Rocky Shores        Of all the intertidal shores, those composed of hard material, the rocky shores-particularly those of the temperate zones-are the most densely in­habited by macroorganisms and have the greatest diversity of animal and plant species. They con­trast sharply With the almost barren appearance of the surface of sand and mud shores. It is these densely populated, topographically diverse, and species-rich rocky areas that have fascinated ma­rine biologists and ecologists for many years. In the last 30 years, these areas have been the subject of several classic studies that have enhanced our understanding of how these associations of species interact to maintain or change the commu­nity. Most of the dominant organisms of intertidal rocks are solitary or clonal animals, whereas subtidal rocks seem dominated by colonial encrusting animals.

 

Zonation

One of the most striking features of any rocky shore anywhere in the world at low tide is the prominent horizontal banding or zonation of the organisms. Each zone or band is set off from those adjacent by differences in color, morphology of the major organism, or some combination of color and morphology. These horizontal bands or zones succeed each other vertically as one progresses up from the level of the lowest low tides to true ter­restrial conditions (Figures 6.7, 6.8). This zonation on intertidal rocky shores is similar to the zonation pattern one observes with increasing elevation on a mountain, where the different horizontal zones of trees and shrubs succeed each other vertically until, if one progresses far enough, permanent snow cover is reached. The major difference be­tween these two areas is the scale. Mountain zones are perhaps kilometers in extent as opposed to in­tertidal zones extending a few meters vertically.

 

 

Rocky intertidal zones vary in vertical extent, depending on the slope of the rocky surface, the tidal range, and the exposure to wave action. Where there is a gradual slope to the rock, individ­ual zones may be broad. Under similar tidal and ex­posure conditions on a vertical face, the same zones would be narrow. In the same manner, ex­posed areas have broader zones than protected shores, and shores with greater tidal ranges have broader vertical zones (Figures 6.4, 6.8).

Of course, these striking bands may be inter­rupted or altered in various places wherever the rock substrate shows changes in slope, composi­tion, or irregularities that change its exposure or position relative to the prevailing water movement.

The fact that these prominent zones can be ob­served on nearly all rocky shores throughout the world under many different tidal regimes led Stephenson and Stephenson (1949) to propose, after some 30 years of study, a "universal" scheme of zonation for rocky shores (Figure 6.9). This uni­versal scheme was really a framework using com­mon terms that would allow comparison of diverse areas. It established zones based on the distribu­tional limits of certain common groups of organ­isms and not on tides. It reflects the knowledge of the Stephensons, and other intertidal ecologists, that distribution patterns of the organisms and zones vary not only with tides, but with slopes and exposure. Therefore, under similar tidal conditions there could be different bandwidths due to differ­ent exposures or slopes of rocks. It was this univer­sal scheme that established a standard format for describing shore zonation, replacing a bewildering host of schemes and names established by earlier biologists.

The Stephensons' scheme has three main divi­sions of the intertidal area. The uppermost is termed the supralittoral fringe. Its lower limit is the upper limit of barn4eles, and it extends to the upper limit of snails of the genus Littorina (periwinkles). The dom­inant organisms are the littorine snails and black en­crusting lichens (Verrucaria type). The extreme high water of spring tides reaches part of this zone, but most of its water comes from wave splash. Above this zone is the terrestrial or supralittoral zone.

The middle part of the intertidal is termed the midlittoral zone and is the broadest in extent. Its upper limit coincides with the upper limit of bar­nacles, while its lower limit is the point where zone is often subdivided and contains a host of different organisms. Perhaps the only universally present dominant group are the barnacles.

The lowest zone of this scheme is the infralit­toral fringe, which extends from the lowest low tide up to the upper limit of the large kelps. This is an extremely rich zone composed of organisms that can tolerate only limited exposure to air. It is really an intertidal extension of the infralittoral zone (Stephensons' term) or what we know as the sublittoral area.

Although Stephensons' scheme did set forth a means for describing zonation on rocky shores, it does not offer an explanation of why the zonation occurs. It is this explanation of zonal patterns that intrigues many marine biologists.

 

 

Causes of Zonation

Whereas it is fairly easy to recognize and measure the extent of the zones on a rocky shore, it is more difficult to find suitable explanations for why or­ganisms are distributed in these zones. Physical and biological factors can be considered to explain the phenomenon. We shall take up each in turn.

Upper-limit barnacles

Equinoctial high-water spring tides

,aria

is

 A = Supralittoral (spray) zone A' = Supralittoral fringe

B = Littoral zone

B' = Midlittoral

C = Infralittoral zone C = Infralittoral fringe

 

FIGURE 6.9 Stephensons' universal scheme of zonation for rocky shores. (From R. L. Smith, Ecology and field bi­ology, 3d ed., Figure 8-4. Copyright © 1980 by Robert

Leo Smith. Reprinted by permission of Harper & Row Publishers, Inc.)

large kelps (Laminaria, etc.) reach their upper­most distribution. This

nalus is

Rocky Shores Reading                         

 

  1. How do rocky shores compare with sand and mud shores?

 

  1. What types of animals dominant the intertidal rocks?

 

  1. What types of animals dominant the subtidal rocks?

 

 

  1. What is the most striking feature of any rocky shore?

 

 

  1. How is one zone different from another?

 

 

  1. Compare and contrast rocky shores with mountains?

 

 

  1. What are 3 ways rocky intertidal zones vary?

a)

b)

c)

  1. What conditions may alter the bands or zones?

 

 

  1. In the Stephenson’s scheme, there are three main divisions. What are they?

 

 

10 Describe each zone discussed in question 9