Intertidal rocky shore zoning

principles, physical and biotic factors and marine creatures

By Dr J Floor Anthoni (2007)
In the most inhospitable place of the sea, where the hard sea shore is covered by the sea at high tide, and exposed during low tide, lives a rich and varied community - why?  The answer to this question can be found only by understanding the living conditions and limiting factors in this very narrow maritime zone. Meet the animals and plants living here. Although the examples refer to the situation in New Zealand, the principles behind them apply everywhere in the world. If you don't live in New Zealand, you may still find these pages applicable to your situation. 

The ecological factors that are most influential on the zoning that occurs on the intertidal rocky shore. (on this page)
A vast number of principles to guide our investigations and observations. Living in the sea and in the intertidal in particular, is spectacularly interesting. (on this page, 14 pages)
The reasons why one species lives mainly here while another is found there, does not only depend on physical factors but also on biotic factors such as competition, predation and so on. However, the sea has some surprises in store. (on this page)
Which organisms are so successful that they build the distinguishable zones? Also view some examples, showing what to look for. (4 pages)
Although the rocky intertidal is not a closed ecosystem, due to the vast open sea bordering it, one still finds all methods of feeding here. Also tips for doing a rocky shore study. (3 pages)
Books and references (on this page)
A log of recent changes to this section (on this page)


identifying the creatures shoreid1: the fishes, crustaceans and echinoderms (16p) 
shoreid2: the seasquirts, molluscs, flower animals, sponges and worms. (21p) 
shoreid3: the coralline algae, green, brown and red seaweeds. (10p)
Related chapters Habitat introduction: the physical factors affecting life in the sea (14p) 
Rockpools: the rock pool habitat (4p) 
InDepth/sea hares: the year of the sea hares, the sea hares of New Zealand. (3p) 
InDepth/parchment worm: invasion of the parchment worm (2p) 
Biodiversity: marine biodiversity and why the sea is so different from the land (32p) 
Goat Island: the Goat Island marine reserve under water (88p) 
Glossary: many scientific terms explained. Worth reading in its entirety. (30p) 
Oceanography/waves: all you need to know about waves (18p) 
Oceanography/tides: all you need to know about tides (4p)

Begin your study of the sea at the Seafriends home page or our sitemap.
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The rocky shore is an ideal place to begin studying the sea as it shows how different the sea is from the land. Many of these differences are not obvious or not visible, and it pays to begin your study with the introduction to marine habitats and our chapter on biodiversity which leads to a summary of differences between the marine and the terrestrial world. On this page we'll encounter the principles applying to the intertidal rocky shore, principles that apply everywhere in the world where a tide forms an intertidal rocky shore.

When  one traverses a snowy mountain in a mediterranean or tropical climate from its top down to its base, one comes across several distinctive zones, identified as follows:

In the above zoning example, obviously the snow/frost and the temperature play an important role, and to some degree winds too. Also the amount of sunlight may play a role. The main driving force is the yearly season, and temperatures decreasing with altitude. So what would the main factors be in the sea?
zoning of the rocky shoreThe sea may experience the seasons too, being iced over at times in cool climates. Ice over water reflects and absorbs most sunlight, thus seaweeds will die. But most of the world's seas and coasts do not freeze over, although frost may threaten creatures at low tide. Temperature and the amount of light play a role for ocean productivity, but there does not exist something like soil, except in very sheltered inlets (sand and mud flats). 
The diagram shown here shows typical coastal zoning in a temperate climate. The intertidal (the subject of this chapter) is driven by the tide. No tide, no intertidal, but there may exist a splash zone in case of waves. Thus the intertidal zone is clearly defined by the movement of the tide. Below it extends the infralittoral (photic zone) and circalittoral zone (demarcated by lack of light to grow plants, or inadequate quantity of light). These two zones are also clearly defined as they depend on the clarity of the water. In murky water the photic zone may be as shallow as 3m, whereas in a clear blue sea of 30m visibility, the photic zone may extend down to 35m until also the quality of the light becomes insufficient.

But there is another zone inside the photic zone (infralittoral), demarcated by the worst storms. Severe storms generate deep waves (swell) that can cause severe damage even when the centre of such a storm passes 1000km away from the shore. This powerful swell of some 200m deep (half of the wave length of 400m), arriving at the continental shelf of 200m deep, experiences the shallowing sea bottom as a hindrance, which absorbs the swell's energy. As such deep waves travel further over an ever shallowing sea bottom, they lose more and more energy. Thus the maximum energy that the most severe waves can deliver to the coast, depends on the depth of the sea bottom. The deeper the bottom, the worse the worst waves are. [note that this has not yet been confirmed by mainstream science]. For more about waves, read oceanography/waves/
In New Zealand, as in many other places, such devastating deep waves cause a barren zone between the strong and flexible stringy seaweeds and the more fragile stalked kelp. This zone is then occupied by powerful grazers such as sea urchins (Evechinus chloroticus), abalone or paua (Haliotis spp.) and Cooks turban shell (Cookia sulcata). Such grazers are then able to prevent seaweeds from establishing themselves in the barren zone, in effect keeping this zone barren, while also extending it here and there. Terrestrial grazers such as sheep and cattle do something similar on pasture and hoofed grazers (ungulates) in the tropical savanna habitat.

