Hawaiian intertidal zones
Hawaiian intertidal zones are unique ecosystems located along the coastlines of the Hawaiian Islands, characterized by their diverse habitats shaped by geographic position and local topography. Despite being home to a lower species richness compared to other Pacific Islands, these zones support a variety of native and endemic species, some of which are found nowhere else on Earth. The harsh conditions in these areas, including intense sunlight, high temperatures, and nutrient-poor waters, pose challenges for survival, leading many organisms to seek shelter under rocks or within crevices during low tides.
The intertidal zones exhibit a limited but distinct zonation, with different species thriving at various heights along the shore. For instance, algae and mollusks populate the lower zones, while barnacles and limpets are found higher up. The complex interplay of habitats, including tide pools, sandy shorelines, and estuaries, fosters diverse communities, though these ecosystems face significant threats from pollution, invasive species, and climate change. Rising sea levels challenge the adaptability of local species, further jeopardizing these sensitive environments. Despite the challenges, conservation efforts are underway to protect these vital ecosystems and their unique biological heritage.
Subject Terms
Hawaiian intertidal zones
- Category: Marine and Oceanic Biomes.
- Geographic Location: Pacific Ocean.
- Summary: Hawaii's multivarious intertidal zones are strongly influenced by their geographic position and local topography.
Located in the center of the Pacific Ocean in the extreme northern corner of the Indo-Pacific Triangle and influenced by currents generally unfavorable for larval settlement, the Hawaiian Intertidal Zones' species richness is low compared with that of other Pacific Islands. Steep coastlines and a small tidal range of about 3 feet (1 meter) discourage the development of intertidal zonation and, thus, habitat and species diversity. Nevertheless, Hawaii's intertidal regions support a unique complex of native and endemic (found nowhere else on Earth) species.
Life in Hawaii's intertidal is harsh. Organisms are exposed to intense sunlight and high air temperatures. Waters are often nutrient-poor, and waves can be punishing. Many organisms are small and camouflaged, hiding under rocks or crevices during daytime low tides. Only urchins and limpets can withstand the wave-beaten areas.
In more protected areas, intertidal zonation can occur. The lowest zones support algae; mussel beds and barnacles occur higher up. Limpets and nerite snails scavenge in and above the barnacle band. Above this band, the splash zone and littoral fringe support still others, such as littorine snail species and isopod crustaceans as well as maritime vegetation adapted to saline soils and sea spray. Grapsid crabs scuttle and range throughout these areas as well.
Intertidal Habitats and Biota
While intertidal zonation is generally reduced compared to the Pacific region as a whole, Hawaii's varied topography creates some variety in intertidal habitats. Calcareous (carbonate-based) and basalt shorelines feature conspicuously-zoned horizontal benches that can extend up to 98 feet (30 meters) seaward. Calcareous shorelines also contain pitted limestone pools that are inhabited by a small littorine snail and a blenny fish, as well as thick turfs of algae that support a complex food web of suspension-feeding, herbivorous, and carnivorous mollusks. On basalt benches, alternating growths of algae create colorful mosaics, while out in the surf zone, algae- and coral-encrusted basalt boulders host a variety of wave-tolerant worms, mollusks, echinoderms, and fishes.
Tide pools occur on these calcareous and basalt shorelines. More exposed pools have a sandy bottom bound with blue-green algae; they typically contain two or three species of small mollusks, grapsid crabs, and two fishes: a blenny and a goby. Seaward, pools become progressively more densely turfed with algae, hosting a variety of worms, mollusks, crustaceans, and echinoderms. Especially common are the spaghetti worm and the bandana shrimp. Seaward pools also serve as incubators for juvenile fishes.
Sandy shorelines provide a different range of habitats, with beach slope and sand-grain size largely determining community composition. The upper beach, including the vegetation line, provides habitat for amphipods, isopods, and ghost-crab males, while female ghost crabs and males of a different species occur lower down. The lower beach, continually awash, supports scavenging mole crabs and four species of gastropods that prey on spionid polychaetes—a family of small, thin, filter-feeding worms that build tubes of sand and other particles.
Estuarine ecosystems occur where rivers and streams meet the sea. In Hawaii, these brackish ecosystems support many endemic species, especially fishes and mollusks, and are the primary habitat of the edible Samoan crab. They are also important nursery areas for several species of in-shore marine fishes. Native oysters occur in estuaries that receive significant freshwater infusions. Many Hawaiian estuaries, unfortunately, are threatened by invasive exotic plants and animals.
Mangrove habitats are important and imperiled worldwide, but in Hawaii mangroves are introduced, lacking natural predators to control their growth. Thus, they often invade coastal regions, out-competing native plants, trapping sediment, and destroying habitat for native plants and animals. Mangrove-eradication efforts are under way statewide.
Though reefs are generally subtidal, outer edges and reef flats may be exposed at low spring tides, creating intertidal conditions. Hawaii's reefs are composed primarily of crustose coralline algae, tolerant of the relatively lower temperatures found at these latitudes. Outer reef edges are densely fringed with frondose algae, while reef flats are patchy and diverse, containing calcareous algae and cnidarian corals, stands of frondose algae, rubble, and sand patches. Sand patches host burrowing mollusks and spionid polychaetes. Other types of mollusks, echinoderms, and fishes are common among the rubble and living coral, and a large fan worm occurs in pockets and crevices.
Environmental Threats
Threats to Hawaii's intertidal include pollution, invasive species, and climate change. Intertidal zones are believed to be significantly impacted by climate change. Changes in sea levels brought about by climate change have threatened the delicate ecosystems of the intertidal. Experts explain that species learn to live in certain parts of the intertidal zone where they best benefit the ecosystem. However, rising sea levels mean they must be able to adapt more quickly, which is impossible for many species. This will harm ecosystems because these species will become extinct and the number of invasive species will increase. Invasive fish like tilapia and guppies also prey on native species, like the opae 'ula, and have disrupted the balance of the ecosystem, which has posed yet another environmental threat.
Bibliography
“Hawaii Coastal Resilience.” Coastal Resilience, coastalresilience.org/project/hawaii/. Accessed 23 Oct. 2024.
Inouye, Byron. “About Hawai'i's Intertidal and the OPIHI Program.” College of Education, University of Hawaiʻi at Mānoa, coe.hawaii.edu/opihi/about-opihi/. Accessed 23 Oct. 2024.
“Intertidal.” National Park Service, 13 May 2016, www.nps.gov/subjects/oceans/intertidal.htm. Accessed 23 Oct. 2024.
Kay, E. A. “Marine Ecosystems in the Hawaiian Islands.” In A Natural History of the Hawaiian Islands, Selected Readings II, E. Alison Kay, ed. Honolulu: University of Hawaii Press, 1994.
"'Opihi Age, Growth, and Longevity Influenced by Hawaiian Intertidal Environment." Science News, 30 Aug. 2021, www.sciencedaily.com/releases/2021/08/210830163930.htm. Accessed 23 Oct. 2024.
Ward, Rebecca J., et al. “Spatial Variation and Antecedent Sea Surface Temperature Conditions Influence Hawaiian Intertidal Community Structure.” PloS One, vol. 18, no. 6, 2 June 2023. doi:10.1371/journal.pone.0286136. Accessed 23 Oct. 2024.