South Coast Rock Pools

Murray Hosking

Rock pool

Small rock pool high in the intertidal zone, coralline crusted with a patch of young Hormosira

Spare a thought for the marine life which inhabits rock pools on the South Coast or anywhere on the coastline. The South Coast has many rock pools to explore, on the rock platforms raised by the old earthquakes of previous centuries. It is a bit of a hazard for a marine creature to be caught in a rock pool high up on the coastal reefs. As the storm or high tide retreats the pool may be exposed to summer sun, and high water temperatures may well result. As the seawater evaporates the salinity can go sky high, or conversely, with a shower of rain may be quickly diluted with freshwater. As the sun shines the photosynthesis of algae in the pool can generate oxygen, which you can see as filamentous algae are buoyed up with bubbles of oxygen, raising the pH levels . That is likely to reverse as the sun goes down at night. However, if the pool stagnates, organic decay can drop the oxygen levels rapidly and raise hydrogen sulphide. It takes a lot to survive all of that change.

 Rock pool oxygen

 Oxygen bubbles in a warm, high rock pool

With a bit of luck for marine life, the pool is low enough in the intertidal zone to be replenished with seawater on the rising tide, though the slap or surge of seawater into the pool may cause a sudden drop in temperature again stressing biota. Close to the low tide level the replenishment is regular, and if the pool is large enough, there will likely be a good growth of algae to shade the water from the worst of the sun. The largest pools are likely to be part replicas of the shallow sub-tidal shore at large.

Nevertheless, both algae and the many animals associated with the pool have been elevated to levels higher than normal for the range of the species and need to adapt to survive long in rock pools. On the South Coast there are very often fringes of Hormosira (Neptune’s Necklace), along with coralline red algae both as flat algal ‘paint’ or crust on the rocks, or as stiff, multibranched fronds, which  form the hidden habitats for a multiplicity of small marine life. Corallina officinalis is one of the most common. In larger pools one can see larger brown algae such as Carpophyllum and tufted Halopteris species, and also green sea lettuce (Ulva) where nutrient levels are high.


 Corallina officinalis below Hormosira

The smallest pool animals graze amongst the coralline and other algae – not that the corallines themselves offer much nutrition, but rather the tiny, epiphytic algae that secrete themselves within. These can be grazed by cat’s eye snails (Turbo species), crabs, worms (such as the flame red proboscis worm – Sacoglossus), isopods, amphipods and other ‘pods and ‘cods.  Many are cryptic, requiring a microscope to study if you can locate them. There are often shrimps to be seen though, the most likely to spot is the rock pool or glass shrimp Palaemon affinus, which usefully predates the larvae of the NZ saltwater mosquito. Sea stars and crabs are there too.

 Small fish are a feature to look for in the medium to large pools, though few are larger than about 10 cm. These need to adapt to the conditions through mechanisms such as rapid growth rates, short life spans and early maturity. The commonest fish are the cling fishes, with a sucking disc to attach to substrate and algae. The most commonly seen are the triplefins, such as the common triplefin, Forsterygion lapillum, observed from its darting movements in pools.

 Large pool

 Large pool low in the intertidal, regularly replenished at high tides and during storms
Common Triplefin
Common triplefin 

Rock pools are always worthy of study – to note the differences between the smallest and highest, and the larger and lower pools. See what you can find when you pick up rocks and shells and examine the undersides – but always replace them as you found them.


This note was drawn from material in ‘The Taputeranga Marine Reserve’ edited by Jonathan Gardner and James Bell, second edition, 2008, and from ‘Seashore Ecology’ by John Morton,2004. All photos by M Hosking.