15.7: Beyond the Sun's Light

Below the surface layers of the oceans, organisms must be adapted to extremely low light levels, to uniformly low temperatures, to a detritus-based food supply that decreases steadily with depth, and to increasing pressure. These factors interact to give pelagic and benthic communities of the bathyal zone and abyssal zone characteristics that are very different from those of communities that live near the surface.

Relatively little is known about bathyal and abyssal communities because the vast volume of the ocean that they inhabit has been visited only for fleeting moments by research submersibles, which many denizens of the deep undoubtedly avoid. More is being learned from ROVs but even ROVs are expensive to operate at these depths, and likely also avoided by many species, so progress is slow. In addition, collecting samples from the deep oceans is very difficult and expensive. 

The creatures that live in the deep oceans zones range from rather familiar forms that resemble fishes and invertebrates of the shallower ocean to incredibly bizarre-looking creatures that would be well suited to science fiction movies. Figure 15-17 shows just a small selection of the fishes that inhabit the aphotic zone.

Figure 15-17. Mid-water fish species are often very strange-looking and have unusual adaptations, such as unhingeable jaws and hugely expandable stomachs, strange eyes, lures, and light-emitting photophores. Many of these adaptations aid these species to cope with the scarcity of food and difficulty of locating mates and prey.

Organisms living below the photic zone have four potential sources of food: particulate detritus that sinks slowly through the water column, carcasses of large animals that sink rapidly to the ocean floor because of their size, prey species that live in the aphotic zone, and prey species that live in the photic zone above. Of these, only prey species in the photic zone are abundant, so many deep-water species migrate to this zone to feed. All other deep-ocean biota must be adapted to a low and uncertain food supply.

Many species of crustaceans (such as shrimps, copepods, and amphipods), other invertebrates (such as squid), and numerous fish species live in the part of the water column immediately below the photic zone, between about 200 and 1000 m. Many of these species migrate vertically up into the photic zone at night to feed and then return to the depths during the day. Other species prey on species that live in their own depth zone. Vertical migrants include the unique Nautilus (Fig. 12-24e,f), which has a chambered shell to provide buoyancy. To avoid problems with pressure changes as the Nautilus migrates vertically, the rigid internal buoyancy chambers contain gas at a low pressure. To adjust its buoyancy it pumps small amounts of water into and out of its internal chambers. Since this increases or decreases its mass, and density is mass divided by volume, this allows the nautilus to control or change its depth. Oceanographers profiling floats achieve the same buoyancy control by applying the same principle but changing their volume rather than their mass by pumping small amounts of oil between their interior and an external expandable membrane.

There is very little or no light below 200 m, and no red light penetrates to this depth. Many organisms of this zone are red or dark colored (Figs. 15-17, 15-18), so they do not reflect any of the ambient light, and this makes them difficult to see in the near darkness. However, many fishes and other species that live at these depths have greatly enlarged eyes to enable them to hunt prey in the dim light.

Figure 15-18. “Dumbo” octopuses named for their floppy ear-like fins that they use to move through the water, are members of a genus of octopi called Grimpoteuthis. The largest Dumbo octopus so far recorded was about 1.8 m in length and weighed about 5.9 kg. They live in the hadal and abyssal zones and have been observed as deep as 7000 m in the Java Trench. As food is rare they eat almost anything including worms, crustaceans, mollusks, and small fish. Unlike other octopuses, they can not swim by jet propulsion and do not produce ink to evade predators. These are likely adaptations that reduce metabolic energy expenditures needed for these functions in response to the relative absence of predators and light for them to hunt by, and the scarcity of food supplies for the octopuses themselves.

The easiest way to hunt visually for prey in waters below about 200 m where light still penetrates is to look upward into the very dim light filtering down from above and search for the dark silhouette of the prey species. Consequently, many fish species of this zone have eyes that look directly upward. To counter this hunting strategy, many prey species have a series of light-producing organs called “photophores” arrayed along the underside of their body. By illuminating their underside with these photophores, they can reduce the sharpness of their silhouette and blend better into the dimly lit background above. Many species also have photophores arrayed on other parts of their body, presumably as devices to identify members of their own species or to attract prey species.

With increasing depth below 1000 m, fewer and fewer species are present that migrate vertically. Many organisms are brightly colored, although, in the absence of light, color does not advertise their presence and so is irrelevant. Eyes become less prominent and absent in many species, but some species that live in the absolute darkness of the deep oceans do have eyes that, at least in some fishes, are adapted to detect bioluminescent light produced by many animal species and bacteria in this otherwise lightless zone, likely for recognition or mate attraction, and used by predators for prey location. Adaptations, found in some fish species suggest that they may be able to detect a single photon of bioluminescence and even to distinguish between wavelengths (see colors) which no other animals are known to be able to do at such low light levels.

Because of the low abundance and widespread dispersal of organisms, all fishes of mid and deep waters must be adapted to take advantage of any prey species they encounter. Therefore, many of these fishes have unhingeable jaws and soft expandable bodies, so they can swallow and digest prey species as large as, or larger than, themselves. Some also have highly sensitive chemosensory (chemical-sensing) systems that allow them to detect even slight changes in chemical traces coming from live or decaying prey.

On most of the deep-ocean floor, the food supply comes from above as a rain of detrital particles and as occasional carcasses of large animals. Much of the detrital material has already been subjected to substantial decomposition during its slow descent through the water column. Hence, what remains is primarily material that is difficult to digest and that decomposes only very slowly. This material has little immediate food value for animals. This detritus is normally consumed by bacteria in the surface sediment, and it is the bacteria that become food for larger animals. Because the suspended particulate food supply near the deep seafloor is at very low concentrations and is of low nutritional value, there are few suspension feeders. Most animals of the deep seafloor are deposit feeders, including many species of sea cucumbers and brittle stars. The particulate food supply in some areas near the continents may be supplemented by detritus carried to the deep seafloor in turbidity currents.

Although we have little direct information about such events, it is known that bodies of large animals must sink rapidly to the seafloor with some frequency. When bait is lowered to the deep seafloor, a variety of fishes, sharks, crustaceans, and other invertebrate scavengers (Fig. 15-17) arrive to feed on the bait within as little as 30 min. Once the bait has been eaten, these animals disappear into the darkness. Some bait, such as a massive whale fall can create an ephemeral, yet long lasting ecosystem that can provide energy for decades. They can also act as “stepping stones” across the barren seafloor, providing energy for the gathering scavengers to disperse and colonize new areas. How these scavengers find the bait or their normal food is not known, but the speed at which they appear suggests that they must have extremely sensitive chemosensory organs for this purpose. It has been observed that many of these scavengers can sense, but cannot find, bait if it is suspended even a meter or less above the seafloor, which demonstrates that they are completely adapted to feeding on material that lies on the seafloor.

The speed with which food is found by deep-ocean scavengers after it reaches the seafloor indicates fierce competition for food, because food does not decompose rapidly at such depths. Bacterial decomposition is inhibited by high pressures, so food items that fall to the deep-ocean floor would decompose only very slowly if they were not immediately consumed. Inhibition of bacterial decomposition by high pressure was first discovered when the research submersible Alvin sank unexpectedly in more than 1500 m of water. A lunch box containing an apple and a bologna sandwich was inside the submersible when it sank. A year later, when Alvin was recovered, these food items were wet but almost undecomposed. They did decompose within weeks after their return to the surface, despite being refrigerated. Inhibition of bacterial decomposition by high pressures has been confirmed by a number of experiments conducted since the Alvin discovery.