Ocean School

Life Science - Coastal Habitats and Inhabitants - Rocky Intertidal

By Susan Kynast, Ocean School Marine Biologist

What is a Rocky Shore?

 

Along the edge of the Gulf of Maine the land drops into the sea in a series of cliffs and steps of bedrock or large boulders. A rocky shore can be as steep as a sheer cliff wall or as flat as a gently sloping rock shelf. It can be completely exposed to violent wave action like Schoodic Point or hidden in the corner of a protected cove. Rocky intertidal habitats don’t move with wave action. This in effect makes any firm structure a rocky intertidal habitat – harbor pilings, wrecks, bridge foundations, and …to the annoyance of boat owners …moorings and boat hulls.

 

Rocky intertidal habitats are relatively free of suspended sediment. However, a cliff wall that ends in a muddy bottom could have wave action stirring up sediment which will alter the plant and animal communities that can live on the rocks. So a rock in a mudflat is really a separate coastal habitat.  

 

The term intertidal refers to the zone over which the tide moves - it is above water at certain times of the day and under water at others. Above the intertidal is the splash zone which gets wet from waves washing over it but is never completely submerged except during exceptionally high tides or storm surges. Below is the subtidal which is always under water. To have a complete rocky intertidal community the rock face should extend down into the subtidal since some mobile organisms move up and down with the tide in order to stay under water. If they run out of rock and end up in sand or mud as they move down with the tide their able to live or survive may be impacted. Organisms that live on the rocky intertidal often have an attachment mode similar to our attempts to stick something to a tiled bathroom wall. An organism trying to move through sand or mud using such an attachment mode (think suction cup for example) will likely be unable to get a hold, get flipped upside down, washed away and eaten.

Finding a Place and Getting Stuck

 

The key to living on a rocky shore is attaching yourself or getting stuck. To any organism a rocky shore is much like a cliff wall to a climber – you need to find a way to hold on to it. Because rocks are one of the few structures that make good attachment points, a lot of organisms like to live there. This leads to competition with space being the limiting resource.

Why is attaching yourself to a rock such a popular concept? If you are a filter feeder, you might need to get away from the sediment. Just like breathing in a dust storm, filter-feeding in sediment-laden water simply does not work very well. Filter-feeders generally use mucus-nets or very fine slits to trap small edible material in the water column. If these get clogged up with sand – you have a problem. So these critters want to get off the sediment, and stay attached to their place to avoid dropping or washing back into it.

 

If you are an organism which is adapted to moving around on rocks, you might have a suction-cup type device as your foot. Should you fall off and end up in soft sediment you might not be able to move at all or not fast enough to move back up on the rocks before the current carries you off. Depending on the amount of wave action on that particular stretch of shore getting washed off might also mean getting washed up on the beach where you might dry out, be eaten by seagulls, ground up by pebbles, getting washed way off shore into dark deep water with little food and large predators.

 

If you are a plant-eating animal (herbivore) you also want to live where plants live, and plants only live where there is light (the photic zone), a fairly narrow strip from the intertidal down a few feet into the subtidal. Below that is the barren landscape dominated by life forms which eat what falls down from above.

 

And finally if you are a plant you need to attach yourself simply because you cannot move. You fall off and that’s it – you become food for some hungry herbivore in the subtidal or fertilizer on land.

 

Not every organism is equally good at attaching itself. The strategies are endless. Blue mussels use a type of dental glue. Barnacles cement their whole body permanently to the rock. Sea anemones use giant suction cups. Starfish use a hydraulic-type system. Crabs use clams and pinchers, snail use a foot with a mucus film, marine worms use all kinds of bristles, and seaweeds use something called a holdfast which may look like anything from the conventional root system of a land plant to a suction cup. If you are good at sticking yourself to smooth rock you have a distinct advantage over your competitors: you can glue yourself to any rock surface that becomes available regardless of how smooth and regardless of how exposed to wave action. If you are really good at it you might even be able to live in a place where your predators can’t go at all which would be perfect. Open patches might become available because others have gotten washed off (especially in the winter when wave action is strongest), ice might have damaged the community, or all the previous occupants got eaten or destroyed by some other disaster (such as humans trampling an area).

