1. Intertidal Communities
Chapter 13
Intertidal Communities

2. Key Concepts
The intertidal zone is that part of the marine environment alternately exposed and submerged by tides.
Organisms that inhabit intertidal zones must be able to tolerate wave shock, desiccation and radical changes in temperature and salinity.

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. Intertidal Communities
Found within the shoreline area reached by waters of the highest high tide and uncovered at the lowest low tide
Stressful environment due to interaction of physical factors, e.g., wind, waves, sunlight
Organisms must be able to withstand desiccation, searing heat and freezing

6. Rocky Shores Composed of hard materials
More densely and have greater diversity of algae and animals than sand/mud intertidal regions
Found from California to Alaska on the west coast and from Cape Cod northward on the east coast of North America

7. Adaptations to Life on Rocky Shores
Organisms on rocky shores are faced with challenges not encountered by organisms in the deeper ocean
alternately submerged by incoming tide and exposed to air by ebbing tide
exposed to salinity fluctuations due to rain and evaporation
pounded by high energy waves

8. Avoiding Overheating and Desiccation
Organisms attached to high intertidal locations face greatest challenge to maintain suitable body temperature and avoid water loss – adaptations include:
large body exposes less surface area, e.g., knobby periwinkle
light color reduces heat gain
special adaptations of kidneys
aggregate in large clumps
mobile animals (crabs) move down as tide retreats into crevices of sheltered areas
close shells at low tide, e.g., barnacles and mussels
clamping down tightly to rock, e.g., limpets and chitons
produce gelatinous covering, e.g., rockweed

9. Coping with Cold
Organisms in high latitudes produce anti-freezing type compounds during winter

10. Avoiding Wave Shock
Animals exhibit compressed or dorsally flattened bodies or shells
Adhere tightly to rock
barnacles cement themselves
limpets, chitons and snails have enlarged foot for attachment
sea stars use tube feet
rock urchins hollow out cavities
mussels attach with byssal threads
intertidal algae are flexible and attach with holdfast organ

11. Adaptations to Salinity
Rainstorms can flood intertidal tide pools decreasing salinity and evaporation on hot days can elevate salinity
many organisms have adapted to tolerate these changes or move to other areas

12. 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

13. Figure 13-7 ROCKY SHORE ZONATION.

14. COMMUNITY ZONATION PATTERN TIDE ZONES
Supralittoral
zone
Supralittoral
Fringe
EHWS
Midlittoral
Zone
(barnacles,
rockweed,
mussels)
Littoral
zone
Infralittoral
Fringe
ELWS
Infralittoral
(subtidal)
zone
Stepped Art
Fig. 13-7, p. 362

15. Rocky Shore Zonation
Zone system for rocky shore proposed by Alan and Anne Stephenson
supralittoral fringe (splash zone): uppermost area covered only by the highest (spring) tide, usually just dampened by spray of crashing waves
supralittoral (maritime) zone: area above high water that may extend several miles inland

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

17. Figure 13-8 (a) VERTICAL INTERTIDAL ZONES.

18. Figure 13-8 (b) VERTICAL INTERTIDAL ZONES.

19. Figure 13-5 (b) COPING WITH WAVE SHOCK

20. Figure 13-5 (c) COPING WITH WAVE SHOCK

21. Figure 13-6 INTERTIDAL ALGAE AND WAVE SHOCK.

22. Figure 13-11 THE INFRALITTORAL (SUBTIDAL) ZONE.

23. Tide Pools tide pool organisms
e.g., algae, sea stars, anemones, tube worms, hermit crabs, molluscs
many are filter feeders

24. Ecology of the Rocky Shore
Physical factors interplay with biological factors determining which organisms occur at a given location within intertidal
Competition, grazing and predation
competition for space is dominant biological factor in the organization of intertidal communities
predation can vary over a predator’s geographical range, e.g., keystone predator (an animal that is responsible for maintaining species diversity within an area) Pisaster, or with environmental variables e.g., heavy wave action
in tropical systems, predation is very strong and spread over a number of consumers, hence no keystone predator
grazing by limpets and other molluscs can reduce algal cover and allow barnacles and others to colonize rocks

25. Figure 13-19 BIOLOGICAL INTERACTIONS IN TIDE POOLS.

26. 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

27. 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 is determined by interaction of waves, sediment particle size, and relationship of swash and backwash

28. 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, receive less wave action and have gentle swash
reflective beach: wave energy is directly dissipated on the beach
usually steep with course sediment deposited as swash and backwash waters collide

29. 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
Coarse sand beaches drain well, dry out quickly, hence support relatively fewer organisms

30. Table 13-1 Characteristics of Sandy Beaches at Various Levels of Exposure

31. Comparison of Rocky and Sandy Shores
Sandy shores lack readily apparent pattern of vertical zonation and superficially appear devoid of life
wave action is most important factor in determining organism distribution
on exposed beaches, few large organisms, inhabitants are either mobile or deep burrowers
temperature has less effect because of insulating properties of sand and water retention
hence desiccation and salinity fluctuations are less severe than on rocky shores

