Animals and seaweeds become much more diverse at the low and sub-tidal zone. Compared to the upper shore, this habitat is more protected and predictable: environmental fluctuations are small, organisms are not in danger of drying out, and there is a constant supply of dissolved nutrients to support a rich array of red and brown seaweeds.
Pink paint and coral turf
Like the black lichen above them, some red seaweeds resemble a grand-scale paint job. Corallina officinalis and related seaweeds begin their life as flat sheets that extract calcium from the sea and deposit it as lime within their cell walls, forming a pink crust on the rocks, known as pink paint. Later they develop into a tufted form that resembles coral fans.
Coralline seaweeds extend from mid-tide level to well below low tide, where they provide an important habitat for sponges, shellfish such as young cat’s eyes, Cook’s turbans (Cookia sulcata), pāua (Haliotis species) and kina (Evechinus chloroticus), a sea urchin or sea egg. They are also known to produce chemicals that influence the development of the larval stages of many marine animals.
Large brown seaweeds dominate the low-tide zone. Neptune’s necklace (Hormosira banksii) often accompanies Corallina in tidal rock pools throughout New Zealand, but does not extend into the subtidal zone. A few other large brown seaweeds form extensive underwater forests that support an extraordinary diversity of animals and smaller seaweeds. Common kelp (Ecklonia radiata) and flapjack (Carpophyllum maschalocarpum) are the key species of northern coasts. Bull kelp (Durvillaea antarctica) and bladder kelp (Macrocystis pyrifera) are the dominant southern browns.
By the armload
The reef star (Stichaster australis), a starfish about the size of a man’s hand and with 10–12 orange arms, lives on the west coast mussel beds, tearing mussels off the rock before prising them open to eat.
Mussels and oysters
Mussels and oysters are filter-feeding shellfish that cement themselves to tidal rocks. They extract microscopic particles of food from the water that passes over their gills. Beds of edible mussels often develop on exposed coasts between the mid-tide and low-tide zones. Blue mussels (Mytilus galloprovincialis) favour colder southern waters and green-lipped mussels (Perna canaliculus) are the common northern species. These two species grow to over 10 centimetres and should not be confused with the little blue-black mussel (Xenostrobus pulex), which is a quarter their size.
Pacific oysters (Crassostrea gigas) and rock oysters (Saccostrea cucullata) are also found at mid-tide levels on northern coasts. Pacific oysters arrived in New Zealand around 1950, quickly spreading around northern coasts and reaching the top of the South Island in 1977. They are twice the size of the native rock oyster and grow rapidly, spawning several times a year. They were an attractive proposition for oyster farmers who now cultivate this species rather than the native rock oyster.