All animals need to be able to have access to food. If animals do not eat, they die. In an intertidal environment, food is not available to all creatures all of the time.
When the tide is low, most high- and mid-shore animals uncovered by the receding water, must either hide themselves away, close their operculums across their shell opening, secrete a mucous ring around the edges of their shell, to protect themselves from the desiccating rays of the hot sun.
Most intertidal animals can only feed when the tide is in. The larger filter feeders, such as the sponges, large barnacles such as the Giant Rock Barnacle, Balanus nigrescens, and large bivalves, Edible Mussel, Mytilus edulus planulatus, occur on the lower shore so that they are submerged for long periods of time.
Smaller filter feeders, such as the Honeycomb Barnacle, Chamaesipho tasmanica, and the Six-plated Barnacle, Chthamalus antennatus, can occur quite high on the shore, only covered by the tide for a few hours each day.
There are a number of feeding strategies that algae and animals of the rocky ocean shore have adopted. Some of these are:
- Nutrient absorbers: these are the energy and nutrient absorbing algae. These are the Primary Producers.
- Grazers and Browsers: these are the molluscs which eat micro- and macroalgae. These are the Herbivores, or should they be called Algavores.
- Suspension feeders: These are the Plankton feeders.
- Deposit Feeders: These are the Detritus feeders.
- Carnivores: These are the meat-eating hunters. First-level Carnivores eat Herbivores. Second-level Carnivores eat First-level Carnivores. Carnivores includes Parasitism.
- Omnivores: Eat a range of food types, but mostly carrion and debris.
Algae absorb sunlight by photosynthesis and convert solar energy into chemical energy which they use in growth or to store within the cell. Algae are the primary producers within this marine ecosystem.
Unlike land plants which obtain their nutrients from the soil by absorption through roots, algae absorb the nutrients they require directly from the seawater that surrounds and supports their fronds. Algae do not have absorption roots, their holdfast only holds them down onto a firm surface.
The large algae that we see on a shore are called macroalgae, and unusually there are surprisingly few shore animals that live entirely by eating algae fronds.
The other algae group is the microalgae, consisting of single-celled plants, spores, and minute juvenile plants which occur in their millions suspended in the water as plankton or coating the rocks as part of the deposited slime, or gaining a foothold to grow into a larger plant.
Large algae plants produce thousands of cells in each frond and on the frond surface. Under the constant swashing of the waves, these cells erode away from the plant. These cells become suspended in the water and are subjected to bacterial action. Others are filtered and consumed by other animals.Some examples of green algae are:
Sea lettuce, Ulva lactuca
Green Sea Velvet, Codium fragile
Caulerpa, Caulerpa filiformis
Neptune's Necklace, Hormosira banksii
Leather Kelp, Eklonia radiata
Strap Weed, Phyllospora comosa
Some examples of red algae are:
Coralline Seaweed, Corallina officinalis
Encrusting Corallines, Corallinaceae species.
Browsers & Grazers
A major source of food on shores is algae. Some molluscs graze on the fronds of the larger green, brown and red algae. One of these is the Common Warrener, Turbo undulata. It grazes on algae at low shore levels.
Here are some Common Variegated Limpets, Cellana tramoserica, with a covering of algae on their shells, but no algae on the rock surface. Why is this so ?
It is estimated that for every square centimetre of limpet, about 75 square centimetres of encrusting algae is necessary to maintain its life during its first year of growth.
On many rock surfaces you can see the small round home scars of these limpets, and if you look carefully you can pick out the area which is grazed by them. If you remove the limpets from an area, you will notice that in a short time tiny algae clumps begin to sprout and grow. In areas where the limpets remain, the rock remains bare.
As these molluscs move across the rock they rasp their tongue, called a radula, across its surface. The radula is a file-like ribbon of small horny teeth that all gastropods and some other molluscs possess. It can rasp either vegetable matter, or flesh in some carnivorous species, and convey the particles of food into the mouth.