32. Comparison of Rocky and Sandy Shores
oxygen levels may be low beneath the sand owing to lack of water exchange
oxygen availability in sediment may be limited when respiration is high and exchange of water with the sea is reduced, e.g., sand flats with fine sediments
anoxic conditions result below uppermost layer when oxygen is severely limited
anaerobic bacteria are common in anoxic sediments and produce hydrogen sulfide gas

33. 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)

34. Figure 13-21 SANDY SHORE ZONATION.

35. Lugworm Auger snail Mole crab Coquina Sand sea stars Ghost crab
Sea urchin
Blue crab
Killifish
Heart clam
Stingray
Sand dollar
Bristle worm
Supralittoral
Midlittoral
Subtidal
Stepped Art
Fig , p. 376

36. Sandy Shore Zonation Supralittoral Fringe of Sandy Shores
stretches from the high tide line to the point where terrestrial vegetation begins
sand dunes may border uppermost extent
below is zone of drying sand, where moisture reaches only during the highest tides and gradually evaporates
inhabited mostly by infauna—organisms that burrow in the sand to survive dry periods and intense heat from the sun

37. Figure SAND DUNES.

38. Sandy Shore Zonation
Temperate inhabitants: insects, isopods, amphipod crustaceans
Ghost crabs and fiddler crabs replace amphipods in the tropics
Ghost crabs have gills, but only make short forays into water to wet them
Ghost crabs live in burrows, and are nocturnal scavengers

39. Figure GHOST CRAB.

40. Sandy Shore Zonation Midlittoral Zone of the Sandy Shore
Vertical zonation
less extensive zones of dry and drying sand than supralittoral zone
zone of retention: retains moisture at low tide because of capillary action of water
inhabited by isopod crustaceans worldwide
zone of resurgence: water is retained at low tide
supports crustaceans, polychaete worms
zone of saturation: constantly moist; supports greatest diversity of organisms

41. Figure 13-24 VERTICAL ZONATIONOF THE SANDY INTERTIDAL ZONE.

42. Zone Dry sand Drying sand Retention Resurgence Saturation Isopod
crustaceans
Dry sand
Drying sand
Polychaete
worms
Retention
Polychaete
worms
Coquina
Tellin
Amphipod
crustaceans
Resurgence
Saturation
Stepped Art
Fig , p. 378

43. Sandy Shore Zonation Midlittoral Zone of the Sandy Shore (continued)
Animals of the midlittoral zone
echinoderms (e.g. sand dollars, sea stars)
snails (e.g. moon snails, olive snails)
moon snails feed on bivalves by drilling a hole in the shell and inserting their proboscis
lugworms
deposit feeders that leave coiled, cone-shaped casts during low tide

44. Figure ANIMAL TRAILS.

45. Figure (b) LUGWORM.

46. Figure 13-28 (a) FEEDING BEHAVIOR OF MOLE CRABS.

47. Figure 13-28 (b) FEEDING BEHAVIOR OF MOLE CRABS.

48. Figure 13-28 (c) FEEDING BEHAVIOR OF MOLE CRABS.

49. Stepped Art
Fig , p. 380

50. Figure 13-30 SANDY SHORE FOOD WEB.

51. Sandy Shore Zonation The Infralittoral Zone of Sandy Shores
subtidal zone is truly marine, exposed only during lowest spring tides
variety/distribution of organisms primarily influenced by sediment characteristics
seagrass beds occur in the subtidal zones of some coasts
many species of fish live here
pace of life is constant relative to that on the beach

52. Sandy Shore Zonation
Relationship Between Tides and the Activity of Midlittoral Organisms
during high tide, bivalves project their siphons to filter feed and bathe their gills
carnivorous snails hunt bivalves
echinoderms emerge in search of food
mole crabs and coquinas move up and down the beach with the tide, filter feeding
predators come in with the tide as well

53. Meiofauna
Meiofauna
microscopic organisms that inhabit the spaces between the sediment particles of intertidal and subtidal zones
pass though a 0.5-mm screen but are retained by a 62-mm screen

54. Figure MEIOFAUNA.

55. Meiofauna Factors affecting the distribution of meiofauna grain size:
coarse-grain sediments have greater interstitial volume that allows larger organisms to move between the particles
fine-grain sediments have less space and exhibit more burrowing forms
water circulation
fine sediments can inhibit water flow and produce anoxic conditions

56. Meiofauna Factors affecting distribution (cont.) oxygen availability
also lower at greater depths
temperature
upper layers are more variable
salinity
wave action
can suspend sediments along with organisms, making them more vulnerable to predation

57. Meiofauna Characteristics of the meiofauna
invertebrates from many phyla
generally elongated with few lateral projections
many are armored to protect them from being crushed by moving sand grains
include predators, herbivores, suspension feeders and detritivores
most exhibit brood protection because they produce a small number of offspring

58. Meiofauna Factors affecting the size of meiofaunal populations
seasons (peak during summer months)
protection from wave action = greater abundance
predation can have severe effects in the upper layers of sediments

59. Ecology of the Sandy Shore
Fauna is less abundant than in rocky shores, and does not occupy all available space
Competition is not a major factor in determining distribution
Predation is less important – fewer predators among invertebrates
Predation and disturbance important on protected sand flats
Greater exposure = less influence of competition and predation, more influence of abiotic (physical) factors, e.g., waves, particle size and slope