Survival in Coral Reef Habitats

Clarification Statement: Examples of the environment affecting a trait could include that normally tall plants grown with insufficient water are stunted and that a pet dog given too much food and little exercise may become overweight.

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This activity builds on the content below.

Variation of Traits

Variation among individuals of the same species can be explained by both genetic and environmental factors. Individuals within a species have similar but not identical genes. Although genes control the general traits of any given organism, external environmental factors can modify an individual’s specific development, appearance, behavior, and likelihood of producing offspring. Differences in where they grow or in the food they consume may cause organisms that are related to end up looking or behaving differently. The set of variations of genes present, together with the interactions of genes with their environment, determines the distribution of variation of traits in a population. 

All About Corals

Corals are simple invertebrate animals. The corals that comprise coral reefs are colonial animals that are composed of hundreds and hundreds of individual polyps. Each coral polyp has a mouth surrounded by tentacles that leads to a stomach. Coral polyps living in a colony are connected to each other through their stomachs by a specialized tissue layer termed coenocarc. The tentacles capture smaller animals and plants that are floating in the water column and bring the food to the mouth to be digested. The tentacles contain special stinging cells called cnidocytes. These special cells release stinging structures called nematocysts that aid in the animals’ defense against predators as well as their own capture of food organisms. Reef building corals also house single-celled marine algae called zooxanthellae in their tissues. These small plants use sunlight and the animal’s waste products in photosynthesis to make food for the coral. This is the main reason why coral reefs are only found in shallow, warm, sunny parts of the ocean, as the zooxanthellae need light in order to photosynthesize. The relationship between the zooxanthellae and the coral is called a mutualism. It is a symbiotic relationship where both organisms benefit. The zooxanthellae live within the coral and receive nutrients (especially nitrogen) from the coral, while the coral receives food that the zooxanthellae make through photosynthesis.

Coral Reproduction

All corals, whether hard or soft, reproduce by both sexual and asexual strategies. Reef-building corals are colonial organisms that form colonies through asexual reproduction. Asexual reproduction can occur in many forms of which the most common types will be discussed here. To understand the formation of coral reefs, we must start with an individual coral polyp. Budding and fission occur when a single coral polyp slowly cleaves itself in half to form two distinct polyps that are genetically identical. When polyps continue this process, they eventually form a coral colony of many genetically identical individuals, which are also termed clones. Although coral colonies start small, some of them, over time, can become massive structures as they continue to reproduce clones through budding. Fragmentation, another type of asexual reproduction, occurs when wave action, or some other disturbance, causes a portion of the coral colony to become separated, or break off the original parent colony, and continues to grow on its own. Over time, these modes of asexual reproduction function to create numerous coral colonies that collectively form a coral reef.


Sexual reproduction also exists in corals, and takes place in colonies whose polyps become sexually mature. Sexual reproduction in corals is not easy considering they are sessile animals. They get around this limitation by releasing their gametes into the water column for external fertilization, a process termed broadcast spawning. Broadcast spawning is usually synchronized between individual species, and often involves a seasonal, monthly, or daily component. Corals can be simultaneous hermaphrodites, meaning that a polyp can release both eggs and sperm as bundles, or they can be gonochoric (gone-o-KOR-ik), releasing either eggs or sperm, but not both. For example, the endemic Hawaiian mushroom coral Fungia scutaria is a gonochoric species that spawns in the early evening within 1–3 days of a full moon in the summer months of June–September. After fertilization occurs, the coral larvae, also called planula (PLAN-u-la), may stay in the water column, drifting with the plankton until they find a suitable habitat to settle and metamorphose into a juvenile coral polyp, a process termed larval recruitment. Once the planula has settled and metamorphosed, the single polyp will soon start to reproduce asexually by budding, creating a new colony, and the process starts again. Important mechanisms in the formation of a coral reef include the dispersal of planktonic planula, and the rafting of small colonies that essentially hitch a ride by settling on floating marine debris. Some coral planula can be in the plankton for up to 100 days, allowing plenty of time to find new habitats. Rafting in particular is thought to be key in the arrival of many of Hawai‘i’s marine organisms. Bottles, sandals, pumice stone, wood, and many other types of floating objects have been found to host small coral colonies, as well as other sessile organisms like algae and sponges, that have settled on the object.