However, most browsers and grazers do not fed on large macroalgae fronds. They feed on microalgae, algae spores and small plants trying to gain a foothold on the rock surface in moist depressions and pools.
Some browsers at the Splash Fringe Level and High Tide Levels are:
Noddiwink, Nodilittorina pyramidalis,
feeds on lichens at high shore levels
Australwink, Nodilittorina unifasciata,
feeds on lichens at high shore levels.
Petterd's Limpet, Notoacmea petterdi
Mid Shore Browsers and Grazers
Some grazers and browsers of the mid-tide levels are:
Zebra Top Shell, Austrocochlea porcata,
Ribbed Top Shell, Austrocochlea constricta
Wavy Top Shell, Austrocochlea concamerata
Black Nerite, Nerita atramentosa
Striped-mouth Conniwink, Bembicium nanum,
Variegated Limpet, Cellana tramoserica
Other grazers at the low-tide level and Low Fringe level are:
Black Keyhole Limpet, Amblychilepas nigrita
Elephant Snail, Scutus antipodes.
Common Warrener, Turbo undulata.
Plankton is a broad grouping of all the animals and plants which drift around in the ocean currents, independent of their size. The animals and plants range from microscopic one-celled organisms to giant jellyfish.
Plankton includes tiny green plants called diatoms, They have a beautifully patterned shell made of silica. Plankton also includes creatures that spend all their lives drifting around, including the juvenile stages of many shore creatures such as crustaceans, molluscs, echinoderms, worms and ascidians.
There are absolutely millions of billions of them. Every cup of surface seawater teems with plankton. There are probably hundreds of plankton in every drop of water.
Some of the countless single-celled plankton animals are the Dinoflagelates, Formanifera and Radiolarians.
- Dinoflagelates have chlorophyl, so can create their own energy but they also have animal characteristics.
- Formanifera are tiny floating cells that live their entire life in the ocean. They have a shell of lime.
- Radiolarians are single-celled animals with a shell of silica. In some parts of the ocean there are deep deposits of radiolarian ooze, created when dead radiolarians drift down from the surface.
It is the plankton food engine that drives much of the food-web ecology for a tremendous range of other creatures in the sea and on the shore.
On a rocky shore there are many plankton feeders, including all of the barnacles, many species of worms such as Galeolaria, oysters, sponges, mussels, pipi, cunjevoi, etc. They are all known as filter feeders because they have adaptations such as specialised mouthparts, brush-like combs, or secrete a slime from their gills to capture plankton food. As see Sponge feeding.
Bivalves and some other molluscs use their gills to obtain oxygen and their food as well as release carbon dioxide. They feed by drawing in the current of water and sifting the plankton and nutritive particles out using a mucous coating on their enlarged ciliated gills.
The entire fishing industry is almost totally dependent upon the fact that most species of commercial fish eat plankton during one or more stages of their life cycle.
Other not quite so friendly plankton are the harmful "bluebottle" jellyfish, and the potentially deadly box jellyfish.Deposit Feeders
For marine animals there is another source of food which is available to them that is unavailable to land animals. This is the thin layer of ooze which coats the surface of rocks, sand and mud after the tide has gone out.
This ooze, called detritus, consists of bacteria and the remains of finely chopped up algae and decaying fragments of animals, as well as countless numbers of stranded microscopic, one-celled green plants which are called diatoms. This is a rich, thin, soup. Animals which eat detritus are called detritivores.
Detritus is a very rich food and there are many types of marine and intertidal animal groups which are fully adapted to feeding upon it. These animals have special adaptations for dealing with the food contained within the muck.
This the detritus-coated clumps of muddy-sand so that its feeding mandibles can nibble off the nutrient rich detritus food.
Some Nassarius molluscs bulldoze through the sediments. Others like this Semaphore Crab, Heloecius cordiformis (above) has feeding claws shaped like spoons to shovel up and manipulate the mud. Using their mouthparts they sift through the slimy muck to gather the food nutrients, discarding the left-over mud or sand, or quickly passing unwanted sediments through their bodies. Many worms and molluscs leave excreta trails behind them.
