Intertidal Zonation1

• Introduction
• Zone 1: The Supralittoral Fringe
• Zone 2: The Upper Midlittoral Zone
• Zone 3: The Lower Midlittoral Zone
• Zone 4: The Infralittoral Fringe

Introduction

As you work your way down a rocky shore from the high-tide line to the low-tide line, you will observe that the distribution of most of the more obvious animals and plants is not at all general. Even organisms that have a wide vertical distribution tend to be concentrated at certain levels. This phenomenon, called intertidal zonation, is due partly to the influence of physical conditions on each species. Animals and plants in the higher reaches of the intertidal zone must be able to withstand exposure for long periods. In winter they may be subjected to rain and to temperatures decidedly below that of the sea water; in summer they may be exposed to bright sunlight and warm air, and some of them will lose water by evaporation. Among the intertidal organisms that are adapted to a nearly terrestrial existence are the periwinkles, little snails of the genus Littorina. If put into a jar of seawater, they promptly crawl out and remain well above the water line. Few animals and plants from lower levels of the intertidal zone can survive prolonged exposure, especially during warm and dry weather.

Checkered Periwinkle (Littorina scutulata)

Zonation is not, however, entirely the result of adaptations to physical conditions. There are biological factors, especially competition and predation that have to be considered. If a particular predator, for instance, can live comfortably at certain tide levels, then the population of a species on which it feeds will be kept in check. If the prey species can flourish at higher levels than the predator can, then it may become abundant at those levels. Well-documented cases of this sort involve snails of the genus Nucella and certain barnacles belonging to the genera Balanus and Chthamalus. The barnacles are capable of living over a rather wide vertical range, but predation by Nucella keeps them in check over most of it. At higher levels, where Nucella drops out, the barnacles grow thickly.

Giant Acorn Barnacle (Balanus nubilus)

The vertical distribution of these barnacles is also affected by competition for space. Although Chthamalus can live at levels lower than those at which it is normally abundant, it does not compete well with Balanus. The fact that it can prosper at levels too high for Balanus gives it an advantage in the uppermost part of the intertidal region. So, the distribution of both Chthamalus and Balanus results from the combined effect of the competition between the two barnacles, the predation by Nucella, and the ability to survive at progressively higher levels.

Discussion of the fauna and flora of rocky shores will be based to a large extent on the zonation shown by some of the more conspicuous organisms. But since the intertidal region is not one continuous, smooth sheet of solid rock, there will be sections dealing with life in crevices, in tide pools, and other specific situations. Some of the trophic relationships, such as those of prey to predator, and various kinds of symbiotic associations, will also be pointed out.

To follow this basic framework it will be convenient to divide the intertidal region into four numbered zones: "1" will indicate the uppermost zone, "4" the lowermost. In many treatments of the subject of intertidal zonation, the term "supralittoral fringe" applies to our zone 1, and "infralittoral fringe" to our zone 4. The midlittoral zone, then, corresponds to our zones 2 and 3. For this introduction to rocky intertidal areas, it will be most instructive to use one in which the substratum is composed to a large extent of massive and more or less coherent rock formations (not just an accumulation of small rocks or boulders), and one that sustains at least a little wave action. Rocky shores of this type are found on the west and south side of San Juan Island and elsewhere in the San Juan Archipelago. What one learns from such a shore can be adapted to various other kinds of situations. One can expect, however, that generalizations formulated on the basis of observations of one particular area will be violated elsewhere. Moreover, in many places zonation is simply not well marked. Vertical bulkheads and sea walls generally show more pronounced zonation than is evident on slopes.

No matter how strong the zonation may be, two features found on most rocky shores will tend to blur it to some extent. Permanent tide pools, especially larger ones, and gullies in which water sloshes up and down after waves break make it possible for certain organisms to live at appreciably higher levels than they would be able to do otherwise. Thus, although surfgrass (Phyllospadix) and coralline red algae are considered typical of the lowermost zone of the intertidal region, in tide pools and gullies they may be found well above this zone.

Back to Top

Zone 1: The Supralittoral Fringe

This zone is affected only by higher tides, and its uppermost portion may not often be wet except from spray or rain. To relate it to data given in a tide table, in the San Juan Archipelago its lower limit is usually about 7 feet (2.1 meters) above 0.0 feet (Mean Lower Low Water); in much of Puget Sound, its lower limit is about 9 feet (2.7 meters) above 0.0 feet. Of course, it passes rather imperceptibly into the next zone below. Remember that a zone is characterized by an assemblage of organisms, not by the finite distribution of one species. In any case, zone 1 has relatively few conspicuous species, but those that do occur here may be present in large numbers.

