What is it? What do we know about it? How do we explore it? Where can I learn more?

What is the "Twilight Zone"?

When most people hear the term "Twilight Zone", they probably think about Rod Serling's classic television series about strange and bizarre events. The term has a very different meaning to biologists. In fact, it even represents slightly different concepts to different researchers. For biologists who study organisms that live in caves, the "Twilight Zone" is the region just at the edge of where visible light from the entrance of a cave reaches. To marine researchers, the term has different meanings depending on what part of the ocean is being discussed. In the pelagic (open-ocean) realm, the "Twilight Zone" refers to the depth range between about 500 and 1,500 feet (150 and 750 meters) beneath the surface; whereas in coral reef environments, it generally refers to somewhat shallower depths. The common thread among all of these meanings in biology is that the term represents a transition from a region that receives sunlight during the daytime, to a region that remains in perpetual blackness. Because most life ultimately derives energy from sunlight, this transition is of relatively great ecological significance.

The coral-reef Twilight Zone is roughly defined as coral-reef habitat at depths between about 200 feet (60 meters) and 500 feet (150 meters). The upper limit represents the approximate maximum depth to which stony corals tend to dominate the reef structure, and the lower limit represents the maximum depth at which significant photosynthesis occurs (the maximum depth to which the living coral reef extends). The reason the coral-reef Twilight Zone is shallower than the open-ocean Twilight Zone stems mostly from the difference in water clarity between the two habitats. In the open ocean, the crystal-clear water allows sunlight to penetrate considerably deeper than around coral reefs, where the water is often teeming with plankton. Therefore, the biologically important transition zone between light and dark exists at somewhat shallower depths around coral reefs.

The depth range of the coral-reef Twilight Zone happens to correspond with a region that, because of limitations of technology, has been almost entirely ignored by marine biologists. The most popular method researchers use to explore coral reefs is conventional Scuba gear. However, because of the physiological limitations of breathing air under pressure (see Physics and "Fizzyology" page), divers using conventional Scuba are limited to a maximum safe working depth of about 200 feet*. To explore deeper regions of the undersea environment, biologists have used deep-sea submersibles. However, these devices are extremely expensive to operate - often costing $25,000 or more per day! They are also not widely available to all researchers. Consequently, most research submersibles are used to explore much greater depths - almost always in excess of 500 feet (150 meters). Thus, the coral-reef Twilight Zone has remained largely unexplored.

*Note: The American Academy of Underwater Sciences (AAUS) has established 190 feet as the maximum depth to which scuba-diving researchers should descend. The vast majority of marine biological exploration by scuba-clad researches has been conducted at much shallower depths.

What do we know about the coral-reef Twilight Zone?

In order to understand the context of what we know about the coral-reef Twilight Zone, it is helpful to understand something about shallow coral reefs as well.

Shallow Coral Reef
Most of us are familiar with a typical coral reef environment: brightly-colored tropical reef fish swarming among vast expanses of large reef-building corals. These stony corals harbor in their tissues symbiotic algae called zooxanthellae, which many corals depend on to maintain their health. These algae need sunlight for photosynthesis, thus the reef-building stony corals dominate only those reefs shallower than about 200 feet (60 meters). Although stony corals can be found at greater depths, they do not represent nearly as significant a role in the overall coral reef community.

Coral-Reef Twilight Zone
Below depths of about 200 feet (60 meters), the large reef-building corals give-way to more soft-bodied marine organisms, such as sponges, soft-corals, tunicates, and other sessile invertebrates. In this depth zone, clear tropical waters allow enough sunlight to penetrate for photosynthesis to occur, but not as robustly as it does on the shallow reefs. The very nature of the reef community takes on a different structure. We know remarkably little about life in this region. Several studies involving research submersibles have provided tantalizing glimpses of how much remains to be discovered in the coral-reef Twilight Zone. Besides the wealth of undiscovered species, the ecological dynamics and biological community structure of this zone are of particular interest, because it spans the gap between two very distinct habitats: the lush coral reefs, and the relatively barren abyssal depths.

Abyssal Depths
Below a depth of about 500 feet (150 meters) on coral reefs, light levels are insufficient to sustain photosynthesis. This results in a dramatic shift in biological communities. Extending from this depth to the very bottom of the sea, biological communities are relatively sparse. Except for regions in close proximity to deep-sea hydrothermal vents (where a fascinating ecosystem ultimately derives its energy from geothermally heated mineral-rich water, rather than sunlight), most organisms dwelling in the perpetually black deep sea must ultimately rely on nutrients sinking from above.

How can we explore the coral-reef Twilight Zone?

The reason we know so little about the organisms which dwell in the coral reef Twilight Zone stems from limitations in previously existing technology. Conventional Scuba is limited in the depth to which it can be safely used by problems associated withdiving physics and physiology. Deep sea submersibles can dive to much greater depths, but are limited by their extremely high costs. In this era of dwindling funding for scientific research, opportunities to use submersibles are becoming fewer and farther between. When funding is available to support submersible research, the tendency is to descend straight past the coral reef Twilight Zone and concentrate on more abyssal depths. Even when submersibles have been used to explore the deep coral-reef, restrictions in collecting tools have often limited their ability to adequately explore the complex habitat typical of coral reefs.

