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Survival in the Open Ocean Habitat

Clarification Statement: Examples of evidence could include the needs and characteristics of the organisms and habitats involved. The organisms and their habitats make up a system in which the parts depend on each other.

This activity build on the content below.

In the above teacher guide pdf, there are presenter notes in yellow boxes in the upper, left corner.

Adaptation to the Environment

Organisms often require specific abilities and adaptations to survive in a given environment or habitat. A polar bear, for example, wouldn't survive well in a desert, just as a kangaroo wouldn't do well in polar regions. Even more drastically, a dolphin wouldn't survive at all in a rainforsest! Organisms have traits (morphologicalphysiological, or behavioral) that provide an advantage in a specific environment. Natural selection provides a mechanism for species to adapt to changes in their environment and the resulting selective pressures influence the survival and reproduction of organisms over many generations. Adaptation can lead to organisms that are better suited for their environment because they will pass desired traits on to their offspring, whereas individuals with traits that are less adaptive produce fewer or no offspring. For example, peacock flounder can camouflage into the surrounding sediment to avoid predation, ensuring their survival and reproductive success (Fig. 1). Sometimes, adaptation over time can lead to the formation of new species. In some cases, however, organisms may not be able to adapt to changes in the environment and the species becomes extinct. Adaptive changes due to natural selection have strongly contributed to the planet’s biodiversity.

<p>Fig. 1. This flounder in Kona, Hawai'i can blend in with a variety of backgrounds so it can remain hidden from predators to survive another day.</p><br />

Zones of The Ocean

In the vast and complex ocean, there are zones from the surface to the deep sea that are characterized by differing physical and biological properties. The open ocean habitat (or the oceanic zone) generally begins at the outer edge of the continental shelf, extending from the surface down to the deepest depths of the ocean. The water environment of this region, collectively referred to as the pelagic zone, is further subdivided into the epipelagic, mesopelagic, bathypelagic, and benthopelagic zones. Each of these zones is delineated by both depth and by the relative penetration of sunlight through the water.

In the epipelagic zone (0–200 meters, 0-657 feet), penetrating sunlight allows for relatively high photosynthetic activity of phytoplankton (generally, microscopic algae), providing energy and food sources for other organisms. This “lighted” region is therefore also encompassed by, and referred to as the photic zone. Below the epipelagic (or photic zone), deeper water organisms must rely primarily on energy sources generated from the photic zone above. It is at this depth that the mesopelagic (200–1,000 meters, 656-3281 feet) begins, followed by the bathypelagic (1,000–4,000 meters, 3281-13123 feet), and finally the benthopelagic (4,000–6,000 meters, 13123.4-19685 feet) zone. In these regions, little to no light penetrates, and therefore all of these deeper and darker depths fall into the region known as the aphotic zone

Creatures of the Open Ocean

The open ocean is vast, and large areas are lifeless while some areas are teeming with life. Currents flow like rivers under the surface of the ocean. These currents influence everything, from the concentration of marine life to weather systems on land. There is also a huge vertical variety of animals and fish, from sunlit surface waters, down through the disphotic (“twilight”) zone, to almost complete darkness and crushing pressures below 1,000 meters. Animals higher up the food chain need to travel long distances to find food in order to survive. Some fish predators are large, powerful, and fast, allowing them to cross the oceans in search of food. Sailfish are the fastest fish in the ocean, reaching speeds of up to 75 mph (120 km/h). Bluefin tuna are almost as fast, and can accelerate faster than a Porsche sports car.

Many pelagic species can be characterized as continuous swimmers, form schools, and/or have countershading that make them difficult to see from above or below. Schooling behaviors of many fish are thought to be a protective behaviors that help them avoid predation by confusing potential predators. Large numbers of fish, such as mackerel scad (‘opelu) commonly found in Hawai‘i, form a school and have abrupt movements that almost look choreographed help them to survive. The sharks (mano), rays, dolphins, and whales are darker on the body’s dorsal (top) side, and lighter on the ventral (under) side (i.e. countershading). This makes these fish more difficult to spot from above (they blend in with the darker deep water below them), or from below (they blend in with the clearer shallow water above them).

Tunas and sword fishes are fast, long-distance swimmers, having efficient circulatory systems that provide the necessary energy to cover the vast expanse of ocean to find scattered prey resources. Many smaller pelagic fish can fly out of the water to escape predators. The flying fish, for example, can actually double their escape velocity while airborne. Many whale species migrate thousands of kilometers each year, between their warm breeding grounds and their rich Arctic and Antarctic feeding grounds. Marine turtles make long voyages across the oceans, between their nesting beaches and feeding grounds.

