressure research on isolated biochemical systems, focusing on enzymatic proteins and lipoprotein membranes.
Denaturation of proteins resulting from pressure induced conformational change is well known and adaptation to the high pressure and low temperature conditions prevailing in the deep sea isrequired. Temperature also acts as a thermodynamic parameter with decreasing temperature decreasing chemical reaction rates; rates change by a factor of two to three for each 10uC temperature change. Successful adaptation to low temperature and high pressure habitats involves increased enzyme concentration, adoption of enzymes with greater efficacy and inclusion of modulator compounds thatfacilitate enzyme reactions and references cited therein). Increased hydrostatic pressure and decreased temperature also reduce the fluidity of bio-membranes necessitating homeoviscous adaptations in membrane structure and composition. In the absence of these adaptations, the effects of high pressure and low temperature are sufficient to affect biological processes at all levels of organization. Theseeffects appear to contribute to limited thermal tolerance through oxygen-limitation, i.e. by reducing oxygen supply. Given the analogous effects of low temperature and high pressure it has recently been proposed that pressure effects on oxygen supply similarly determine tolerance to pressure. Since respiratory rate has previously been used as an indicator of metabolic rate in marine ectotherms...