The shape of the shore also depends on wave action, and like landscapes, seascapes adjust their shapes for minimal loss in either erosion or opportunity for life. Read our Least Loss Landscapes hypothesis [which is not yet backed by mainstream science]. The effect of this is that shores with massive wave exposure (and thus deep sand bottoms) develop into vertical walls that bounce waves back while absorbing only the least of their energy. Ironically, these steep rock walls provide shelter as waves roll up and down without much shearing (currents). At the most sheltered shores we find a shape that shears the wave at its top, as in a swimming pool's wave-absorbing channel along all sides. Waves then shear over shallow platforms that are covered in life, thus minimising erosion of the rock and damage to life.
It must also be noticed that the sunlight has a profound influence on the shape of the shore, such that shaded shores (less life) are always steeper under water than sunlit ones (abundant life).

The intertidal is defined by zones caused by the moon tide and sun tide. When they work together, the tide is larger, which is called a spring tide. In between occur the neap tides. For more about tides, read oceanography/tides/. Note that most places in the world have two tides each day, but there exist places with only one tide each day, and even ones with no tide at all. Most places have between 1 and 3m of tide swing, but places exist with up to 10m between high and low tide.
Thus the intertidal zones can be defined clearly:
wave exposure modifying the littoral zones
  • splash zone or maritime zone: the zone above high spring tide, that is regularly sprayed or washed by waves. On exposed shores the splash zone is wide, whereas on sheltered shores it becomes essentially absent. Above the splash zone begins the terrestrial world, marked by lichens (white, black, orange crusts) that are resistant to salt spray. Characteristic of the splash zone is that sometimes for many days in succession, during calm weather, the sea does not get there.
  • upper tide zone (supra littoral fringe): the water comes here only twice each month during spring tides.
  • mid tide zone (eulittoral zone): the water comes here twice daily but also retreats twice daily.
  • lower tide zone (sublittoral fringe): the water recedes from here only twice each month during spring tides.
The diagram shows how the three zones are substantially altered by wave action, which also blurs their distinction.

Compared to mountain zoning which covers several kilometres of altitude, the zoning on the shore happens within a few metres. Also the living conditions can change profoundly over short distances, due to irregularities like ledges, cracks, stones and so on. Of course, waves can make this fine distinction of zoning somewhat vague, and so can the shape of the shore. Here are some new factors to consider:

One would have expected, that because all life begins with plants, most of the intertidal would be covered in seaweeds, but it takes only one glance to see that this is not true. Why?
It is caused by the massive influence of the sea outside, and the planktonic food it contains (plant and animal plankton and their dead bodies). Thus all organisms living from this source, can colonise the intertidal in competition with seaweeds, a match that they win in their respective zones. This makes barnacles, mussels and oysters invasive chalky colonisers as their abundant plant source resides in the open sea. Within their chalky houses, these animals are exceptionally equipped to prevent dehydration when the tide is out.
surface meets bottom
Planktonic organisms keep themselves suspended either by swimming or by some buoyancy control mechanism. But when the organisms die, they either sink down or float up. Thus the two places where the food collects, are the surface and the bottom. It so happens that the intertidal is the ONLY place where this happens, thus planktonic food (phytoplankton, zooplankton and detritus) is aplenty. In tidal inlets where wave action is weak, detritus abounds, whereas on the exposed rocky shore, the surface 'scum' is a rich source of food.
But the surface can also become a menace in the case of oil spills and other forms of pollution that are surface-bound. This may explain the often sudden disappearance of many organisms such as the red beadlet anemone (Isactinia tenebrosa).
veges & pizzas
When studying the organisms of the rocky shore, one must distinguish clearly those that live from the food produced on-site and those that live from imported food. The first are like families living from their garden plots of vegetables; the second are like families ordering their pizzas and fried chickens from far away. The first live on low quality food (veges), the latter on high quality food (proteins). The first cannot be as productive as the latter. Remember though that even those living from locally grown vegetables, still benefit from imported nutrients.

