is probably the variable most difficult to intuitively understand. Ocean pressure evokes thoughts of dark and forbidding depths, of submarine movies in which the captain and heroic crew must take their craft to depths far greater than she was built to withstand, to there lie on the bottom, evade the enemy, and hope to survive. The great pressure causes the sub to creak and groan, bolts to pop like bullets out of the hull, and leaks to sprout before the ordeal can be successfully ended. However, World War II submarines could not get very deep at all, <300 m, and even modern nuclear subs do not get out of the mesopelagic zone (200–1000 m). Our view on pressure from those movies is one where pressure is acting on gas‐filled spaces. A submarine is quite a large gas‐filled space and must be immensely strong to withstand even the modest pressure of a dive to 100 m: 11 atm, 162 psi, or 11 143 kPa. In point of fact, most of the species that live under pressure do not have gas‐filled spaces, and thus the effects of pressure are far more subtle, especially in the upper 1000 m where much of the ocean’s pelagic biomass resides. In our mind’s eye though, the pressure associated with even the average depth of the ocean must be a formidable challenge to life.
Pressure at the deepest point (10 916 m) in the ocean, the Challenger Deep in the Mariana Trench near the Philippines, is 16 046 psi (1.11 × 105 kPa). Pressure at the average depth of the ocean of 3800 m (Sverdrup et al. 1942) is 5586 psi (3.85 × 104 kPa). Those are big numbers and are part of the reason we have always held pressure in such high regard as an operator influencing life in the ocean. In fact, respect for pressure has colored some of the history of man’s exploration of the sea.
In the 1840s, an eminent British scientist by the name of Edward Forbes was conducting research on the bottom fauna in the coastal waters of the British Isles using an oysterman’s dredge. Depths were 600 feet (183 m) and less, and he found the fauna varied and abundant. His reputation as an ocean scientist earned him an invitation to journey to the Mediterranean to do some similar dredging. He found a very sparse bottom fauna in the Mediterranean, but the depths at which he was sampling, 1400 feet (428 m), were about twice those he had sampled off the coast of Great Britain. He assumed that the depth difference between the two regions was responsible for the change in the abundance of the bottom fauna. He decided that if the trend in declining abundance with depth continued as observed, in short order, the bottom fauna would disappear altogether. Based on his results, he declared that below a depth of 1800 feet (550 m), no life would exist and he termed those depths the “azoic zone,” the zone without life.
As time went on and exploration of the ocean continued, animals were recovered from greater and greater depths, but rather than accepting the possibility that life existed at the ocean’s greatest depths, the azoic zone just kept getting pushed deeper and deeper. In 1860 just prior to the voyage of the Challenger, the most famous oceanic expedition of all time, a submarine telegraph cable was brought to the surface for repair from a depth of 2000 m in the Mediterranean. The cable was covered with encrusting fauna, animals like barnacles and corals that build calcareous structures. Because those fauna would not have had time to create their structures on the cable during its rapid journey to the surface, the azoic theory was pretty well laid to rest at that point. The theory was further discredited by the discoveries of the Challenger over the next 5 years. Yet, there was still some doubt that life could exist at the very deepest points in the ocean, like the Challenger deep. The challenge to life caused by pressure was at least partially responsible for those doubts, along with the cold, the dark, and the distance from the sun and plant life.
The final demise of the Azoic Theory came in 1960 with the voyage of the submersible Trieste to the deepest point in the ocean. It was one of the great moments in the history of man when Jacques Piccard, son of Auguste Piccard, the submersible’s designer, and Lt. Don Walsh of the US Navy descended to the deepest point in the ocean, the Challenger Deep in the southern part of the Mariana Trench. The onboard instrumentation recorded a depth of 11 521 m, which was later revised to 10 916 m. Measurements since then have revised the estimate both up (11 034 m) and down (10 896 m) using different instrumentation, but they are all very close to the original estimate. While at depth, Piccard and Walsh observed swimming shrimps, thus showing that life can exist at the deepest point in the ocean.
Early Work
Unlike temperature or salinity, which can be readily adjusted in the laboratory, conducting experiments with pressure requires specialized instrumentation in the form of pressure vessels. The larger the vessel, the more expensive it is to manufacture, which has limited the amount of research done on the effects of pressure. Despite the difficulties of working with it, enough research has been done on pressure for us to know its basic effects and its main sites of action. In fact, pressure research has a fairly long history, encompassing taxa ranging from bacteria to metazoan invertebrates and fishes. We will cover the points that most directly apply to pelagic fauna.
Regnard (1884, 1891) was the first scientist to study the effects of hydrostatic pressure on invertebrates and fishes. Inspired by the voyages of the Challenger (1872–1876) and the Talisman (1882–1883) and their discovery that life existed at the great depths of the ocean, Regnard decided to bring the environment of the deep sea into the laboratory. He tested the effects of pressure up to 1000 atm on various freshwater and marine animals using a hydraulic pump and rapid pressurization. He found that decapod Crustacea and bony fishes were less resistant to pressure than anemones, echinoderms, mollusks, and annelids. His results on the responses of species to pressure are valid to this day.
Ebbecke (1944) continued the work of Regnard to its logical conclusion also using surface‐dwelling and intertidal species. He composed a list of relative pressure tolerance in animal groups going from highest tolerance to lowest: (i) Anemones, (ii) Starfish, (iii) Sea urchins, (iv) Scyphozoan medusae, (v) Gastropods, (vi) Polychaetes, (vii) Shrimp, and (viii) Teleosts.
In addition, he did some behavioral observations of animals exposed to stepped pressures and found that behavioral responses could be divided into three distinct classes.
Phase I. (50 atm) Phase I was characterized by a state of excitement or increased activity as if nervous. Low pressure thus seemed to act as a stimulant.
Phase II. (150 atm) Phase II was characterized by a state of moribundity, almost as if the animal were paralyzed.
Phase III. (200 atm) Phase III caused tetany (maximal contraction of muscles) in shallow invertebrates, and fish were killed immediately. Therefore, surface‐dwelling fishes cannot cope with a pressure equivalent to a depth of 2000 m.
Work on tolerance to pressure continued into the late 1960s using similar techniques: shallow‐dwelling species exposed to high pressure using small pressure vessels and rapid pressurization. Authors well known for pressure research of this kind were R.J. Menzies, R.Y. George, V. Naroska, C. Schlieper, and H. Flugel (Flugel and Schlieper 1970). An interesting new twist to pressure research was the use of the hydrowinch on an oceangoing research vessel (Menzies and Wilson 1961) as a mechanism for applying pressure. Animals were placed in net‐covered jars affixed to the hydrowire and sent to depths ranging from 469 to 3480 m. The lined shore crab Pachygrapsus crassipes succumbed to trips below 867 m but survived lesser depths with severe tetany from which they eventually recovered. Mussels (Mytilus edulis) were more resistant, surviving trips to 2227 m, but all succumbed to a round trip to 3480 m. It is worth noting that the authors also exposed both species to the lower temperatures they experienced at depth, with no ill effects observed.
At the same time that interest was developing in tolerance to pressure, pressure effects on rate functions were being explored. Regnard initiated this type of research by observing animals through the window of his pressure vessel. The best of the early pressure physiology was by Fontaine (1928) who was the first to study the effects of pressure on the oxygen consumption of plaice (Pleuronectes platessa), a European flatfish popular as a menu item. His results mirrored those of Ebbecke in that low and moderate pressures had a stimulatory effect