The Deep Sea

There are some land features in the open ocean: underwater mountains known as seamounts. Formed by volcanoes or sinking islands, seamounts rise steeply from the ocean floor. Deep-water currents are forced up their sides, bringing nutrient-rich water to the surface around shallow seamounts. This allows plankton to thrive, thus supporting complex food chains, and making these areas islands of life in the open ocean. Some fish species periodically gather at seamounts to feed and spawn, while marine turtles and whales stop at shallow seamounts for food and shelter during their long migrations.

Animals living in the disphotic zone have various adaptations for living in the dimly lit waters. Some species have enormous eyes to find food. To avoid being eaten, many disphotic zone animals are transparent, including squid and crustaceans. Some fish have silvery reflective scales to help make them invisible.

Life in the deep-sea is scarce, so predators need large teeth and mouths to cope with whatever crosses their path.The fanged-tooth fish, for example, has the largest teeth of all marine animals in relation to its body size, while gulpers can swallow preys at least as large as themselves.

Open Ocean Vocabulary
  • Adaptation: a change or the process of change by which an organism or species becomes better suited to its environment.
  • Aphotic zone: the deeper regions of the water environment in which no light penetrates; generally the bathypelagic and benthopelagic zones
  • Bathypelagic zone: follows the mesopelagic zone; generally 1,000–4,000 meters in depth, low to no light penetration; together with the benthopelagic zone falls into the aphotic or midnight zone
  • Benthic zone: the ocean floor
  • Benthopelagic zone: follows the bathypelagic zone; generally 4,000–6,000 meters in depth, no light penetration, and falls into the aphotic or midnight zone; sometimes referred to as the abyssal zone
  • Benthos: the bottom or bed of a body of water, including the sand, mud, silt, and organisms that live there; also benthic community
  • bioluminescence: the production of light by a living organism, usually through a symbiotic association with light-producing bacteria
  • camouflage: the blending of an animal with its surroundings
  • carnivores: animals that eat other animals
  • continental shelf: the shallow, gradually sloping seabed around a continental or island margin, usually not deeper than 200 meters
  • convergence: the evolution of similar heritable traits attributable to natural selection from pressures in similar environments, as opposed to resulting from common genetic lineage
  • Countershading: type of camouflage coloration commonly found in animals and means that the animal's back (dorsal side) is dark while its underside (ventral side) is light. This shading helps an animal blend in with its surroundings
  • “darkness” zone: bottom most layer of the pelagic zone, where light does not reach
  • Disphotic or “twilight” zone: the poorly lit zone of the ocean below the photic zone
  • Epipelagic zone: the upper region of the pelagic zone where light penetrates, typically 0–200 meters in depth; also referred to as the photic zone; in common discourse used interchangeably with pelagic zone
  • evolution: the process through which a population of organisms accumulates genetic changes over generations that can lead to adaptations to environmental conditions
  • habitat: a place in which organisms live
  • herbivores: animals that eat plants
  • Mesopelagic zone: follows the epipelagic zone; generally 200–1,000 meters in depth, with low light penetration; commonly referred to as the disphotic zone
  • Morphological: relating to the form or structure of things.
  • Natural Selection: the process whereby organisms better adapted to their environment tend to survive and produce more offspring. The theory of its action was first fully expounded by Charles Darwin and is now believed to be the main process that brings about evolution.
  • Oceanic zone: the deep ocean waters, away from the in uence of land, generally beginning at the outer edge of the continental shelf
  • Open ocean: deep ocean waters that are not close to land masses
  • Pelagic zone: the water environment of the open oceanic zone
  • Physiological: relating to the way in which a living organism or bodily part functions.
  • predators: animals that hunt and kill other animals
  • prey: an animal that is hunted, killed, and eaten by another animal
  • Photic zone: the upper region of the water environment in which light penetrates, allowing for photosynthesis, generally in the epipelagic zone; sometimes referred to as the sunlight zone or euphotic zone
  • Phytoplankton: the plant forms of plankton; generally small microscopic algae and diatoms
  • plankton: small, usually microscopic organisms that drift, or swim in the water column
  • Selective pressure: Any cause that reduces reproductive success in a portion of a population, driving natural selection. Examples of selective pressures include competition, predation, land clearance, pollutants, diseases and illnesses, climate change and parasitism.
  • Schools: organized groups of sh that move and turn together as one
  • “sunlight” zone: the upper zone of the ocean with enough photic for plants to survive
  • “twilight” zone: the poorly lit zone of the ocean below the photic zone
  • venomous: an animal which possesses poison that can be delivered into the body of another animal by a bite or sting
  • zooplankton: small, usually microscopic, organisms that drift in the ocean


NERRS: Survival of the Fittest

Representative Image: 
Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.