Some detritus feeders are the nereid worms and the heart urchin which prefer more calm habitats.
Some small gastropod molluscs are also browsers, grazers and detritus-eaters. They move along the moist pool surface, scraping off the surface scum as well as microalgae living there.
estuarine shore crabs, such as the Semaphore Crab, Heloecius cordiformis,
shown above, and the Mudflat Sentinel Crab,
Macropthalmus setosus, at the left, as well as the tropical beach-living
Sand Bubbler Crab, Scopimera inflata, are detritus feeders. The
Sand Bubbler crab carves out grooves in the sand as it chomps along, quickly
sifting out detritus particles with its highly adapted mouthparts, leaving
distinctive, straight trails of neatly rolled up balls of sand behind.
There are a number of carnivores which live on our shores. Some are swift, efficient hunters, roaming over the rock surfaces hunting down their prey. Fish such as Snapper, Chrysophrys auratus, Yellow Fin Bream, Acathopagrus australis, hunt when the tide is high. The Cyan-coloured Octopus, Octopus cyaneus and Rocky Shore Blue-Ringed Octopus, Hapalochaena sp. which are also found on rocky shores are efficient hunters. If you are fortunate enough to see an octopus hunting down a crab, you will see just how clever and efficient it is.
Birds such as the Sooty Oyster Catcher, Haematopus fuliginosus, hunt when the tide is low.
There are other hunters too. Not so fast, but no less efficient. I have seen the Reef Crab, Ozius truncatus, eating nematocyst-charged Blue-bottles, Psysalia ultriculus. That would give a "sting" to the meal.
There are a number of carnivorous molluscs of various sizes. The largest ones of the low shore, low fringe level and marine zone are the Cart-rut Shell, Dicathais orbita, and Spenglers Rock Whelk, Cabestana spengleri which eats Cunjevoi, Pyura stolonifera.
Another smaller, wide ranging carnivorous mollusc is the abundant Mulberry Whelk, Morula marginalba, found on eastern shores.
The Mulberry Whelk eats the Common Variegated Limpet, Cellana tramoserica, and a wide range of barnacles including the Six-plated Barnacle, Cthamalus antennatus, Rose-coloured Barnacle, Tesseropora rosea, Surf Barnacle, Catomerus polymerus and others.
The Mulberry Whelk has the ability to drill a hole through the shell of their prey, stick their saw-like radula tongue in through the hole, to chop up their prey. They then suck the juices out. It is believed that they place a drop of stomach acid onto the lime of the prey's shell, which assists the carnivore to drill its deadly hole. This process is believed to take only two high tides.
Many of the worms are effective predators, having mouthparts that lunge outwards to capture unsuspecting prey.
Other slow movers, but no-less efficient hunters are the predatory Sea Stars, including the Eleven-armed Sea Star, Cosinasterias calamaria.
Some other carnivores do not actively hunt their prey, but wait for a straying animal to come to them. Examples are the anemones, including the Waratah Anemone, Actinea tenebrosaSand Anemone, Oulactus muscosa. Anemones have stinging nematocysts and sticky tentacles, which are hard to escape. andParasites:
A specialised group of carnivores are the parasites. They do not deliberately kill off their prey, but attach themselves to their host in a way so that they can feed off their tissues, but not actually kill their host. One example is the parasitic barnacle and the Smooth-handed Crab, Pilumnopeus serratifrons.
Omnivores are a group of animals that are not specialised in their feeding preferences. They will eat virtually anything they can find.
The Eight-armed Seastar, Patriella calcar, will eat algae, small molluscs and crustaceans, as well as the remains of dead animals.