 

Barnacles are as good as any at attachment in the animal world. There are however problems if you are the best at attachment. As soon as you attach, you have created a rough surface that allows somebody else to live on top of you, and somebody on top of them. If you are a filter feeder you quickly starve. Even if the higher level attach-er resides on top of your unlucky neighbor, it now breaks the wave action, makes things calmer, and gives protection to predators wanting to eat you. However the animal or plant attaching itself to someone else is not in the most ideal position either. Let’s say the bottom layer is a barnacle. A blue mussel glued itself on top of that, and a kelp grows on top of the blue mussel. Kelp grows to several feet in length and its long blade gets moved around by wave action with significant force. Certain other seaweeds and / or animal colonies might also grow on the blade of the kelp, further increasing the drag. Sooner or later a big wave will pull hard enough that either the long-dead barnacle on the bottom or the glue attachment of the mussel fails. The whole clump falls off, and the cycle starts again. If remnants of the barnacle or the holdfast from the seaweed remain, attachment for whoever comes next will be easier. If the whole area is scraped completely clean – as can happen in ice scour – the cycle starts anew.

 

Another expert in attachment are lichens which are found in the splash zone. These are the first colonizers of newly formed land during a process called primary succession. Some forms of lichens can grow on completely smooth rock, bonding themselves to the rock face so hard that they cannot be removed with a wire brush. There are places in Maine where you can see the lichens which grew in what used to be the splash zone thousands of years ago when the water level was much lower. They are now under water but still attached as strongly now as they were then. However, further up on land lichens run into the barnacle-type problem. The first lichen makes the rock just a little bit rougher, and a bigger lichen manages to attach to it, and then an even bigger one. Finally a moss grabs a hold, overgrows the lichens, and the process of building soil has begun.
 
Because of the problem with becoming overgrown many animals use some form of trade-off between mobility and the perfect attachment. Total, permanent attachment only works if you are a filter feeder and can have the food basically come to you. If you are a predator or grazer (herbivore) you need to be able to move around. But even some filter feeders can move. Blue mussels probably have the best system. They glue themselves to the rock face with glue fibers (byssal fibers), which they can themselves dissolve again if they want to move. In fact they use those fibers themselves for moving by shooting them out, attaching them to something, and pulling them back in, in the process pulling their body along. Of all predators, sea stars (commonly called starfish), have the most elaborate system consisting of hundreds of individual suction cups pressurized by a hydraulic system that they can be individually tightened or released to hold on, move, and even kill.

 

The snails use a mucus-covered foot to attach themselves and move while protecting themselves with a shell. You can be bigger while still exposing only a small part of yourself to predators. However, a bigger shell gets grabbed more easily by wave action and water movement, so the chance of you falling off is greater. Therefore you will generally find the smaller, flatter snail-type creatures further up where the wave action is strongest. Limpets have taken this concept to the extreme by using a symmetrical hat-type shell. The larger snails generally live further down, are highly mobile, and only come up into the intertidal when it is fully flooded by the tide.

Planning for Attachment – the Question of Reproduction

 

If your life strategy makes you a dweller of the rocky intertidal, you need to somehow get there at the beginning of your life. If you are going to attach yourself permanently (i.e. you are a sessile organism) you want to have some sort of sensory system in place which allows you to choose your spot with care. Thus many animals living in the intertidal start out their life as a free-swimming planktonic organism. Barnacles for example start out their life as little crab-type creatures before they select their spot and attach themselves.

 

Tunicates (also known as sea squirts) are even more alien to our type of thinking. As urochordates they are one of the most highly developed invertebrates, evolutionarily only one step behind fishes. And indeed, their larva looks much like a fish and is generally regarded to move through the process of neotony (reproducing as a larva) giving rise to the fishes.

 

Tunicate larva are free swimming and have both vision and the sensory system to actively react to and explore their environment. However, once they have selected their ideal attachment spot, they permanently attach themselves with their head, degenerate their nervous system, and become a filter feeder with little more sensory or motile capabilities than a plant. Natural selection simply selects for maximum reproductive output through maximum food intake and conversion over the maximum number of years.

Reproductive strategy involves mating, which can be hard if you cannot move. Many sessile organisms are simply broadcast spawners. Here one or a few individuals will start spawning, which puts hormonal cues into the water column which causes even more organisms to spawn. Eggs and sperm mix in the water column and development of the planktonic life stage begins. The great problem with this life strategy is threshold density. If there are not enough individuals in an area the hormonal cue in the water column does not become strong enough, and a mass spawning will not take place. Instead every individual will spawn on its own time schedule. The concentration of gametes in the water column will not become high enough for fertilization, and reproduction will have failed.