Heredity explains why offspring resemble, but are not identical to, their parents and is a unifying biological principle. Heredity refers to specific mechanisms by which characteristics or traits are passed from one generation to the next via genes. Genes encode the information for making specific proteins, which are responsible for the specific traits of an individual. Each gene can have several variants, called alleles, which code for different variants of the trait in question. Genes reside in a cell’s chromosomes, each of which contains many genes. Every cell of any individual organism contains the identical set of chromosomes. When organisms reproduce, genetic information is transferred to their offspring. In species that reproduce sexually, each cell contains two variants of each chromosome, one inherited from each parent. Thus sexual reproduction gives rise to a new combination of chromosome pairs with variations between parent and offspring.


Very rarely, mutations also cause variations, which may be harmful, neutral, or occasionally advantageous for an individual. Environmental as well as genetic variation and the relative dominance of each of the genes in a pair play an important role in how traits develop within an individual. Complex relationships between genes and interactions of genes with the environment determine how an organism will develop and function.


Genes and the Environment

Many characteristics of organisms are inherited from their parents. Other characteristics result from individuals’ interactions with the environment, which can range from diet to learning. Many characteristics involve both inheritance and environment.


Types of Corals

Common hard corals found in Hawai‘i include:

Finger coral (Porites compressa) pōhaku puna: 

This coral is aptly named because of its stalky branches forming finger-like shapes that dominate vast swaths of reef in the reef slope zone. The finger corals grow fast where wave action is mild and light penetration is sufficient. These corals quickly out-compete other species, becoming the dominant coral on the reef slope zone.

Lobe coral (Porites lobata) pōhaku puna:

Related to the finger coral, lobe coral is a massive, mounding coral. It grows slow, forming large lobes instead of discreet fingers, and can reach great sizes covering several meters or more. Because of its mounding shape, it can withstand high wave action and predominates in the reef bench zone where the faster growing, out-competing finger coral cannot grow.

Cauliflower coral (Pocillopora meandrina):

This coral has sturdy, thick, flat-leafed branches and grows in small, discreet tufted colonies. It is most common in high-energy wave areas, but can be found in all three reef zones.

Rice coral (Montipora sp):

This coral takes on a variety of forms, growing vertically into fragile finger-like projections in calm, shallow, well- lit habitats of the reef slope zone. It can also grow into large protruding plate-like structures in calm, deeper, less well lit lower slope zones to obtain sunlight.

Coral Reef Zones

Reef Crest Zone:

The reef crest zone is the highest (most shallow) part of the reef, and lies between the shoreward, protected back reef zone and the outer fore reef zone. This zone is characterized by a relatively uniform bottom topography made up of coral sand and loose coral rubble and exists from a depth of approximately 0–5 meters. Shallow growing corals may be exposed during extreme low tides. The wave action in the reef crest zone is medium to high, and light penetration is also high. The sea surface temperature can be variable in this zone and can be influenced by day-to-day weather conditions. For example, on a cloudless day, the sun’s intensity can warm the water in this shallow zone more so than if clouds were present. Winds also affect the temperature; on windy days, the water cools faster than on days when only light breezes are blowing. Sediments can become suspended in the water column because of wave action and land influences, affecting the clarity of the water in the reef bench zone. The dominant corals present include the massive lobe coral that can tolerate the higher energy parts of the reef. Below the immediate shallows, an assortment of Hawaiian corals thrive in this zone, including the finger coral, cauliflower coral, mushroom coral, lace coral, and rice coral.


Fore Reef Zone:

The fore reef zone (often called the “reef front”) begins at the seaward base of the reef crest. The bottom topography characteristic of the reef slope zone is a gentle sloping bottom with fine grain sand, sediments, and coral rubble interspersed between large colonies of rice coral and finger coral to a depth of 30 meters. The wave action is low to absent, and the light penetration is minimally reduced because of the depth at which this zone resides. The temperature in this zone is more uniform but because of the light reduction, it is common to see large coral colonies in this zone competing for open access to light. Suspended sediments tend to rest here because of the lack of wave action; the water clarity is generally better than in the reef crest zone. The finger coral dominates this part of the reef. However, most of the other coral types previously described can also be found here. (Except for lobe coral, the reef gets out-competed by finger coral because of differences in their respective growth rates.)