Other animals are scavengers, and eat any debris they can find lying on the shore. Crabs such as the Ghost Crabs, Ocypode cordimanna, of the eastern and northern coasts, and the Burrowing Shore Crab, Leptograpsus octodentatus, of southern shores, amphipods and isopods such as the Marine Slater, Ligia australiensis, Sea Gulls, Larus novaehollandiae, as well as visiting Australian Ravens, Corvus corinoides, will eat the remains of dead animals such as the carcasses of Muttonbirds, and fish, thrown onto the shore by the waves.Trophic Levels
On the land the typical Trophic Levels pattern is:
In the ocean, and on intertidal shores, this Trophic Levels pattern is a little different. There are a few other important sources of food to be taken into consideration.
Firstly, there is Plankton which has animal, both very small and some large, and single-celled plant components. Plankton occurs in absolutely countless numbers and volume. In southern waters, it is the energy engine that drives much of the Marine Food Web.
Then there is detritus, which is the broken down ooze of dead and decaying plant and animal remains, being fed on by bacteria. Many animals are adapted to feeding on detritus, such as the Shore Crabs and many worms. The bacteria which eats the detritus is also another very important source of food. Detritus is left as a thin film on a shore when the tide receeds.
Along with detritus, more bulky debris consisting of birds and fish carcases, torn off algae clumps, and all sorts of rubbish, also left by the tide.
The First Trophic Level consists of marine plants. This is made up of single-celled phytoplankton, algae and some seagrasses. They bring carbon into the food chain by fixating energy from the sun, and release oxygen into the water and atmosphere. The plants are the Primary Producers. Some of the algae of the shore are the green algae, brown algae and red algae.
The Second Trophic Level are the browsers and grazers of algae, and phytoplankton eaters. Some of these are the Top Shells, such as the Zebra Top Shell, Austrocochlea porcata, Ribbed Top Shell, Austrocochlea constricta, the Common Variegated Limpet, Celana tramoserica and the Black Nerite, Nerita atramentosa. The Luderick, Girella tricuspidata, or Black Fish is an algae feeder and prefers Sea Lettuce, Ulva lactuca.
We also include the Filter Feeders at this level. They eat both photoplankton and zooplankon, which are the small animals and juvenile stages of many marine creatures drifting in the oceans. Filter Feeders strain the plankton from incoming water with their gills, while some also secrete a slime which sticks to the plankton, and cilia take the food to the mouth. Some Filter Feeders are the Serpulid Worm, Galeolaria caespitosa, the Sponges, as well as Bivalves such as the Edible Mussel, Mytilus edulis planulatus, and the Pipi, Donax deltoides.
A third group added to this level are the Deposit Feeders which eat detritus ooze from the ocean floor, as well as the slime which coats rocks and mud shores when the tide has gone out. Mixed up with the detritus are huge volumes of bacteria, breaking it down. The bacteria and detritus are a rich source of nutrients for many specialised animals that eat nothing else. Some of these are the various estuarine shore crabs such as the Semaphore Crab, Heloecius cordiformis and the Mudflat Sentinel Crab, Macropthalmus settosus.
The Third Trophic Level are the First Level Carnivores. They hunt down and eat the herbivores, or is it algavores. They are the predatory molluscs such as the Mulberry Whelk, Morula marginalba, and the Cart-rut Shell, Dicathais orbita and relations. Yellowfin Bream, Acanthopagrus australis, come into the rocky shore at high tide to eat crabs and molluscs. The mollusc and bivalve-eating Sooty Oystercatcher, Haematopus fuliginosus, is a First Level Carnivore.
The Fourth Trophic Level are the Second Level Carnivores. They hunt down and eat both First Level Carnivores and Herbivores. They are usually swift, voracious hunters, because they have to capture a lot of prey to gain the energy they require. Some of these are the Eastern Blue Groper, Achoerodus viridis, and the Port Jackson Shark, Heterodontus portusjacksoni.
Shepherd, S.A. & Thomas, I.M. (1982). Marine Invertebrates of Southern Australia. Pt. 1:. Sth. Aust, Govt. Printer, Adelaide.
Vaughan, H. (1984). The Australian Fisherman's Companion. Lansdown Press.