one cannot reproduce the rocky shore inside an aquarium because
the dominant influence of the open sea cannot be simulated

One would also expect that intertidal zoning is fixed and predictable, as the zonation diagrams shown above, suggest. But after some investigation, one finds that this is not so. Why?
Of all the abiotic (physical) environmental factors discussed above, the tidal range, exposure, shore shape and substrate are the most important ones. Because each has its own 'sorting sequence', the resulting zones are quite variable.

spring low tide at a sheltered place
0701180: located behind a barrier rock, which shelters this coast, stringy seaweeds are threatened by drying out at spring low tide. The greenish zone is the mid intertidal. Pied shags are drying themselves after a day's fishing. The reef flat is rich in organisms and tide pools abound with seaweed species. The seaweeds are cartilage weed (Xiphophora chondrophylla), flapjack (Carpophyllum maschalocarpa) and just visible stalked kelp (Ecklonia radiata).
spring low tide at an exposed place
0701183: Never Fail Rock is an exposed rock in a 50m deep sea). The spring tide covers no more than 2m (compare with the birds). It is a very calm sea now at spring low tide and stringy seaweeds (Carpophyllum maschalocarpa) are having a hard time. This is the sheltered side of the rocks, and white bird excrement shows a curved splash zone where it is washed away. The greenish zone is the eulitteral.
spring low tide at a sheltered place
0701157: the intertidal in a healthy  sea: high biodiversity of seaweeds and other life. Every bit of rock is covered in long-lived sea life. Stalked kelp dares to grow in the lower littoral fringe. East coast Mercury Islands.
predatory starfish drying out while reaching for black mussels
f218219: the intertidal in a degraded sea: low biodiversity. Reef stars (Stichaster australis) surviving desiccation (drying out) while reaching for the black band of flea mussels. Much of the shore is not covered in long-lived sea life. No coralline pink paint! West Coast.
low tide in a healthy place
f009525: rock pools with seaweeds while all exposed rock is covered in life, a healthy environment at Goat Island marine reserve. This reef flat is found at the mid to lower littoral and there is no upper littoral or splash zone. There are many broken rocks, some large, accommodating high biodiversity. Featherweed and flapjack in the pools. Abundant coralline pink paint.
a degraded rocky shore, Long Bay
f991016: a degraded rocky shore at low tide, showing little variety and poor cover. The pink paint (Lithothamnion sp.) is almost entirely absent, making the shore subject to rapid erosion. Long Bay marine reserve. The white crusts consist of rock oysters and plicated barnacles. 
Whangaparaoa degraded shore
f219610: a potentially interesting rocky shore with rocky undulations capturing large rockpools and boulders littered everywhere. Yet this is a very degraded shore with poor diversity and cover, due to the release of treated sewage at the point in the distance. Whangaparoa Peninsula. Very little pink paint.
f219722: a low tide platform at a very exposed shore has gutters full of seagrass, caused by a high input of mud and nutrients. Near Napier.
a kelp wash-up: natural disaster?
f222332: a seaweed washup after a storm, although natural, can cause tremendous loss of life as seaweeds rot away and decomposing bacteria kill sensitive life. A natural disaster such as this has a man-made component: sea urchins died by eating a highly poisonous dinoflagellate slime and this invited the kelp to grow in the urchin barren zone where storms would take them out. The dinoflagellate Ostreopsis slime was caused by unnaturally high nutrient loads in the water which is man-made.
zoning on an evenly sloping shore
f219317: intertidal zoning on a sheltered evenly sloping shore continues in the vegetation above. Note how low the barnacle zone is relative to the sublittoral fringe of brown seaweeds. Note also the grey, orange and black lichens so close to the top of the intertidal zone. Entrance to Whangarei Harbour.

Apart from the obvious physical factors that cause the zoning on the shore, there are a number of other principles to consider, which makes the study of the rocky shore so interesting.

species interaction
Whenever a species finds a place favourable for living and reproducing, it discovers that other species think so too. Competition begins. After a long time of evolution, species differentiate such that each survives on its particular skills that the other does not have. Thus evolution tends to minimise competition. Likewise, predators develop skills to catch and eat others, each predator species with slightly different skills so that they do not need to compete. At the same time the prey develops better defences.