Back to Top

Zone 2: The Upper Midlittoral Zone

In the San Juan Archipelago, this part of the intertidal region ranges from about 7 feet (2.1 meters) down to about 4 feet (1.2 meters); in most portions of Puget Sound that are characterized by rocky shores, it is a little higher. Among the brown algae, Fucus distichus, the rockweed, stands out. It is a substantial seaweed with a flattened thallus that keeps branching dichotomously until it reaches a length of about 30 centimeters. The midribs of the branches are continuous and prominently raised. The hold fast is not much more than a thickened button. At least some of the terminal branches become swollen and warty; these are the sites of egg and sperm production. Fucus luxuriates in late spring and summer, and where it grows well, it may cover just about everything else.

Brown Algae (Fucus distichus)

The hermit crabs must be close to the top of everyone's list of favorite seashore animals. The idea of living in an empty snail shell has been explored by several groups of invertebrates, including some other crustaceans, but hermit crabs have made a real success of it. As a hermit crab outgrows one shell, it must find another, sometimes running into serious competition for available homes in the right size range. Once the crab has solved its housing problem and has safely tucked away its soft, coiled abdomen, it will not look again for some time. On the whole, unoccupied shells, especially larger ones, are in short supply wherever hermit crabs are found, but the intensity of competition varies depending on circumstances. In any case, hermit crabs will fight for exclusive rights to empty shells, and this behavior can be observed in aquaria as well as in tide pools.

Hairy Hermit Crab (Pagurus hirsutiusculus) Photo Credit: Keoki Stender

Back to Top

Zone 3: The Lower Midlittoral Zone

Between about 4 feet (1.2 meters) and 0.0 feet (Mean Lower Low Water), the flora and fauna show some distinctive components. Where there is some real wave action--as is typical of the west side of San Juan Island and the southern part of Lopez Island--there will be beds of the California mussel, Mytilus californianus, and of the goose barnacle, Pollicipes polymerus. Neither of these will be found in Puget Sound proper, and they are absent from most islands of the San Juan Archipelago. Thus only the more exposed shores in the archipelago can come close to showing a lower midlittoral zone of the sort seen on the outer coast.

The California mussel sometimes attains a length of about 20 centimeters, but specimens 15 centimeters long can be considered large. Like the edible mussel prevalent in quiet waters, it is tightly attached to the rock by means of a byssus and tends to be aggregated. The masses are sometimes several feet across. The California mussel has a good flavor when it is cooked by boiling or roasting, but it can be very poisonous. "Mussel poisoning" is caused by a toxin derived from a microscopic organism, a dinoflagellate called Gonyaulax catenella.

When Gonyaulax is abundant in the plankton, it is ingested along with other organisms, and the toxin accumulates in the tissues of Mytilus. The paralyzing effect of this substance is somewhat similar to that of curare, which has long been used on darts and arrows by certain South American Indians. The summer months are apt to be the only really dangerous ones, although the poison stored in the tissues may remain for some time after the Gonyaulax has disappeared from the diet of mussels. In any case, one should avoid gathering the California mussel for food in the summer months. Toxicity can be established by injecting laboratory mice with an extract of mussel tissue, but this is obviously not practicable for every lover of seafood. Cooking does gradually destroy the toxin, but even high heat over a long period cannot be trusted to do the job thoroughly. Other bivalves, including the littleneck clam of protected situations, store up the toxin at dangerous levels, but the California mussel is the worst offender on the open coast.

Back to Top

Zone 4: The Infralittoral Fringe

This part of the intertidal region is exposed only by very low tides--those that bring the water level down into the range between about 0.0 feet (Mean Lower Low Water) and -3.5 feet (-1.1 meters). The fauna and flora of zone 4 are apt to be frustratingly rich for a beginner, so they will have to be approached selectively, with a view to laying a solid foundation on which one can continue to build as one's background and interest deepen.

Back to Top

Reference Cited

1. Kozloff, Eugene N. 1973. Seashore Life of the Northern Pacific Coast. University of Washington Press. Seattle, WA.