Over the past decade, an increasing number of non-commercial civilian divers have been experimenting with mixed-gas diving technology, to extend the depth limits of conventional scuba. Perhaps more significantly, modern closed-circuit rebreathers -- diving devices which have been around for more than a century but have been notoriously unreliable and restricted in use primarily to military and commercial diving operations -- are now becoming more reliable, and available to the public. Armed with this advanced form of scuba, highly trained scientific researchers can begin to explore the deep coral reefs with the flexibility and "intimacy" of scuba diving, at a mere fraction of the cost of submersibles.

Future research will combine submersible and mixed-gas diving technology, along with other useful tools such as Remotely Operated Vehicles (ROV's), to harness their complementary capabilities. The future of exploration on the deep coral reefs is very bright.

Further Reading

Ambrose, G. 1996. Breathe Deep: Isle divers test new gear that recycles air, allowing them to probe deeper and stay longer. Honolulu Star Bulletin April 3, 1996:A-1,A-8. (Related articles: Ambrose, Greg. 1996. Rebreather opens up a new ocean frontier. Honolulu Star Bulletin April 3, 1996:A-8; Ambrose, Greg. 1996. ‘Twilight Zone’ yields to crystal clear waters. Honolulu Star Bulletin April 3, 1996:A-8.).

Avent, R.M., M.E. King, and R.H. Gore. 1977. Yopographic and faunal studies of shelf-edge prominences off the central eastern Florida coast. Int. Revue ges. Hydrobiol. 62(2): 185-208.

Brock, V.E. and T.C. Chamberlain. 1968. A geological and ecological reconnaissance off western Oahu, Hawaii, principally by means of the research submersible "Asherah". Pac. Sci. 22(3): 373-394.

Colin, P.L. 1974. Observation and collection of deep-reef fishes off the coasts of Jamaica and British Honduras (Belize). Mar. Biol. 24: 29-38.

Colin, P.L. 1976. Observations of deep-reef fishes in the Tounge-of-the-Ocean, Bahamas. Bull. Mar. Sci. 26(4): 603-605.

Colin, P.L., D.M Devaney, L. Hills-Colinvaux, T.H. Suchanek, and J.T. Harrison, III. 1986. Geology and biological zonation of the reef slope, 50-360 m depth at Enewetak Atoll, Marshall Islands. Bull Mar. Sci. 38(1):111-128.

Dennis, G.D. and T.J. Bright. 1988. Reef fish assemblages on hard banks in the northwestern Gulf of Mexico. Bull. Mar. Sci. 43(2): 280-307.

Earle, S.A. 1991. Sharks, squids, and horseshoe crabs - the significance of marine biodiversity. BioScience 41(7): 506-509.

Fricke, H.W. and B. Knauer. 1986. Diversity and spatial pattern of coral communities in the Red Sea upper Twilight Zone. Oecologia. 71:29-37.

Fricke, H.W. and H. Schuhmacher. 1983. The depth limits of Red Sea Stony corals: An ecophysiological problem (A deep diving survey by submersible). PSZNI Mar. Ecol. 4(2): 163-194.

Gilliam, B. 1992. Bishop Museum Deep Project, Hawaii. p. 154-156. In: Deep Diving: An Advanced Guide to Physiology, Procedures and Systems. (Gilliam, B., R. Von Maier, J. Crea, and D. Webb, eds). Watersport Publishing, Inc., San Diego. 255 pp.

Gilmore, R.G. and R.S. Jones. 1992. Color variation and associated behavior in the epinepheline groupers, Mycteroperca microlepis (Goode and Bean) and M. phenax Jordan and Swain. Bull. Mar. Sci. 51(1): 83-103.

Ginsburg, R.N. and N.P. James. 1973. British Honduras by submarine. Geotimes, 18: 23-24.

Grassle, J.F. 1991. Deep-sea benthic biodiversity. BioScience. 41(7):464-469.

Halstead, B. 1996. Hi-Tek Adventure. Scuba Diver, September/October 1996: 61-64, 6 figs.

Hartman, W.D. 1973. Beneath Caribbean reefs. Discovery 9:13-26.

Hills-Colinvaux, L. 1986. Deep water populations of Halimeda in the economy of an atoll. Bull. Mar. Sci. 38(1):155-169.

Itzkowitz, M., M. Haley, C. Otis and D. Evers. 1991. A reconnaissance of the deeper Jamaican coral reef fish communities. Northeast Gulf Sci. 12(1): 25-34.

Lang, J.C. 1974. Biological zonation at the base of a reef. Amer. Sci. 62:272-281.

