Marine biology

The earliest known life forms were marine. The oceans cover 71 percent of Earth’s surface and have an average depth of 3,800 meters, meaning they represent 99 percent of the living space on the planet. Marine biology is the study of all the organisms that occupy this space. Despite having 99 percent of the planet’s living space at their disposal, only 250,000 of approximately 1.8 million described living species (14 percent) are marine. While the oceans may lack diversity at the species level, they are home to members of thirty-one of approximately thirty-four animal phyla, about twice the number of phyla that are represented on land or in freshwater.

Because of their vastness and humans’ inability to easily visit deep waters, the oceans remain the least studied habitats on Earth. For example, scientists know that giant squid eighteen meters in length, larger than any other invertebrate, exist because they have been found in the stomachs of sperm whales and washed up on beaches. However, no human has ever seen one of these creatures living in their natural habitats. Yet despite the comparatively small amount of study in marine biology, research in these systems has contributed greatly to the understanding of living systems.

Oceanographers divide the ocean into zones that have distinct physical characteristics, which, in turn, select organisms that are adapted to those characteristics. The benthic realm refers to the sea floor. It extends from the high intertidal zone, where the ocean meets terrestrial land, to depths of eleven kilometers at the oceanic trenches, the deepest parts of the ocean where the ocean floor slowly sinks back into the interior of the Earth. Organisms that live in, on, or near the ocean floor are considered benthic organisms and represent 98 percent of all marine creatures. The pelagic realm of the ocean refers to the open ocean, basically the space between the benthos and the sea surface. Pelagic organisms, representing the remaining 2 percent of marine species, live in the water column. The pelagic zone can be divided into the photic and aphotic zones, a distinction that is especially important for photosynthetic organisms. The photic zone is the shallow part of the ocean that receives enough sunlight to support photosynthesis, which is about two hundred meters deep in the clearest waters, and as shallow as three meters in turbid coastal waters. The aphotic zone is where there is not enough light to support photosynthesis; it extends from the bottom of the photic zone to the ocean floor.

Organisms of the Open Ocean

Pelagic creatures can be further categorized into plankton, which drift around in the ocean currents, and nekton, which can swim against currents. Photosynthetic plankton live in the photic zone and are called phytoplankton. These algae form the base of the food web in the open ocean and are eaten by zooplankton, or heterotrophic plankton, a category that includes protists and small crustaceans such as copepods. Zooplankton are eaten by small fishes, which are eaten by larger fishes, which are eaten by sharks, birds of prey, and people. The open ocean food web is one of the longest food webs known, partly because it starts with the smallest photosynthetic organisms.

The largest animal migrations on Earth occur every day in the open oceans, in a process called diel vertical migration. Zooplankton, midwater fish, squid, and krill migrate to shallow waters at night and then return to the dark depths during the day. The main reasons for this daily migration are probably to feed and to avoid being eaten. Food densities are highest in the shallow, productive waters, so these predators move to the shallows to feed at night. Because they would be susceptible to their own visually oriented predators in the well-lit shallows, they return to the dark depths during the day to avoid being eaten.

In the aphotic zone, there are only heterotrophic organisms that are supported mostly by organic material that rains down from the lit environments above. These animals live in darkness, with the exception of light produced by the animals themselves, called bioluminescence. It is common for sea creatures, especially ones at intermediate depths, to house luminescent bacteria within their tissues, which can produce light to communicate, lure prey, or illuminate the creature's bottom surface to conceal its silhouette against the dimly lit background from above. Anglerfish are deep-sea predators that attract prey near their mouths by dangling a bioluminescent lure in front of their heads.

The density of organisms in the deep sea is low. Because of this low density, a long period of time can pass between meals or between encounters with the opposite sex. To deal with the problem of infrequent meals, deep-sea creatures are often gigantic compared to their shallow-water counterparts. Their large size allows for the storage of food reserves that sustain the animals between meals. Predatory fish also have large mouths and stomachs that allow them to take full advantage of any meal, regardless of its size. To overcome the problem of rare mates, the miniature males of some anglerfish have the unusual adaptation of attaching themselves to a female, where they live the rest of their lives as parasitic sperm producers.