The science of ecology teaches the following classical species interactions:

Interactions within a species population (intra-specific interactions) can be: Obviously, most of the inter-species interactions mentioned above relate to higher organisms, except for competition where the amount of food and the distance to neighbouring competitors is critical.
kite diagrams of competing grazing molluscsOne method of scientific study of the intertidal zone is by laying out a measuring tape and measuring species densities along the tape. The species counts can then be depicted in the form of a kite diagram as shown in this example of grazing molluscs. The width of the black kite represents the number of individuals found within a defined quadrat. (Note, however, that the kite diagram shown here is more conceptual than representing actual numbers.) Students are then asked to explain why certain species are found where they are, why some overlap with one another and others don't, using all the knowledge explained before. In our example, all species shown are grazers which suggests that competition should be a major biotic factor as well as exposure to desiccation (drying out).
However, also some important information is missing. To begin with, it does not show where the high and low tide marks are. One should also know that this is a shaded and reasonably sheltered shore, reason why the grazing slug Onchidella is found here, crawling out of their hideouts, grazing the sea weeds above. So the zone builders not shown on the diagram are important biotic factors as well. For instance Siphonaria is usually associated with the blood crust Ralfsia or Gelidium. One should also know that this is an almost vertical shore, reason why Cellana ornata is found and Melagraphia is lower down than usual. The message here is that there are many hidden clues before one can understand the ecology behind this diagram of grazers. But there is more to consider.

If one were to study the same area of shore over a long time period (longer than 10 years), one may notice that the kite diagram changes seasonally and annually, and often quite suddenly too. So in the end, our entire toolbox of ecological factors proves to be of no use. None of the explanations we can muster holds true over a sufficient span of time. Why? Why does ecology in the sea appear to make no sense?

The most important property of the sea is that it is strange; stranger than we can imagine;
nothing works the way we anticipate.

There are a number of invisible ecological factors that we do not find on land:

The result of all this is that the intertidal shores (and somewhat the sea) are rather counter-intuitive (paradoxical), resulting in patchiness (randomness) with large variations over time.

Now we've come full-circle with the most important ecological factor saved for last. Read this chapter again, but now from the paradigm (way of thinking) that failure (death) is more important than success (survival). Also consider this:

The most overlooked ecological factor is TIME;
when we look at a situation for long enough, our knowledge becomes a riddle. - Floor Anthoni 2009

Further reading
References in blue are available from the Seafriends Library

Adams, Nancy M (1994): Common seaweeds of New Zealand. Canterbury University Press.
Adams, Nancy M (1994): Seaweeds of New Zealand. Canterbury University Press.
Ayling, Tony & Geoffrey J Cox (1982): Collins guide to the sea fishes of New Zealand. Collins.
Cometti, Ronald & John Morton (1985): Margins of the sea: exploring New Zealand's coastline. Hodder&Stoughton.
Dell, R K (1963): Native crabs. (Nature in New Zealand series). AH & AW Reed.
Enderby, Jenny and Tony (1998): Goat Island marine reserve, Leigh, New Zealand. Enderby.
Fell, Barraclough H (1962): Native sea-stars (Nature in New Zealand series). AH & AW Reed.
Forest J et al. (2000): Paguridea (Decapoda Anomura) exclusive of the Lithodidae. NIWA Biodiversity Memoir 114
Foster, Brian A (1978): The marine fauna of NZ: Barnacles (Cirripedia: Thoracica). NZ Oceanographic Inst Memoir 69.
Francis, Malcolm (1988-2001): Coastal fishes of New Zealand - an identification guide (3rd edn). Reed Publishing.
Gunson, Dave (1984): Collins guide to the New Zealand seashore. William Collins.
McLay, Colin L (1988): Crabs of New Zealand. Leigh Laboratory Bulletin 22.
Morton, John and Michael Miller (1968): The New Zealand sea shore. Collins. 
Morton, John (2004): Seashore ecology of New Zealand and the Pacific. David Bateman. 
Paulin, Chris & Clive Roberts (1992): The rockpool fishes of New Zealand. Te ika aaria o Aotearoa. Museum of New Zealand.
Penniket, J R and Geoff Moon (1970): New Zealand seashells in colour. AH and AW Reed.
Powell, A W B & B J Gill (Ed) (1947-1998): Powell's native animals of New Zealand (4th edition). David Bateman
Powell, A W B (1979): New Zealand mollusca; marine land and freshwater shells. William Collins.
Schiel, David R (2006): Guide to common intertidal species of the South Island, New Zealand. University of Canterbury.
Stace, Glenys (1998): What's around the rocks, a simple guide to the rocky shore. Viking.
Walsby, John (1990): Nature watching at the beach. Wilson & Horton.


What's new?
20090414 - species interaction added with hidden ecological factors
20070710 - Pages made smaller for ease of access and printing.
20070703 - All links on this page were corrupted but have now been corrected. Oops.
20070330 - Further refinements and more photographs.
20070318 - Final touches but many photos still to come slowly. Published on the Web.
20070302 - Seaweeds part3 published, but more work needs to be done.
20070226 - First published on the Web, including identification part1 and part2.