Chapter 13

1. Intertidal Communities Chapter 13

2. 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.

3. 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.

4. Key Concepts • Biotic factors are most important in determining the distribution of organisms on rocky shores, but physical factors are most important on sandy shores.

5. 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

7. 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

8. 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, dissication • lower regions exposed only a short time before tide returns to cover them

10. 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

11. Rocky Shore Zonation • Zone system (continued) • midlittoral zone—the true intertidal zone; extensive part that is regularly exposed during low tides and covered during high tides • infralittoral fringe—area extending from the lowest of low tides to the upper limits reached by large kelps (laminarians) • subtidal (infralittoral) zone—the region of shore covered by water, even during low tide

14. 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

15. 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

21. 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

22. 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

23. 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

24. 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

26. 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

28. 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

30. Tropical Rocky Shores • Less temperature variation and seasonal rainfall • higher temperatures are more stressful • less temperature variation and fewer storms are less stressful • Zonation patterns on intertidal rocks • supralittoral fringe and midlittoral fringe each have major subdivisions evident in tropical areas

31. Supralittoral Fringe • White zone—the true border between the land and the sea • hermit crabs, isopods, knobby periwinkles • Gray zone • knobby (and other) periwinkles • nerites – an exclusively tropical group that tends to replace limpets in higher intertidal zones • farthest zone from low tide line where macroscopic marine algae grow

32. Supralittoral Fringe • Black zone • immersed only at the highest spring tides • lacks knobby periwinkle • several species of algae and cyanobacteria dominate • smaller periwinkles, other nerites, fuzzy chitons

33. Midlittoral Fringe • Yellow zone • microscopic boring algae covering its surface give it a yellow or green color • barnacles, limpets, fuzzy chitons, rock snails, irregular worm snails

35. Midlittoral Fringe • Pink zone • sometimes underlies the yellow zone • characterized by widespread encrustation of coralline algae • irregular worm snail, mats of anemones, keyhole limpets, gastropods

37. 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

38. 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

39. Comparison of Temperate and Tropical Rocky Intertidal Systems • Higher stress and predation in tropical environment • Abundant mobile invertebrates, but fewer sessile ones in the tropics • Large body size or residence in higher areas more important means of avoiding predators in temperate areas • Macroalgae have less impact on community structure in the tropics

40. 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

41. 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

42. Ecology of the Rocky Shore • Rocky shores in the tropics • total predation is strong, and control of competitively dominant species is spread over a number of consumers • Top-down and bottom-up factors • top-down factors—factors whose effects flow down the food chain: competition, herbivory and predation • bottom-up factors—factors that affect the basal level of food chains: nutrient availability, recruitment

43. Sandy Shores • Many temperate and tropical shorelines consist of sandy beaches • Extend almost continuously from Cape Cod south to the Gulf Coast on the east coast of North America

44. Role of Waves and Sediments • Sediment particle size influences the beach’s nature, porosity of sediments, ability of animals to burrow • Wave action influences sediment type: • heavy wave action = coarse sediments • little wave action = fine sediments • Beach slope results from interaction of waves, sediment particle size, and relationship of swash and backwash

45. Role of Waves and Sediments • swash—water running up a beach after a wave breaks • backwash—water flowing down the beach • Types of beaches: • dissipative beach—wave energy is strong but is dissipated in a surf zone some distance from the beach face • usually flat with fine sediment • reflective beach—wave energy is directly dissipated on the beach • usually steep with course sediment

46. Role of Waves and Sediments • On all sandy beaches, a cushion of water separates the grains of sand below a certain depth • especially true on beaches with fine sand where capillary action is greatest • Fine sand beaches have a greater abundance of organisms • greater water retention • sediment is more suitable for burrowing

48. Comparison of Rocky and Sandy Shores • Sandy shores lack readily apparent pattern of zonation • wave action is most important factor in determining organism distribution • temperature has less effect because of insulating properties of sand and water retention • oxygen levels may be low beneath the sand owing to lack of water exchange

49. Sandy Shore Zonation • Less defined pattern of longitudinal zonation • Vertical zonation exists among organisms buried in the sand • depends on amount of water trapped at each level • 3 major zones: • supralittoral • midlittoral • subtidal (infralittoral)