Back Reef Zone:

The back reef zone (also commonly called the lagoon) of a coral reef lies immediately shoreward of the innermost margin of the reef crest zone, and extends all the way to the shore. The back reef zone has significantly less coral cover, and the bottom topography consists of mostly sand with large amounts of coral rubble to a depth of 40 meters. Wave action is almost absent in all but the harshest swells and weather but deeper currents can exist. Light penetration is also reduced and the temperature tends to be uniform. In harsh weather or during big swells, the sandy bottom can become churned up, reducing water clarity. The sandy bottom is not the best place for corals to settle; small colonies of lobe and lace coral are found here. Other invertebrates, like sea cucumbers, dominate the sandy bottoms. Because of their young geological age, the eight main Hawaiian Islands generally lack barrier reefs that shelter protected lagoons and give rise to the typical reef flat, reef crest, reef slope nomenclature of reef systems. Kāne‘ohe Bay on the island of O‘ahu is the only local example of these reef types. The geologically older Northwestern Hawaiian Islands (now the Papahānaumokuākea Marine National Monument), on the other hand, have many barrier reefs, and the reef systems that exist there are more characteristic of reef systems around the world. The three reef zones described above reflect more local geological reef systems typified by the main Hawaiian Islands.


Coral Vocabulary

  • Allele: one of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome.
  • Chromosome: a threadlike structure found in the nucleus of most living cells, carrying genetic information in the form of genes.
  • Gene: a unit of heredity which is transferred from a parent to offspring and is held to determine some characteristic of the offspring.
  • Heredity: the specific mechanisms by which characteristics or traits are passed from one generation to the next via genes.
  • Mutation: the changing of the structure of a gene, resulting in a variant form that may be transmitted to subsequent generations, caused by the alteration of single base units in DNA, or the deletion, insertion, or rearrangement of larger sections of genes or chromosomes.








In the above teacher guide pdfs, there are presenter notes in yellow boxes in the upper, left corner. This presentation comes from the original SEA curriculum.

Survival in a Habitat

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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.

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Fig. 1. Image courtesy of Wikimedia Commons

Organisms often require specific abilities and adaptations to survive in a given environment or habitat. 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, org


Shoreline Habitats

Shoreline habitats, also referred to as coastal ecosystems, are often further categorized into rocky intertidal, sandy shoreline, and wetland ecosystems (Fig. 1). The conditions within each ecosystem varies, therefore creating specific environments in which creatures must be adapted to survive.


Rocky Intertidal Zone

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Fig. 1. An example of a dynamic coastline on the island of O'ahu.

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Image courtesy of Darold Massaro, GoodFreePhotos

The area along our coastlines where land and sea intersect is known as the intertidal zone. The the area between the high and low tide line. Included in the intertidal zone are tide pools, which are pools of water isolated from the rest of the ocean during low tide, and the splash zone, which is above the high-tide line but gets occasional splash from waves and salt spray from wind. Several factors are important in determining the types of organisms found in a given intertidal community, including: air/sun exposure, substrate type, salinity, and wave action.

The intertidal zone can be divided into three different regions, depending on the amount of time they are covered by water. The high intertidal zone is covered by water during high tide only. This can be a harsh environment to live in because of long periods of exposure to the sun and air, high salinity levels, and often high wave exposure. The middle intertidal zone is covered by water about 50% of the time (per tidal cycle). Temperatures in this zone are less extreme because of shorter exposure times to the sun, and consequently, salinity levels are only slightly higher than the sea. The low intertidal zone is only uncovered during low tide and is the least variable habitat of the intertidal zone. As one progresses from the high intertidal zone to the low intertidal zone, the diversity of organisms tends to increase. 

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Fig. 2. This diagram clearly depicts the areas on a coastal shoreline. 

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Image courtesy of the original SEA curriculum

The tidal range, which is the difference between high and low tide, is highly variable between locations and collectively defines the size of the intertidal zone. For example, the tidal range at the Bay of Fundy in Nova Scotia can be as much as 17 meters while some areas in the Caribbean can have a tidal range as low as 0.5 meters. Because Hawai‘i’s daily tidal fluctuations are small, the tidal range in Hawai‘i is only about 1 meter. 

Some areas of Hawai‘i get regular swells that result in a large splash zone. In these cases, it is wave action, not tidal flux, which influences what animals can live there. In other areas of Hawai‘i where the shoreline is protected from waves, such as O‘ahu’s Kahana Bay, there is a predictable assemblage of organisms at different tidal heights, a phenomenon called tidal zonation. Turf algae dominate the low mark, or low intertidal zone. Higher in the intertidal zone, one finds successively: the mussels Isognomon californicum and Brachidontes crebristriatus (nahawele li‘ili‘i), and the introduced species of barnacles Balanus amphitrite and Chthamalus proteus. Higher in the intertidal zone, above the barnacle band, one finds the limpet Siphonaria normalis (‘opihi ‘awa) and the Nerite snail Nerita picea (pipipi). In the splash zone, above the high tide mark, are various littorine snail species (Periwinkles), and an isopod (a type of crustacean with seven pairs of legs and a flattened oval body). Research in the intertidal zone has received little attention in Hawai‘i, and much work still needs to be done to characterize this habitat.