 

Another option (if you can get close enough) is to actually mate and then release the fertilized gametes into the water column. This eliminates some issues with threshold densities, but creates other problems if not enough conspecifics survive around you. I.e. it is a good strategy for organisms with a patchy distribution occurring in groups, while broadcast spawning works better for organisms which have a pretty uniform distribution over a wide area. Barnacles employ the mating strategy. In order to increase their chance of successful mating they are hermaphrodites (i.e. both sexes at once), and they use long penises which allows them to reach the barnacle next to them.

 

The planktonic stage of many sessile organisms is faced with the task of finding a suitable habitat for the rest of their life. In general the choice is whether to join an existing colony or to colonize a new patch of ground. Again this choice is made based on hormonal cues excreted from conspecifics. If you plan to actually mate later in life, you need to settle extremely close to others of your kind. If you are a broadcast spawner, your choice would be based on other considerations such as food availability (lots of you in a small area will decrease the amount of food in the water column) versus safety in numbers (interestingly even some sessile organisms such as blue mussels can fight off predators). Planktonic stages of sessile animals also carefully test the substrate in an attempt to locate the best possible surface.

 

Many of the seaweeds employ a somewhat different strategy. Lacking a nervous system it is not advantageous for a plant to have an extended planktonic stage since it cannot make choices of where to go. Also extended time in the water column greatly increases the chance of getting eaten. The gametes or zygotes therefore attach themselves to the rock face quickly. Because the process is basically passive, this is extremely dependent on weather conditions. Some seaweeds need perfectly calm conditions shortly after their gamete release which is driven by astronomical factors, light, and water temperature. These triggering conditions might occur once every couple of decades or even centuries, and as the earth’s climate changes, even less frequently.

If you are mobile you have almost unlimited options for reproductive strategies, including external or internal fertilization, water-column development (eggs) or brooding (live young), or even cloning yourself. Sea anemones for example are simply able to walk away from part of their body which then becomes a new sea anemone. Cloning is an advantageous strategy if you have a design that works (why change it). It is also useful if you are toxic and live in close proximity to your conspecifics – your toxin cannot hurt your clone. The disadvantage of course is that evolutionarily speaking cloning is a dead end – you can no longer change your genetic material to create new and sometimes perhaps better designs. Therefore most organisms which can clone themselves are also capable of some form of sexual reproduction. It is interesting to note here that since we as humans are essentially preventing natural selection from acting on us as a species (mostly through medical care) or are even selecting for what we consider less advantageous designs (reproduction tends to be higher in population segments with lower intelligence), cloning would probably be the reproductive method of choice for us from an evolutionary point of view.

High Energy Versus Low Energy – The Question of Wave Exposure

 

Wave action affects intertidal species’ home. Rocky shores are classified as high energy or low energy environments. High energy environments are exposed to significant swell action while low energy environments are more protected, quieter, with more basic wave structure. If you live in a high-energy environment, waves can rip you off the rock. Holding on tight won’t help if you have a soft body – your body parts could get torn off, or rocks or other debris thrown around by the waves might smash or flatten you. However, you have lots of oxygen,  water exchange, and fresh food if you are a filter feeder. If you are sessile, mobile predators might not be able to get to you if there is a lot of water movement, giving you a definite edge on survival. Strategies for surviving high-energy environments include, permanent attachment methods, thick shells which are very smooth and therefore reduce drag, and behavioral strategies which minimize wave exposure for mobile species. The dogwinkle, the prime predator of higher energy environments preys on the barnacle, the primary sessile organism of the high-energy intertidal. Barnacles have thick, sturdy shells and permanent attachment methods so they can settle anywhere  in the intertidal as long as there aren’t loose rocks which grind them up. Dogwinkles, which have small, thick, very smooth shells are mobile, but not as adept at hanging on to the rock face and instead tend to hide in cracks where wave action is diminished. These factors create halos of barnacle-free rock around cracks and crannies packed with dogwinkles.

 

Plants also have their own strategies of dealing with high energy environments. Thicker stems and blades, and solid holdfasts are the first line of defense. While seaweeds growing in low-energy environments are sometimes only a few cells thick, seaweeds in high-energy environments have thick, leathery blades. Long strings instead of wide blades are common, and where blades are used they might be split lengthwise or have holes to reduce drag. In case the whole plant is torn off, some seaweeds develop extremely tough holdfasts from which the plant can regrow. These holdfasts might be centuries old while each blade may last only a few years. Other seaweeds compensate via their lifecycle. The large form of the plant is annual, growing and reproducing fast and dying in the winter when ice scour and wave action make life particularly hard. However, a very small form is also part of the lifecycle. This form is encrusting, plastered against the rock face, and virtually indestructible by wave action. The job of the large form is to assimilate energy and reproduce, while the job of the small form is to persist.

