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Structure to Function

Clarification Statement: Examples of structures could include thorns, stems, roots, colored petals, heart, stomach, lung, brain, and skin.


Assessment Boundary: Assessment is limited to macroscopic structures within plant and animal systems.

Table of Contents

This activity builds on the content below.

Structure and Function

A central feature of life is that organisms grow, reproduce, and die. Organisms have unique characteristic structures, functions, and behaviors. Coral reefs are ecosystems that provide examples of specific structures, or body designs, that serve designated functions. The reef  structure creates numerous and varying habitats that allow a variety of organisms to exist and fulfill roles, or niches, in the reef community. Fish and other animals in the reef habitat have evolved various feeding methods and body structures that allow them to survive and thrive in reef environments. 

To learn more about fish life in general, check out these additional resources!

Reef Fish Feeding


There are many sizes, shapes, and colors of reef fishes living in coral reef habitats (Fig. 1). Fishes have a variety of methods and adaptations for feeding, depending on the area of the reef they inhabit and the composition of their diet. Reef fishes live in areas above the reef, on the surface of the reef, in reef crevices and caves, and on the sandy bottom near the reef. Reef fishes often have special mouth features to help them feed—which they use to feed in specific style and eat a particular type of food.

Herbivores feed on marine plants called algae. Algae usually grow on the surface of the reef; some fishes even farm their own algae beds. Herbivores have mouth forms and teeth designed to scrape the algae off of rocks and coral. Reef fish that eat zooplankton, invertebrates, and other fish are called carnivores. Carnivores have unique mouth forms and teeth, depending on the type of animal they eat (Fig. 2).



Some carnivores have small mouths useful for plucking zooplankton from the water column. Other carnivores have large mouths that help engulf their prey, allowing the fish to swallow its prey whole. Many carnivores have unique mouth features that help them feed in a particular area of the reef, or on a particular type of prey. For example, the longnose butterflyfish has a very long snout that helps it probe into deep crevices to find worms and other invertebrates (Fig. 3); the goatfish has structures hanging from its lower jaw (barbels) that help it dig in the sand to find invertebrates. 

Reef fish that are omnivores eat both algae and animals. Omnivorous fish do not have as specialized a diet as fishes that are strictly herbivores or carnivores. Omnivores are sometimes called generalists.

Feeding Structures

Some of the unique modifications in mouth shapes and body features found in common Hawaiian reef fishes are described below:

Butterflyfishes (Chaetodontidae):


These brightly colored reef fish have very short, small mouths that allow them to graze on algae growing on the reef. butterfly fishes also eat small invertebrates like coral polyps and anemones. Butterfly fishes are considered omnivores. However, some butterfly fish have very specialized, long snouts they use to reach into the coral and eat coral polyps (e.g., Longnose butterfly fish, lauwiliwilinukunukuoi‘oi) (Fig. 3).

Damselfishes (Pomacentridae):


Damselfish are often found in small groups hovering right above the reef in search of tiny animals and plants floating in the plankton. They have short snouts but relatively big mouths that allow them to suck the plankton out of the water column (e.g., sergeant major fish, mamo, Fig. 4). Many damselfishes are also gardners of algae; the fish weed out algae they dislike and protect the algae they prefer to eat.

Moray Eels (Muraenidae); puhi:


Eels are carnivorous ambush predators that wait in cracks and crevices in the reef to capture passing prey. Eels possess big sharp teeth and have large mouths (Fig. 5).

Goatfishes (Mullidae); weke‘ā:


Goatfishes have mouths on the bottom of their snouts. Goatfishes also have two barbels, or feelers, for tasting and searching the sandy bottom for worms and other soft-bodied invertebrates that live in the sand (Fig. 6).

Parrotfishes (Scaridae); uhu:


Parrotfishes have large teeth that are fused together and strong jaws to enable them to scrape algae and coral from the reef. Parrotfishes are primarily herbivores (Fig 7).

The above article from Wired showcases the absurdidites of the parrotfish adaptations for survival! See the original link below:

Absurd Creature of the Week: This Goofy Fish Poops out White-Sand Beaches by Matt Simon


Body Structures for Defense

Life on a reef can be dangerous for many organisms—because there is always the chance that a predator may be lurking. The evolution of defense structures has resulted in many unique features that help reef organisms survive. Examples of defense structures in common Hawaiian reef fishes, and associated reef organisms, follow:


Surgeonfishes are named for the knifelike spines, or scalpels, on either side of the base of their tails. One swipe of the tail can slash an enemy or intruder (Fig. 8).


Butterflyfishes have many spines disguised in their fins. They use very vivid coloration to announce their potential danger to a predator or enemy (Fig. 9a). Many different types of butterflyfishes, like the millet seed butterfly fish (lauwiliwili), also have big false eye spots near their tails to confuse predators concerning their actual size (Fig. 9b).


Porcupine pufferfishes not only inflate their bodies with water to avoid being eaten, but they also have large spikes that poke out from their bodies to prevent capture (Fig. 10).


Scorpionfishes use coloration to blend into their habitat, effectively hiding themselves by looking like a part of the reef. Because these fish are ambush predators, they also use this advantage to capture unknowing prey. Scorpionfishes also have poisonous spines in their fins (Fig. 11).


Sea snails and nudibranchs use bright coloration patterns to warn potential predators that they may taste bad or are poisonous (Fig. 12).


Corals also have specialized defense mechanisms called nematocysts. Nematocysts are stinging cells that are released when the coral is harassed by predators. Since corals cannot move or shake off any sediment or other particles that may fall on them, they also continually shed mucous to slough off anything that might try to settle on them (Fig. 13).


Crabs have large claws that they use to defend themselves against predators. Crabs also use their claws to catch food. Certain types of hermit crabs often keep anemones on their shells for protection (Fig. 14).


Anemones have special stinging cells in their tentacles, just like corals. When a predator tries to eat the hermit crab, it gets a mouthful of nematocysts instead.



Adaptations Vocabulary
  • Algae: simple one-celled or many-celled aquatic plants that lack true roots and leaves.
  • Ambush predator: an organism that hides and waits for prey to pass, rather than actively chasing prey.
  • Barbel: a whisker-like organ near the mouth found in some fish. The structure and function of barbels varies.
  • Carnivore: an animal that feeds on other animals.
  • Function: an adaptation that contributes to evolutionary fitness.
  • Herbivore: animals that eat plants.
  • Nematocysts: a specialized cell in the tentacles of jellyfish, corals, and related organisms. Nematocysts contain barbed and venomous coiled threads that can be projected in self-defense or to capture prey.
  • Niche: the ecological role of an organism in a community.
  • Omnivore: animals that eat both plants and animals.
  • Prey: organism that is caught and eaten by a predator.
  • Structure: the arrangement of and relation between the parts of something complex.
  • Zooplankton: animal plankton, consisting of small animals and the immature stages of larger animals.




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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.