Organisms of the Bottom

Most marine species are found on the ocean floor. They occur in such familiar marine habitats as mud flats, sand flats, beaches, coral reefs, kelp forests, and the rocky intertidal. The main primary producers in benthic habitats are macroscopic seaweeds that grow attached to the bottom or microscopic algae that grow within the tissues of animals such as corals, sponges, and bryozoans. Benthic animals include mobile creatures such as fish, crabs, shrimp, snails, urchins, sea stars, and slugs. Additionally, there are numerous animals that, unlike familiar terrestrial animals, never move around as adults. These sessile animals include barnacles, sponges, oysters, mussels, corals, gorgonians, crinoids, hydroids, and bryozoans.

The commonness of sessile animals in the marine benthos suggests that it is a successful way of life. These animals’ lifestyle combines aspects of plant and animal lifestyles. Sessile invertebrates are plantlike in that they obtain some of their energy from sunlight (the animals themselves do not photosynthesize, but they house photosynthetic symbionts), are anchored in place, and grow in a modular fashion, like the branches of a tree. They are animallike in that they capture and digest prey and undergo embryonic development, often involving metamorphosis. In fact, nearly all benthic animals start life in the pelagic realm, drifting around as planktonic larvae and dispersing to new habitats as they develop and feed. After a few hours to several weeks of pelagic living, they sink to the ocean floor to complete life as adults.

Being stuck in one place presents special challenges for sessile animals, including food acquisition, predator avoidance, and mating. Sessile animals feed by having symbiotic algae and by filtering organic particles from passing water currents. Like plants, sessile animals use structural and chemical defenses against predators and have tremendous regenerative abilities to recover from partial predation events. Most benthic animals mate via external fertilization: sperm and eggs are spawned into the water column, and fertilization occurs outside the female’s body. Amazingly, sessile barnacles must copulate to achieve internal fertilization. These animals increase their reproductive success by being hermaphroditic, thus ensuring that any neighbor is a potential mate, being gregarious to assure a high density of mates, and having a penis long enough to deliver sperm to an individual seven shell lengths away.

Trophic Cascades and Keystone Predators

Marine organisms live in environments that are foreign to humans, and they have lifestyles that are unique to them and different from terrestrial creatures. However, the study of marine organisms has led to advances in ecological theory that have proved to be useful in understanding terrestrial communities as well. The concept of keystone species, where a relatively rare species has a disproportionately large effect on the community structure, was discovered from research conducted on rocky intertidal habitats. Professor Robert Paine removed starfish from rocks off the coast of Washington State. He observed that mussels soon crowded out seaweeds and barnacles, resulting in about a 50 percent reduction in species richness. He concluded that starfish are keystone predators because their predation prevented mussels from excluding less competitive species from the habitat.

Not far offshore from the rocky intertidal in kelp forests, another keystone species was shown to influence the community by means of a trophic cascade, in which the effects from a top predator “cascade” down to lower trophic levels. Sea otters, by preying on sea urchins, protect the kelps that make kelp forests, an important habitat that many marine species rely on. In the 1800s, hunters greatly reduced the number of otters by harvesting their thick furs. As a result, sea urchin populations exploded because they were relieved from predation, resulting in the decimation of kelps by herbivorous urchins. Once otters received government protection, their numbers increased, sea urchins decreased, and kelp forests returned, at least in some areas. Interestingly, Aleutian killer whales began preying on otters in the 1980s, causing kelp forests to disappear again. The reason orcas began eating otters was probably due to concurrent declines in seal and sea lion numbers, the normal prey of these killer whales. The ultimate causes of these altered food webs are uncertain, but they almost certainly result from human activity. Even though humans are poorly adapted for a marine existence, the evidence is mounting that humans are altering marine ecosystems in complex, novel, and unpredictable ways. The science of marine ecology is best equipped to study these effects and offer information to protect and manage ocean life. In 2022, marine biologists estimated that only 5 percent of the world’s oceans had been explored; by 2024, just 26.1 percent of world's ocean floor had been mapped.

Principal Terms

Autotrophs: primary producers; organisms that are self-feeding, including photosynthetic and chemoautotrophic organisms

Benthic: the area of the ocean floor; organisms associated with the sea bottom

Epifauna: animals that live on the sea floor

Heterotroph: consumers; organisms that must acquire energy by consuming organic material

Infauna: animals that live on the sea floor

Invertebrates: animals that lack backbones

Littoral: the area in the intertidal zone; organisms that live in the intertidal

Nekton: organisms that are strong swimmers and can move against ocean currents

Pelagic: the area of open water in the oceans; organisms that occur in the water column

Plankton: organisms that drift in the ocean currents because they have limited power of locomotion

Bibliography

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