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Adaptations in the Shoreline 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.

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

<p>Fig. 1. An example of a dynamic coastline on the island of O'ahu.</p><br />
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. 

<p>Fig. 2. This diagram clearly depicts the areas on a coastal shoreline.&nbsp;</p><br />

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
 
 
 

NERRS: Understanding Adaptation

 
 
 
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.