The rocky coast – upper shore
There are recognisable vertical patterns or zones of life at the land’s edge. Plants and animals are specially adapted to conditions at different heights on the shore – from the cliffs down to the low-tide zone.
Exposed coastal cliffs with their nooks and ledges support an open vegetation of grasses, hardy ferns, coastal flax (Phormium cookianum) and sprawling succulents such as horokaka, also known as Māori ice plant (Disphyma australe).
Gentler coastal slopes support lowland coastal forest and scrub. If they are cleared of original cover, pasture grasses and invading shrubs like gorse and mānuka will grow.
Pōhutukawa (Metrosideros excelsa), the distinctive red-flowered coastal tree of northern New Zealand, anchors itself to cliffs with sturdy roots, sending out a number of stems from its base. It is one of the few trees able to colonise bare lava, and forms the dominant cover on volcanic Rangitoto Island. Tolerant of salt winds, fast growing, and spectacular in summer, it is a popular ornamental tree, grown far beyond its natural southern limits in Poverty Bay and Taranaki.
Just above the splash zone – the area constantly exposed to salt spray – flowering plants and ferns fail to thrive. Colourful lichens such as bright yellow-orange Xanthoria, white encrusting species of Pertusaria, and grey-green Ramalina, dominate this area. Lichens are composite organisms, consisting of a fungus in a partnership with algae or bacteria. Beneath the colourful lichens lies a distinctive black band of Verrucaria lichen.
The hardiest maritime animal is the banded periwinkle (Austrolittorina antipodum), a tiny snail with a blue and white conical shell that can withstand days without water. Thousands congregate just above the high-tide zone, where they graze on thin films of seaweed and Verrucaria lichen.
Barnacles occupy the next zone down, around the high-tide level, where they receive a twice-daily dowsing of sea water. Although having the outward appearance of shellfish, barnacles are filter-feeding crustaceans, relatives of crabs and crayfish.
Three shore species are commonly encountered. Column barnacles (Chamaesipho columna) are small, about the size of a rice grain, and occupy the largest section of the barnacle zone. The larger brown barnacle (Chamaesipho brunnea) can be found on very exposed northern shores. The ridged barnacle (Elminius plicatus) is the largest of the common barnacles, about the diameter of a 5-cent piece.
Ornate limpets (Cellana ornata) usually return to the same spot on the rocks after their nightly feed. A marked limpet was observed moving onto its neighbour’s spot. When the resident limpet returned it pushed against the intruder for 20 minutes, then settled some distance away. During the next high tide the offending limpet moved off to feed and the ousted limpet returned directly to its home site, staying put without feeding for the tidal cycle. The intruder returned to its own spot after feeding.
The middle shore is the domain of grazing shellfish (molluscs). When the tide is high during the night, molluscs such as spotted top shell (Melagraphia aethiops), pūpū or cat’s eyes (Turbo smaragdus), limpets (Cellana species) and snakeskin chitons (Sypharochiton pelliserpentis) feed on the algae clothing the rocks. The common cat’s eyes have a dark green ‘trapdoor’ that closes when the animal withdraws into its shell.
When the tide is out during the day, young molluscs shelter in crevices or under rocks, but larger limpets and chitons will withstand full exposure to the elements by clinging to the rock surface with their muscular foot.