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Behavior and Sensory Systems

NGSS PE Linking Sentence
The content and activities in this topic will work towards building an understanding of the individual and group behavior of fish within the world ocean.

The behavior of many fishes can be observed by students who scuba dive or snorkel. You can also observe fish behavior at an aquarium or in a fish tank. A behavior is an organism’s response to a stimulus. A stimulus (plural: stimuli) is a signal detectable by an organism. Fish behaviors are complex and often change as a fish grows and matures.

Behaviors are also influenced by changes in the environment. Fish react not only to other organisms, but also to their habitats. Habitat preference is behavior related to where an organism lives. Some fishes, such as many damselfish, prefer specific substrates, such as rock, sand, or coral, which they behaviorally remain close to.

Fishes can be broadly categorized into two large groups by their habitat preference: pelagic or demersal. Pelagic fishes live and feed in the open ocean or in lakes away from the bottom and shoreline. Examples of pelagic fish species include tunas, anchovies, whale shark, and swordfish. In contrast, demersal fishes live near or directly on the bottom of a water body. Examples of demersal fish species include flat-bodied flounders and reef-dwelling surgeonfish and clownfish.

The most obvious behaviors to observe are the movements of a fish. Some types of movements include swimming, hovering, perching, and lying on the bottom. Some fishes swim all the time—either moving around from place to place or hovering in a single area for long periods. Other fishes stay on the bottom most of the time. Changes in movement by fish are often accompanied by other distinct behaviors.

Schooling is a fish behavior that occurs when there is a bunching or grouping of fish all orienting in one direction and keeping equal spacing. Within a school or aggregate of fish there is often a pecking order, which is the organization among fish by levels of authority. Dominant individuals control the behaviors of other fish within the group.

Stress behaviors are responses to disruptive, disturbing, or unfamiliar stimuli. When fish are threatened, for example, they may exhibit behaviors of aggression. To show aggression, many fishes raise their dorsal fin spines, push out their gill covers, open their mouths, and approach other fish. A response to aggressive behaviors might be fright, when a fish might move away or change color. Fishes may also use camouflage, a behavior used to disguise their presence in an environment, often to protect themselves from predators or to conceal themselves from prey. A fish that changes its appearance to blend with coral, rocks, or sand is camouflaging itself.

Fishes must also forage for food. Fishes exhibit food-searching behaviors before and during feeding. Some fishes actively hunt for their prey, whereas others feed more passively and alongside other species. Some fishes lie in wait to dart out and snatch passing prey. Other fishes may become territorial to defend their living space or their mate or while finding food.

Advertising coloration occurs when fish alter their color to send signals to other animals. Warning coloration can often be seen on poisonous fish and on dangerous spines. Fish can also use movements to signal their wants—these are advertising behaviors. Changes in color and behavior can often be observed during courtship, both before and during mating.

Predation is another type of behavior. Predation occurs when an organism kills and consumes another organism. Mahi mahi that catch and eat flying fish are predators, but so are plankton feeding fishes, like manta rays (Fig. 4.81). To be a true predator, an animal must kill and eat its food. Fish like the male anglerfish, that takes its food without killing the female host, is a parasite, not a predator. Scavengers like hagfish that feed on shed or dead plant and animal material are also not predators as they eat food that is already dead and do not have to kill it.

Image caption

Fig. 4.81. A manta ray feeding on tiny planktonic organisms.

Image copyright and source

Image Courtesy of Gordon Flood 

Nervous System

Behavior is controlled by the nervous system. The nervous system is a complex group of tissues and organs that control most body processes. The nervous system has two major divisions: the central nervous system and the peripheral nervous system. The central nervous system consists of the brain and the spinal cord (Fig. 4.82). The brain receives information from sense organs that monitor conditions both within and around the fish. The brain interprets this information and sends response commands to the body. The brain, a soft and delicate organ, is surrounded and protected by the bones of the skull.

Image caption

Fig. 4.82. Brain and sense organs of a fish

Image copyright and source

Image from Living Ocean, CRDG, University of Hawaii at Manoa

Fig. 4.82. Brain and sense organs of a fish
Felo 7–14 (but note that it is misnumbered in LO as 7-10, on p.81

A fish’s brain is divided into six major parts (Fig. 4.82). Two olfactory bulbs, which control the organs of smell, sit side by side at the end of long stalks at the very front of the brain. They receive information from the nostrils about chemicals in the water. In most fishes, the olfactory bulbs comprise the largest part of the brain, indicating the importance of smell.

The cerebrum consists of two lobes behind the olfactory bulbs. The cerebrum controls the voluntary muscles. It also stores memories. In higher animals, such as humans, the cerebrum is the thinking part of the brain. In general, fishes have limited thinking ability and limited memory—only enough to learn simple tasks and adapt to new environments. Some fishes are specialized to remember their home territories. For example, butterfly fishes and tidepool fishes form mental maps of their territory and can find their way home after being displaced for long periods of time.

Two optic lobes lying just behind the cerebrum control vision. Large nerves called optic nerves connect the optic lobes to the eyes. The pituitary gland, beneath the optic lobes, is the master gland of the endocrine system of the body.

The cerebellum is behind the optic lobes. This portion of the brain is the control center for coordinating skeletal muscles once contraction and relaxation is initiated by the cerebrum. When the command goes out from the cerebrum to swimming muscles, for example, the cerebellum takes over and ensures that the muscles work in proper rhythm and order. The medulla, lying under the cerebellum, connects the brain to the spinal cord. One of its functions is to control the flow of hormones in the fish. It also controls the heart, the smooth muscles of the internal organs, and the rhythmic contraction of gill muscles in respiration.

The other part of the central nervous system is the spinal cord, which runs through holes in the vertebrae and acts like a communication wire. Composed of long nerve fibers, it carries nerve impulses to sensory receptors in the brain and transmits nerve impulses from the brain to the muscles, glands, and other tissues.

The peripheral nervous system is the network of nerves that connect muscles and sensory organs to the central nervous system. There are two types of nerves: sensory nerves, which carry information from the sensory organs to the spinal cord or brain, and motor nerves, which send commands from the brain or spinal cord to the muscles and glands.

The sensory organs send the information they receive to the central nervous system and the brain. Fishes use their sensory organs to detect changes in their bodies and in their environment. Sensory organs include the eyes, the ears, the lateral lines, the nostrils, and the taste organs. Each of these sense organs is equipped with sensory nerve endings.


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.