Intertidal Zone

At first glance, they might appear to be home to only a few limpets and some seaweed, but peering into the depths of even the smallest rock pool reveals incredible diversity. And no two are ever the same.

Crabs and other crustaceans are easily spotted throughout pools, while small fish like blennies and gobies dart around the bottom feeding on tiny crustaceans.

Meanwhile, sea stars stick around the edges. Larger species like the eleven-armed sea star and the velvet sea star may be easily spotted, but smaller species like the little sea star which is less than 15 millimetres across, or the southern biscuit star which comes in a variety of patterns and colours are often only found under boulders.

Large red anemones are also fairly conspicuous, trapping amphipods and isopods in their sticky tentacles, but living under boulders are also lots of smaller species.

Shells and snails

Seaweeds, corals, worms, sponges, barnacles, limpets and other molluscs like mussels, snails, whelks, nudibranchs and oysters may be found in the depths of rock pools.

In temperate waters, beachcombers can often see the turban snail, known for its distinctive green-striped shell. Growing up to four centimetres in size, they feed on algae growing on rock platforms.

Limpets, with their distinctive, oval shaped shell, are another frequent sight along rocky shorelines. During high tide they move about and graze on algae, returning to the same place when the tide falls and sealing themselves tightly against the rock to conserve moisture.  Rock pools are teeming with life and there’s a lot happening that we don’t see.

However, you should avoid putting your hands in the water and exploring rock crevices, as not all creatures are harmless. One creature that hides under boulders and in pools that have a sandy floor, is the blue-ringed octopus, which is very common in New South Wales and Victoria. They are able to burrow down and hide in the sand which makes them hard to see. Normally blending into its surroundings, this small brown octopus develops brilliant blue ring-shaped markings when it is threatened. Despite its attractive markings, the blue-ringed octopus is extremely poisonous and should not be approached or handled.

Life in tropical rock pools tends to be less colourful, and is restricted to the few species that are able to tolerate prolonged high temperatures like barnacles and limpets, which are able to seal tightly to the rocks to stop moisture loss.  Even in temperate regions, the inhabitants of smaller pools face the same challenges, so small pools tend to be home to only the most robust species like sea lettuce, Neptune’s necklace and crabs.

Intertidal Walk Little Bay

further details Sydney Shell Club

Cowries of Little Bay

Rock pools at a glance

Southern blue-ringed octopus (Hapalochlaena maculosa)

A relatively common inhabitant of Australia’s rocky southern coast, these venomous blue-ringed octopuses hide under rocks and in the sandy base of rock pools. It is extremely poisionous — a bite can cause paralysis, and in rare cases, be fatal. Despite its reputation, it isn’t aggressive and when disturbed will try to flatten itself out and change colour to blend into its surroundings. Its blue rings light up only when it feels threatened and is about to bite.

 

 

Neptune’s necklace (Hormosira banksii)

Varying in colour from olive green to reddish brown, Neptune’s necklace is a common seaweed of the temperate coast. Growing on flat surfaces such as rock ledges that allow it to be exposed to the air at low tide and covered during high tide, it consists of a row of leathery, spherical air bladders joined together like a necklace. It often grows alongside bright green sea lettuce.

 

 

Blennies (Blennioidei)

Blennies are small, bottom-dwelling fishes, most of which live in shallow coastal waters, including rock pools. About 360 species are known worldwide, with 92 species found in Australia. Growing to approximately 13 centimetres, the Tasmanian blenny is a mottled, orange-brown fish commonly found in south eastern Australia on rocky reefs, in bays and estuaries, under jetties and amidst the rubble at the bottom of rock pools; it has even been found living in old beer bottles.

 

 

Anemones (Actiniaria)

While anemones look like flowers, they belong to the same group as jellyfish — cnidaria. Coming in many different colours and sizes they have a cylinder-shaped body surrounded by stinging tentacles, that immobilise their prey and draw it into the mouth to be digested. While they are usually found anchored to a rock, they can move around if conditions become unfavourable. They also retract their tentacles to avoid direct sunlight when water levels drop at low tide.

 

 

Eleven-armed sea star (Coscinasterias calamaria)

Found across temperate Australia, eleven-armed sea stars may grow up to 25 centimetres across (although they are usually between 10 and 12 centimetres) and will eat almost anything. Like all sea stars, they have an exoskeleton made up of hard plates known as ossicles that are embedded in their skin, giving it a bumpy texture.

 

 

Velvet sea star (Petricia vernicina)

There are several species of large sea stars found in deep rock pools along the temperate coastline, including the velvet sea star which has five arms and varies in colour from red, to mottled orange–yellow, to a purplish mauve. As its name suggests it is velvety to the touch, and like other sea stars digests its food outside its body by everting its stomach and covering its meal. If disturbed while feeding it quickly pulls its jelly-like stomach back into its mouth.

 

 

Biscuit sea star (Tosia australis)

Another temperate species, this sea star grows to only a couple of centimetres across and occurs in a range of colours and patterns which can include mauve, pink, purple, red, brown, cream and black. A row of tube-like sucker helps them to stick to the sides of rock pools, move around and open the shells of bivalve molluscs long enough to slide their stomachs inside.

 

 

Turban snails (Turbo undulata)

Growing up to four centimetres in size, the distinctive striped turban snails are found among seaweeds like Neptune’s necklace in rock pools and on rocky shorelines.

 

 

 

 

Limpets (Cellana tramoserica)

They have an oval shaped shell, usually with a peak near the centre. During high tide they move about and graze on algae returning to the same place when the tide falls and sealing themselves tightly against the rock to conserve moisture.

 

 

 

Purple sea urchins (Heliocidaris erythrogramma)

Sometimes collected for food, this species grows up to 14 centimetres in diameter and can be found in southern Queensland, New South Wales, Victoria, South Australia and Western Australia. Their mouths are located on the underside of their body and they feed on algae or seagrasses.