Features

Marine environment

Horoirangi Marine Reserve location
map

The bluffs overlooking Horoirangi Marine Reserve are more than a dramatic landscape; they are also the source of rocks for the rugged boulder reefs below. A distinctive feature of the reserve, these reefs form a wide intertidal zone and extend offshore for up to 400m and to a depth of around 20m.

The boulders on the upper shore, regularly moved by the waves, are mostly devoid of life. Lower down and subtidally the reefs are much more stable—a matrix of interlocking boulders, some in places measuring metres across, interspersed with occasional patches of bedrock. Bedrock is more common around Mackay Bluff and towards the north of the marine reserve, culminating in the spectacular rocky outcrops of Ataata Point.

A vividly coloured ambush starfish
waits patiently to trap its prey

These intertidal and subtidal reefs are home to a typical array of animals—shellfish, anemones, sponges and sea squirts cling to the rocks; snails, starfish and crustaceans move secretively amongst the boulders; and various fish patrol above. Ambush starfish are unusually common in Horoirangi, their medley of startling colours providing a sharp contrast to the subdued hues of the rocks and most other shallow-reef species.

Brown seaweeds form a dense forest between Mackay Bluff and Ataata Point; elsewhere their leathery fronds occur only as a sparse fringe below the low water mark. Other types of seaweeds are uncommon in Horoirangi, though sea lettuce can be seasonally abundant.

Hermit crabs are common on the
offshore sediments

Soft sediments lie beyond the outer reef edge: mixes of mud, sand, shell and gravel closer to shore, with soft mud dominating offshore. In some shallower areas, wave action has sculpted the sediments into large ripples. Although various burrowing worms, shellfish and shrimp-like animals dominate these offshore sediments, more conspicuous are the surface dwellers, such as hermit crabs, urchins and brittle stars.