 

Low energy environments are hardly the ideal solution for everyone. Water exchange tends to be low, so oxygen levels can drop, temperatures become extreme, and the food supply for filter feeders diminishes. Silt and sand can accumulate, clogging up feeding apparatuses and photosynthetic surfaces. But seaweeds can afford to be more delicate, with wide thin blades which capture a maximum of sunlight. They can also grow right to the surface and therefore capture much more sunlight than those which only persist at deeper levels due to excessive wave action. Thin blades take much less energy to build, so growth is faster.  But grazers will also tend to be plentiful here, eating the seaweeds almost as fast as they can re-grow. If this delicate balance gets disrupted – for example through the introduction of a larger, hungrier grazer, the whole plant community might change. The Common Periwinkle dramatically altered the northeastern US following its introduction in the last century. The tougher species might not do well in low energy environments. Toughness becomes a liability here since it generally serves little purpose (there is no wave action to protect against) and prevents rapid growth and energy assimilation. Slow growth usually means that something else will grow on top of you and smother you. Low-energy environments will allow predators free access everywhere. Even the toughest barnacle for example can not withstand a few days of undisrupted drilling from a dogwinkle - and the dogwinkle has all the time in the world to drill since there are no waves to tear it off.

 

Thus patterns of exposure on different sections of the rocky intertidal will result in distinctive plant and animal communities. The most spectacular organisms are often found on man-made environments where wave action is low, but water exchange high, such as the inside of harbor walls and pilings.
 

Can’t Go Up – Can’t Go Down: Intertidal Zonation

If you look at the rocky intertidal you will notice that species tend to occur in distinct horizontal bands. On the very top is a black stripe, followed by a light green one, a white one, a blue one, another green one, a red one, and a brown one. The patterns are the conflicting forces of predation, grazing, competition and exposure on the other.

 

The black zone is made up of lichens (a symbiotic life form formed by a fungus and an alga). Lichens grow just about anywhere, but are overgrown by land plants. So they move as close a possible to the water’s edge. Further up and they get overgrown, further down and they end up submerged. Hence the stripe.

 

Below the lichens which are a land life form limited by the edge of the sea, are the sea life forms which are limited by exposure to the land, namely dessication (drying out) and overheating. The first band are the delicate light-green seaweeds. They persist on a narrow strip limited by drying out from above and voracious grazers from below. Here live rough periwinkles and little marine snails pushed upward by their larger cousins, the common periwinkles below. Winning here means that something else is more effective at using your food or energy source and eats it all. In that case the common periwinkles eat all the delicate green weeds, leaving none for the rough periwinkles. Luckily they have the ability to move just slightly higher up and exploit a food source which the common periwinkles cannot reach. The delicate green weeds cannot persist where common periwinkles can reach, so common periwinkles after eating them turn to other, tougher food sources not as suitable for rough periwinkles. A species which is outcompeted and cannot move out of the range of its competitor generally goes extinct, as does a species which is overgrazed over its complete range. The fact that the delicate green weeds are removed by the periwinkles opens up free space on the rock for the next group, the barnacles. Barnacles, being filter feeders, need to be submerged for a significant time each day to feed. Any further up they starve and dessicate. Being further down would be ideal, however that is where the dogwinkles live. Dogwinkles fortunately cannot exploit barnacles quite to the top of their range since they can only drill while submerged. The barnacle survives by moving up into the air. In this case predation determines the lower boundary.

 

The blue zone is made up of blue mussels. Being larger filter feeders, blue mussels need water around them. They are generally not bothered by dogwinkles, having developed a very effective defensive against them, so they can persist in the middle of the dogwinkle zone. However, their large predator is the sea star. Sea stars prey by using their suction system to pry mussels open. This system works only under water, and being large and soft-bodies, sea stars cannot afford to be exposed at all (they are also brightly colored and very visible to sea birds which would readily eat them). Therefore the upward extent of sea star predation is limited by the time it takes to pry open a mussel, which is a few hours.

