Intertidal Communities

The Rocky Shore

Key Concepts

• The intertidal zone is the coastal area alternately exposed and submerged by tides.

• Organisms that inhabit intertidal zones must be able to tolerate radical changes in temperature, salinity, and moisture and also be able to withstand wave shock.

Key Concepts

• Organisms on rocky shores tend to be found in definite bands, or zones, on the rocks.

• In contrast to sandy shores, rocky shores provide a relatively stable surface for attachment.

• Tide pool organisms must be able to adjust to abrupt changes in temperature, salinity, pH, and oxygen levels …abiotic factors.

Key Concepts

• Biotic factors are most important in determining the distribution of organisms on rocky shores, whereas physical factors are most important on sandy shores.

Characteristics of the Intertidal Zone

• Daily fluctuations of the environment– organisms must tolerate radical changes in

temperature, salinity and moisture, and endure the crushing force of waves

• Inhabitants are most active during high tide, when area is submerged– water provides food for filter feeders

• As the tide retreats, organisms adjust to exposure to air and sunlight

Rocky Shores

• Composed of hard materials

• Found from California to Alaska on the west coast and from Cape Cod northward on the east coast of North America

• Coasts may be recently uplifted, formed from lava flows, or highly eroded by wind and waves

Rocky Shore Zonation• Zonation—separation of organisms into prominent

horizontal bands defined by color or distribution of organisms

• Rocks provide a stable surface for attachment of organisms

• As tide retreats...– upper regions exposed to air, changing

temperatures, solar radiation, desiccation– lower regions exposed only a short time before

tide returns to cover them

Zonation

Rocky Shore Zonation

• Zone system for rocky shore proposed by Alan and Anne Stephenson– Supralittoral (maritime) Zone — area

above high water that may extend several miles inland

– Supralittoral fringe (splash zone) —uppermost area covered only by the highest (spring) tide, usually just dampened by spray of crashing waves

Rocky Shore Zonation

• Supralittoral fringe of rocky shores– receives very little moisture– exposed to drying heat of the sun in summer and

extreme cold in winter– few organisms inhabit this harsh area– gray and orange lichens composed of fungi and

algae are common– sea hair – a filamentous alga – most common animal = periwinkles, molluscs of

Littorina and associated genera

Lichen zoneGray & Yellow lichen and the Green lichen

– Gray & orange lichens common in this zone– Symbiotic relationship: mutualism between and algae and

a fungus.– Fungus traps traps the water; alga undergoes

photosynthesis and produces food

Rocky Shore Zonation

• Supralittoral fringe (continued)– other inhabitants include limpets and isopods– some periwinkles and isopods breathe air

• Midlittoral zone of rocky shores– inhabitants must avoid desiccation, maintain gas

exchange, and deal with temperature extremes as the tide moves in and out

– wave shock—force of the waves as they crash against the rocks during low tide

Littorina littorea

Periwinkles (Littorina cincta) get through low tide by clustering in a moist, shady crevice. They also seal against the rock to retain moisture.

Rocky Shore Zonation

• Midlittoral zone (continued)– Upper midlittoral zone

• typical organisms = acorn barnacles and rock barnacles

• barnacles permanently attach to surfaces

• barnacles open their shells to filter feed during high tide, and close them to trap water inside during low tide

• barnacles cool themselves by opening the shell slightly and allowing a little water to evaporate

Dog Whelk Snail & Barnacles

Rocky Shore Zonation• Midlittoral zone (continued)

– Middle and low midlittoral zone• oysters, mussels, limpets, and periwinkles dominate

• oysters and mussels survive low tides by trapping water in their shells

• limpets and chitons graze algae at high tide

• common periwinkles bury themselves in seaweed to retain moisture during low tide

• rock urchins survive wave shock by hollowing out a space in the rock and wedging into it

Midlittoral Zone

• Major organism is predominately Mytilus sp.

