Part 1. The zonation of intertidal rocky shores:
Known for its vibrant biodiversity, the Western Cape of South Africa fosters a wide
range of endemic species all living in a relatively small area comprised of different habitats
with unique characteristics. By definition, marine habitats are distinguished by the substrate
of which they are made of, good examples are rocky reefs, sandy bottoms, muddy flats and
kelp forests, all displaying very diverse lifeforms from benthic fauna (living on the seabed) to
open water species moving freely across the water column. Of great importance here in the
Cape Peninsula, rocky shores provide a fixed stable habitat for the ecosystem to thrive. This
is however not the case for other habitats where species have evolved different highly
specialized traits enabling them to fully take advantage of the particular set of characteristics
within that environment.
Let’s begin by focusing on the structure of intertidal rocky shores of the West-coast,
more specifically the different parts they comprise, in other words zonation. The intertidal
zone can be categorized by four distinct zones, all of which have characteristics of their own.
The Littorina Zone is the area constantly above seawater level, even at high-tide, and displays
harsh levels of desiccation due to being constantly exposed to sunlight and air. Of barren
appearance, small-breathing snails such as the African periwinkle but also the purple laver
Porphyra capensis, a thin purplish-brown seaweed that hangs like a plastic bag, inhabit the
area. Below is the Upper Balanoid zone with semi-dry conditions that was once dominated by
granular limpets until alien species like the eight-shell and volcano barnacles invaded the
Cape in the 1990s. This led to the mass colonization of barnacles in the Upper Balanoid to the
expense of removing nearly all granular limpets, leaving just enough room for the sea lettuce
Ulva to share grounds. Moving bellow the Upper Balanoid is naturally the Lower Balanoid, a
more sheltered zone that supports a wide diversity of flat-bladed algae such as the tongue-
weeds Gigartina polycarpa, dead-man’s fingers Splachnidium rugosum and the spotted
mazzaella Mazzaella capensis, with numerous sandy colonies of Cape reef-worm Gunnarea
gaimardi and beds of Mediterranean mussels Mytilus galloprovincialis, another alien specie
that displaced the indigenous ribbed mussel Aulacomya atra. The Lower Balanoid is subject
to more intense wave action which requires species to firmly hold onto the substrate in order
not to be dislodged. The sandy anemone Bunodactis reynaudi for example thrives in mussel
beds by foraging on those dislodged by waves. Following down our intertidal journey comes
the Cochlear (or Argenvillei) Zone, although not officially part of the big four this zone is
worth mentioning as the levels of limpets there are so abundant that barely any other species
can develop. Specifically, the pear limpet Scutellastra cochlear and the Agenville’s limpet
Scutellastra argenvillei dominate the area with densities of 2’600 individuals per m 2, an
amount so dense that juvenile limpets have to live off the back of larger adults, being the only
place available where larvae can settle without being eaten. This extremely high density of
population is seen only in limpets of the Western Cape and nowhere else in the world. Last
but not least is the Infratidal Zone, which as its name implies is at all times fully emerged
underwater. A wide array of invertebrate species benefit from the friendlier conditions offered
in the Infratidal. The majestic sea bamboo, a flagship specie when snorkeling around Cape
Town, forms dense forests alongside the slpit-fan kelp Laminaria pallidia.
Next month we will dive deeper into the specifics of what makes rocky shore
environments so challenging for the many species inhabiting them. Stay tuned!
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