Organisms Found in the Rocky Intertidal Zone

An organism’s ability to withstand exposure to the highly variable environmental conditions of the intertidal is a major factor in determining what zone they are able to live in. Intertidal organisms have developed special adaptations to help them survive. Because oxygen is extracted from the water by marine organisms, a major challenge in the intertidal is the ability to not dry out when exposed to the air and sun during low tides. Different animals have different mechanisms to cope with this. Mobile animals such as crabs, like the ‘a‘ama crab (Grapsus tenuicrustatus), are able to periodically move back into the water to remain moist. They will also move into small crevices in the rock where small pockets of moisture, and shade from the hot sun can be found. Rock boring urchins (‘ina, Echinometra mathaei and E. oblonga) are able to create their own shelter by using their hard spines and scraping jaws to enlarge natural holes in the rock. Sessile animals, like barnacles, limpets and mussels, can not move back into the water, and have other mechanisms to deal with drying out. The Hawaiian mussel (nahawele li‘ili‘i, B. crebristriarus) is able to close its shell tight to remain moist. Limpets (‘opihi, Cellana exarata and C. sandwicensis) have a muscular foot, which allows them to seal their shells to the rocks to remain moist. Slow-moving motile organisms, like the black nerite snail (pipipi, N. picea) have an operculum which they can seal against their shell to remain moist.


Organisms must also be able to resist wave action in the intertidal. Just as the ‘opihi’s muscular foot allows them to remain moist, it also allows them to hold on to its substrate so that it is not swept away by waves. Mussels, such as nahawele li’ili’i, are able to attach themselves to the rocks with byssal threads secreted by their foot. The organisms in the intertidal also tend to be small which decreases their chance of getting swept away by the waves. 

Plants in the Intertidal Zone

Many species of plants are found along the littoral fringe, also known as the splash zone because they are splashed by waves or sprayed by salt carried by the wind. These organisms also have special adaptations to help them survive in the difficult intertidal ecosystem. Leaf hairs and shiny leaf surfaces help to reflect the sun’s rays, and to prevent heating and slow down evaporation. The thick and fleshy tissues of succulent plants help to store water, and their waxy leaf surfaces help prevent water loss. Special leaf arrangements minimize the amount of leaf surface exposed to the sun, helping the plant stay cool. Many plants are low to the ground with small leaves and shallow, spreading root systems which protect them from wind, and keep them anchored in shifting sands or barren rock. Plants found along Hawai‘i’s littoral fringe include the beach naupaka (Scaevola sericea), beach morning glory, pōhuehue (Ipomoea pes-caprae), beach vitex (pōhinahina, Vitex rotundifolia), beach ‘ilima (Sida fallax), false sandlewood, naio (Myoporum sandwicense), and the endangered species ‘ōhai (Sesbania tomentosa).

Sandy Shorelines

Organisms in the Sandy Shorelines

Wetland Habitats

Organisms in Wetlands


Shoreline Habitat Vocabulary

  • Adaptation: a feature of an organism that has evolved over a period of time by the process of natural selection such that it increases its long-term reproductive success
  • Byssal thread: strong threads secreted by mussels to attach to rocks and large, generally heavy objects in the intertidal zone
  • High intertidal zone: area of the intertidal zone covered by water during high tide only 
  • Intertidal zone: the area between low- and high-tide marks and alternately covered by water and exposed to air during each tidal cycle
  • Littoral fringe: area of land adjacent to the intertidal zone
  • Littoral zone: synonym for intertidal zone
  • Low intertidal zone: area of the intertidal zone exposed only during low tide
  • Middle intertidal zone: area of the intertidal zone that is covered by water approximately half the time of each tidal cycle
  • Operculum: a lid-like covering which serves as a protective “door,” sealing the opening to the shell of gastropods when the animal withdraws into the shell
  • Salinity: the amount of dissolved salts present in a liquid
  • Sessile: organisms that remain attached to a substrate
  • Splash zone: area above the high-tide mark in the intertidal zone that gets occasional splash from waves and salt spray from wind
  • Tidal range: the difference between high and low tide; defines the size of the intertidal zone.
  • Tide pool: a pool of water left along the shore as the tide level falls

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