• Attaches firmly to rocks via byssal threads

Mussels & barnacles

Rocky Shore Zonation• Midlittoral zone (continued)

– Seaweeds of the midlittoral zone• rockweeds (brown algae) grow on rocks without full

exposure to the sea

• rockweeds compete with barnacles for space by sweeping the rocky surface with their blades, preventing cyprid larvae of barnacles from settling

• some rockweeds are toxic to deter grazing

• rockweeds produce a gelatinous covering that retards water loss and prevents desiccation

• they form large mats that trap water and provide a haven for animals during low tide

Bladderwrack Fucus vesiculosus

• Bladderwrack Fucus vesiculosus Bladderwrack is the most well known of the wracks. Can be found in large beds on Cornwalls ﾕ rocky shores. With a quick look at bladderwrack it appears that there are two types of bladder. The knobbly bladder like organs at the end of some of the fronds, are receptacles. In spring the female receptacles are covered by orange mucus and the males by green mucus.

Rocky Shore Zonation• Midlittoral zone (continued)

– Tide pools—depressions in the rocks which retain water during low tide

• water loses oxygen as it heats in the sun

• salinity may change owing to rainfall or evaporation of water by the sun

• oxygen in tide pools containing algae may change drastically – high during the day when algae are active, low (+ low pH) at night

• salinity, temperature, pH abruptly returned to ocean conditions when tide reaches the pool

Rocky Shore Zonation• Midlittoral zone (continued)

– Tide pool organisms• e.g. algae, sea stars, anemones, tube worms, hermit

crabs, molluscs

• many are filter feeders

• biotic interactions help structure tide pool communities

• e.g., in New England, Irish moss lives in tide pools where common periwinkles eat the green alga Enteromorpha, which normally outcompetes Irish moss

When the tide goes out, a bit of ocean spray stays in this small tide pool. Organisms like these chitons (Sypharochiton sp.) and snails (Diloma sp.) move to the pools to keep moist. Even here, life is tough as the water often undergoes drastic changes in salinity, dissolved oxygen (D.O.) content and temperature.

Rocky Shore Zonation• Infralittoral fringe of rocky shores

– transitional area submerged except at spring tides

– rich flora and fauna of organisms that can tolerate limited air exposure

– rocks may be covered with seaweeds

– in cooler waters, molluscs, sea stars and brittle stars live among large kelps

– other animals include hydrozoans, anemones, sea urchins, spider crabs

Infralittoral Fringe

• surf zone includes the edge of the lower rocky platform and parts of the reef

• rocks may be covered with algae such as Sargassum

• boring urchins, anemones, sponges, bryozoa, sea cucumbers, keyhole limpets

Subtidal Zone• Relatively barren compared to subtidal zone

in temperate climates

• Small, turf-forming red algae dominate– lack of larger algae present in higher zones

thought to be owing to herbivory

– in experiments, exclusion of herbivores permitted erect algae to establish themselves where they were not found previously

Intertidal Fishes

• Resident species– Typically have special adaptations for surviving

harsh intertidal conditions• small size; absent, reduced or firmly attached scales;

compressed/elongate or depressed body shape; absent or reduced swim bladder; greater body density

• tolerant of temperature and salinity changes• some intertidal fish can leave the water to feed

• Temporary inhabitants– tidal, seasonal and accidental visitors

Ecology of the Rocky Shore

• East coast rocky shores– barnacles dominate upper zones– below the barnacles are mussels– algae and consumers survive on protected shores

• West coast rocky shores– barnacles compete with algae; mussels displace

barnacles by growing over them– balance maintained by ochre sea stars, the

keystone predator

• Rocky shores are the most densely inhabited by macroorganisms.

• Have the greatest diversity of animals and seaweeds.

• On the east coast, rocky shores are found from Cape Cod northward.

• Rocky shores are defined by prominent hoizontal bands, defined by color and distribution of organisms.

• The separation of organisms into definite bands is called zonation.

• Unlike the other habitats we've studied, rocks provide organisms with a stable surface for attachment.

• The characteristic abiotic factors of intertidal zones apply here.

• Organisms present may vary depending upon wave action, tidal cycle, climate, length of exposure, amount of light, shape of shore and type of rock.

• The uppermost area of rocky shore which is covered only by the highest. (spring) tides and is usually just dampened by the spray of the crashing waves is the supralittoral fringe or splash zone.

The Rocky Shore Habitat

REVIEW

• Above this is the supralittoral or maritime zone, an area above the high water mark.

• Below the supralittoral fringe is the midlittoral zone (true intertidal zone).

• Below the midlittoral zone is the infralittoral fringe, which extends from the lowest of low tides to the upper limits reached by the large kelps.

• The subtidal zone (infralittoral) is the region of shore covered by water, even during low tide.