 

So why does complete overgrazing or overpredation work in a marine environment when it doesn’t on land? If a terrestrial species eliminated its prey species as completely as marine species do, the terrestrial species would drive its prey to extinction and then quickly starve. The solution to this apparent paradox is that sessile marine species – plant and animal alike – generally do not choose on which tidal level to settle. Instead settlement takes place over the entire intertidal zone, and predators spend the year living off those recruits which have essentially settled too low. During the next season new offspring will be produced by the adults which have escaped predation or grazing by living where they cannot be reached. This ‘reservoir concept’ is being used in conservation through the creation of marine or game parks where reproduction can take place but no take is allowed in the hope that the level of reproduction will be sufficient to restock empty areas. The marine invertebrates produce literally hundreds or thousands of offspring every year of which only one or two need to persist to the next reproductive cycle. This excess plankton feeds the marine food chain up to animals as large as whales. Few vertebrates are capable of this level of excess offspring production.

 

Below the blue mussels the zonation of seaweeds becomes evident. Here the downward limiting factor is not so much grazing pressure, but light. All plants photosynthesize, i.e. they use sunlight to produce food out of air. Where there is no more sunlight, photosynthesis does not work, and the plant dies. The upper strip of seaweeds is made up of the heavy green strands of Ascophyllum. Ascophyllum has evolved air bubbles as a strategy to float up to the surface and capture as much sunlight as possible (in the process making it much darker down below, which may or may not be a problem depending on the steepness of the slope). Ascophyllum, like all land plants is green.

 

The question why the grass is green can in fact be answered. White light is made up of three components red, yellow, and blue. Red light has the longest wavelength, produces the most warmth, and is therefore used by the plants (absorbed). Blue and yellow light isn’t, so it is reflected as green (what we see). Because red has the longest wave length, it is also the first component of light to drop out in water (the last color to drop out is blue, which is why the deep ocean is that color). Because red light is not available below the water surface, the next band of seaweeds is red. Those plants absorb both blue and yellow light to photosynthesize. Under water that is all there is, so they look gray or black. Exposed to light above the surface, they reflect the red they cannot use and therefore look red to us. Some red algae such as the pink coralline algae which cover the rocky seafloor in the dimness of the subtidal and survive under rocks or thick seaweed cover in the intertidal are so sensitive to red light that exposure to light will kill them. Below the red seaweeds is the zone of the brown kelps, giant plants which capture any available light with their large floating blades. The kelps are at the lowest edge of the intertidal, where exposure to air is extremely short. Below them is the subtidal, where light is no longer sufficient for photosynthesis, and wave action becomes insignificant. Other factors such as temperature, remaining visible light, remaining oxygen, and pressure drive communities here along a gradient which eventually reaches the abyss, where deep-sea trench communities survive without any of the factors which make life possible at or near the surface. Except for the communities of the deep-sea trenches all life throughout the ocean water column depends on energy created in those few feet of surface water where sunlight penetrates enough to make photosynthesis possible.

 

Action:
What you can do:

1. The rocky intertidal is one of the habitats with the greatest density of life found anywhere in the world. Every step you take kills literally hundreds of organisms – some of which may be hundreds of years old.
1. Avoid wandering in the intertidal – take a boat
2. Avoid dragging or crushing moves in the rocky intertidal – use a sand or pebble beach
3. Avoid turning rocks over. Limit lifting up of rocks to look underneath. If you do lift up a rock, carefully replace it exactly the way it was (checking that nothing mobile has moved into the hole in the meantime).
2. Mooring chains, pilings, and wrecks quickly get settled by marine life. Some of those organisms may be new recruits (just have settled out of the plankton), while others have migrated there and may be decades or centuries old.
1. Avoid scrubbing mooring chains across them. Use scuba gear to selectively dislodge large clumps of mussels or large kelps.
2. Don’t pull pilings out of the water. They are often home to ancient individuals such as sea anemones. If you have to replace a piling, consider dropping it to the bottom. If that is not possible, tie it off just below the surface and raise it slowly, giving mobile organisms such as sea anemones a chance to leave. As an alternative you can relocate large organisms by hand.

What needs to be done globally:

1. All marine life depends on plankton, and yet plankton is seriously threatened by pollution.
1. Stop oil spills by requiring double-hulled tankers and stricter safety regulations.
2. Stop open ocean dumping of oil and chemicals.
3. UV light kills plankton. Stop the growth of the ozone hole.
2. The intertidal ecosystem is a delicately balanced species community. Removing or adding any one species can crash it.
1. Before harvesting any one species consider its impact on the ecosystem.
2. Understand the full life cycle and reproductive output of any species before considering harvesting it.
3. Never harvest a species which you cannot reliably age – you may be mining a group of centuries-old individuals.
4. Accept that many marine species should not be harvested because of inadequate surpluses