ROCKPOOLS: MICROOCEAN WORLDS Naturalist Georgina Jones shares the wonderfully intricate goings-on in rockpools

ROCKPOOLS

micro-ocean worlds

Naturalist Georgina Jones shares the wonderfully intricate goings-on in rockpools, where coexistance and predation occur in equal measure, largely invisible to the untrained eye.

South Africa’s rocky shores offer a treasure of rockpools just waiting to be explored.

ONE OF THE TREASURED memories of many childhoods is time spent peering into or splashing about in rockpools. These fascinating places offer a glimpse into the complexity and beauty of the marine world.

Seaweeds, powered by the sun and buoyed up by the ocean’s water, provide the base for life in most rockpools. Grazers, ranging from limpets and snails to roving urchins and pool-dwelling fish, feed on their fronds and spores. Some limpet species are farmers, grazing selectively on a preferred type of seaweed and actively removing others from their gardens. Urchins, feeding under shading fronds, are spiny protectors for juvenile perlemoen, sheltering them from predatory kreef. Although they are safe from hungry crustaceans, urchins protect themselves from the sun by wearing shell or debris hats.

Wave action in rockpools can damage seaweeds, tearing their fronds or ripping them loose completely, making them more easily accessible to adult perlemoen.

SCAVENGERS AND HUNTERS

Some animals living in rockpools sit and wait for food to come to them. Mussels have sacrificed brains in favour of enlarged feeding filters. Sponges can build themselves into turrets and use whip-tailed cells to propel water-borne food into their bodies. Redbaits siphon food-rich water into their feeding baskets. Anemones attach to rocks or hermit-crab shells and snag passing prey with their tentacles. Barnacles have specially adapted feeding legs that extend into the water to sieve out passing snacks.

Then there are the busy detrivores. These include crabs and snails that patrol the rockpools in search of leftovers from other animals’ meals, discarded exoskeletons or the dead and dying.

Rockpools have predators too. Klipfish hunt tubeworms and small crabs. Although some starfish are grazers, carnivores like spiny starfish are enthusiastic predators, tackling mussels, clams and even seafans. Octopuses, the master camouflage artists of the ocean, hide under rocks or in plain sight, disguised as seaweed or even a rock, and wait for unwary prey.

As in any healthy ecosystem, reproduction is an ongoing feature. Some animals rely on the ocean to disperse their offspring, sending millions of eggs and sperm into the water to find their adult homes. Others, like some of the crabs, brood their babies either on their bodies or, in the case of some brittle stars, inside themselves. Cuttlefish and octopuses find choice dens in which to lay their eggs and then ensure the developing embryos are safe and oxygenated while they develop.

Above: A four-colour nudibranch hunting for prey. Left: Orange tangleworms lurk in cracks. Bottom: Unusual stalked jellyfish nestle under overhangs.

COPING STRATEGIES

Rockpools are not the easiest of homes. They are subject to extremes of temperature, water depth and salinity, not to mention wave action and sand inundation. The plants and animals inhabiting them have developed various coping strategies.

Plum anemones live under overhangs and close up tightly when they are exposed to air. Fish must find the remaining water in the pool during low tide.

Wave action is good for filter feeders such as mussels and barnacles, bringing them food – but they have had to develop means of keeping themselves in place while waves break over them. Barnacles have developed a strong cement, whereas mussels attach themselves with strong threads. Mobile animals must find places to hide when waves and storms scour their pools.

Sand inundation is a significant challenge. Some species can survive for up to months under sand, simply shutting their metabolisms down and waiting for the sand to recede. Other species rely on their dispersed larvae to recolonise the pools once they are again habitable. Some reappearances remain a mystery.

Be it a camouflaged octopus, a fish darting for an overhang, a slowly moving starfish or an unfurling tubeworm, rockpools offer life in abundance for observing adults as well as children to delight in.