Lavenberg, R. and S.A. Earle (eds). 1975. Ecology of coral reef invertebrates and plants. Science Bulletin 20, Nat. Hist. Mus. L.A. County. 103 pp.

Macintyre, I.G, K. Ruetzler, J.N. Norris, K.P. Smith, S.D. Cairns, K.E. Bucher and R.S. Steneck. 1991. An early holocene reef in the western Atlantic: Submersible investigations of a deep relict reef off the west coast of Barbados, W.I. Coral Reefs 10(3): 167-174.

Moffitt, R.B., F.A. Parrish and J.J. Polovina. 1989. Community structure, biomass and productivity of deepwater artificial reefs in Hawaii. Bull. Mar. Sci. 44(2): 616-630.

Montres Rolex S.A. 1996. Richard Pyle, United States. Project: Investigate biodiversity in the undersea Twilight Zone (Exploration and Discovery). P. 146-147. In: Spirit of Enterprise: The 1996 Rolex Awards. Secretariat of the Rolex Awards for Enterprise, Geneva, Switzerland.191 pp.

Parker, R.O. Jr. and S.W. Ross. 1986. Observing reef fishes from submersibles off North Carolina. Northeast Gulf Sci. 8(1):31-49.

Porter, J.W. 1973. Ecology and composition of deep reef communities off the Tongue of the Ocean, Bahama Islands. Discovery 9: 3-12.

Pyle, R.L. 1991. Rare and Unusual Marines: So many fish, so little time. Freshwater Mar. Aquar. 14(4):42-44.

Pyle, R.L. 1992a. Deep reef set. aquaCorps 3(1): 17-21.

Pyle, R.L. 1992b. The Twilight Zone. AquaCorps: Mix. 3(1):19, 1 fig.

Pyle, R.L. 1992c. The peppermint angelfish: Centropyge boylei, n.sp. Pyle and Randall. Freshwater Mar. Aquar. 15(7): 16-18.

Pyle, R.L. and E.H. Chave. 1994. First record of the chaetodontid genus Prognathodes from the Hawaiian Islands. Pac. Sci. 48(1): 90-93.

Pyle, R.L. and J.E. Randall. 1992. A new species of Centropyge from the Cook Islands, with a redescription of Centropyge boylei. Revue Fr. Aquariol., 19(4): 115-124.

Pyle, R.L. 1995. Chapter 12. Pacific reef and shore fishes. In: Maragos, J.E., M.N.A. Peterson, L.G. Eldredge, J.E. Bardach, and H.F. Takeuchi (Eds.). Marine and Coastal Biodiversity in the Tropical Island Pacific Region: I. Species Systematics and Information Management Priorities. Program on Environment, East-West Center, Honolulu, Hawaii, pp. 205-238.

Pyle, R.L. 1996. How much coral reef biodiversity are we missing? Global biodiversity, 6(1):3-7 (Published in both English and French versions).

Pyle, R.L. 1996. The Twilight Zone. Natural History, 105(11):59-62.

Pyle, R.L. 1996. A Learner's Guide to Closed Circuit Rebreather Diving. In: Proceedings of the Rebreather Forum 2.0. 26-28 September, 1996. Redondo Beach, CA, pp. P45-P67.

Sharkey, P. and R.L. Pyle. 1992. The Twilight Zone: The potential, problems, and theory behind using mixed gas, surface-based scuba for research diving between 200 and 500 feet. In: Diving for Science...1992, proceedings of the American Academy of Underwater Sciences Twelfth Annual Scientific Diving Symposium. American Academy of Underwater Sciences, Costa Mesa, CA.

Shinn, E.A. and R.I. Wicklund. 1989. Artificial reef observations from a manned submersible off southeast Florida. Bull. Mar. Sci. 44(2): 1041-1050.

Silverstein, J. 1995. Richard Pyle Ph.D. (Phish Doctor): an exclusive interview. Sub Aqua Journal. 5(2):16-19, 4 figs.

Somers, L.H. 1992. Chapter 18. Looking Ahead: Mixed Gas in Scientific Diving. In: Mount, T. and B. Gilliam (Eds.). Mixed Gas Diving: The Ultimate Challenge for Technical Diving. Watersport Publishing, Inc., San Diego. 392 pp.

Starck, W.A. III and J.D. Starck. 1972. From the Bahamas to Belize: Probing the deep reef's hidden realm. Nat. Geogr. 149(12):867-886.

Strasburg, D.W., E.C. Jones and R.T.B. Iverson. 1968. Use of a small submarine for biological and oceanographic research. J. Cons. perm. int. Explor. Mer. 31(3):410-426.

Thresher, R.E. and P.L. Colin. 1986. Trophic structure, diversity, and abundance of fishes of the deep reef (30-300 m) at Enewetak, Marshall Islands. Bull. Mar. Sci. 38(1):253-272.

Vinogradova, N.G. 1979. Vertical zonation in the distribution of deep-sea benthic fauna in the ocean. Deep-Sea Res. 8:245-250.

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