rocky intertidal zonation— what causes zonation?

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  • Rocky Intertidal ZonationWhat causes zonation?

  • Rocky intertidal shore Characteristics1.Physical environmental factors associated with the tides and wave force. 2.Fishing from the rocks is a recent popular recreational activity which may be disruptive to the long-term conservation and preservation of species on rocky coast.3.A complexity and inter-dependence of the organisms on rocky shore. 4.Broad zones of organisms could be found at different levels on the shore and that these were modified according to the local pattern and force of wave-action.

  • Why study rocky shores?1. A suitable platform to analyze the processes governing patterns of distribution, abundance and diversity of natural assemblages as rocky shores are readily accessible from the land at low tide.2. The intertidal rocky shore is an extreme habitat that is in a state of almost constant change when compared to land or the sea. 3. Due to water movements associated with tides, waves and spray, conditions affecting different levels on the rocky shore vary continuously throughout the

  • Expose to air, potential desiccation,Exposure to waves that create shear forces on the rocks and could potentially dislodge organisms. Changes in salinity, dissolved oxygen levels, and substrate type and availability of shelter.

  • Intertidal zones of assemblages 1.Lowest level of shores: algae, ascidian +Limpets->tube worms (substitute by calcareous algae in sheltered shore)->barnacle->snail

  • Topics on rocky intertidal 1.Quantitative characteristics2.Interaction between plants and animals (grazing and the distribution of algae, algae and the lower limit of distribution of limpets, algae and the distribution of sessile animals, sessile animals and the distribution of algae)3.Interactions between sessile and grazing animals 4.Positive interactions involving gastropods

  • 5. Competitive (negative) interactions (competition for space among sessile species, competition for food among grazing gastropods-intraspecific competition and interspecific competition and their relationships, effects of competition on life histories) 6. Predation on rocky shore (e.g., whelks, crabs, fish and birds) (predation by whelks Morula marginalba drill and consume all shelled sessile species and many of the gastropods; are welks keystone species?)

  • 7. Disturbances to intertidal assemblages 8. Physical disturbances on rocky shore 9. Relationship of settlement and recruitment to patterns of distribution

  • Chiton

  • LimpetThis is a limpet that hold on to a rock to prevent itself to be washed away (

  • barnacles is a type of arthropod belonging to infraclass Cirripedia in the subphylum Crustacea, and is hence related to crabs and lobsters;

  • Supplimentary Reading

  • Interaction between macroalgae and animalsMacroalgae could be prevented from extending to higher levels because the grazing sea snails found there consume them. Or Rocks are already covered by encrusting algae.

    Intensive grazing by the gastropods removes virtually all early, microscopic stages of the macroalgae. (Listen to grazing sound of sea snail).

  • Interaction between macroalgaeand animalsAlgae and the lower limit of distribution of limpets:Limpet which feed on microalgae and do not eat macroalgae, tended to migrate away from areas with well-established macroalgae algal beds more than from areas with little macroalgae.

  • Sessile animals and the distribution of algae

    Where macroalgae grow in profusion and occupy most of the space at low levels on shores, they can prevent the subsequent invasion of plankton larvae of sessile animals (e.g., barnacle). In any low-shore area, if barnacle manage to settle and attach to cleared surface, subsequent rapid growth of algae kills them, by growing over and smotheringthem.

  • Sessile animals and the distribution of algae At mid-shore levels, the limpet consumes the spores and early stages of many algae.

    Barnacles may reduce the effects of wave-shock and probably trap moisture reducing desiccation, which might affect the near-by macroalge to survive better.

  • Sessile animals and the distribution of algae Interaction between sessile and grazing animalsSurvival of newly settled tube-worms was negatively affected by grazing activities of snails and limpets. Grazing by limpets kills juvenile barnacles.

    Effects of sessile species (barnacles) on grazers (limpets):

  • Sessile animals and the distribution of algae There are effects of barnacles on limpets. More juvenile limpets recruited to areas where barnacles were present than to areas from which barnacles were absent. Pre-emption of space by barnacles prevents invasion by adult or juvenile limpets.

    Barnacles probably provide protection from the forces of breaking waves so that large sea snail can live in the adjacent space.

  • Sessile animals and the distribution of algae Competition for space among sessile species: There is evidence for direct interference of the tube-worm on the barnacle.

    Competition for food among grazing gastropods: Negative interactions among grazers are wide-spread.

  • Predation on rocky shores Predation is an important determinant of patterns of distribution and abundance of many species on rocky shore. Starfish are considered as keystone species. The sea snail drills and consumes almost all shelled sessile species and many of the gastropods.

  • Whelk The whelks prefer to eat barnacles larger than 4 mm; where these were present, the juveniles survived for longer periods. Other predators include crabs, fish and birds.

  • Mussel Note the drill holes in the Mytilus edulisshells around the whelk Nucella lapillus shown above. only good way to determine when a whelk is drilling is to listen for it.

  • Predation on rocky shores The role of crevices was important. Preys near crevices will be susceptible to considerably greater rates of predation by sea snails than those further away.

    Differential effect of predation: the interaction between the preferences shown by the predator and the density of alternative prey of different preference ranking. These combine to influence the intensity of predation on a given species of prey.

  • Disturbances to intertidal assemblagesStorms and wave-action create space in mussel beds.

    Movements of sand influence the patterns of distribution of algae.

    Human disturbance

  • Factor affecting species diversity:Species-area relationship: Number of species increase with increase in sampled area.

    Intermediate disturbance model: More species in areas of habitat suffering from some, but not excessive, disturbance. The availability of micro-habitats on the boulders also increases the number of species present

  • Tops of boulders tended to be covered by algae; under-surfaces were covered with a variable array of sessile animals (sponges, bryozoans, tube-dwelling polychaetesand tunicates).

    Boulders of different sizes were subject to disturbance of different magnitudes, due to waves and the movement of sand.

  • Relationship of settlement and recruitment to patterns of disturbance1. The surface texture of rocks influenced the numbers and/or types of species. Marine larvae can discriminate very well among different surface and different textures. Some scientists supposed that larva might settle preferentially on some shores rather than others. However, larvae might settle in variable numbers from place to place regardless of substratum.

  • 2. Shaded areas rather than those in direct sunlight have more settled larvae, indicating that larvae seek out shade places to settle.3. Young worms were found almost exclusively at heights occupied by adults.

  • 4. Pre-emptionof space by algae at low levels on the shore can prevent settlement of some species.5. The distributions of adult organisms are entirely determined by the patterns of settlement of the larvae.


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