Nandi Magwadlamba
SANParks communications

The Knysna Estuary and associated wetlands is the most ecologically important estuary in the country. According to several scientific studies relating to birds, fish and plants, it ranks higher than all other estuaries in terms of its natural importance. It hosts 43% of all of South Africa’s estuarine natural life – all the more incredible, because the Knysna estuary is just 1 800 hectares in size.

The sea grass meadows around Leisure Isle, for example, support over 90% of the species occurring in the whole estuarine bay. The sea grass in Knysna also supports a number of high-profile rarities (besides the well-known sea-horse) including various small snails and sea-slugs for which it is the only known place in Africa south of the Sahara where they occur.

It’s then perhaps of little surprise that unfamiliar things still sometimes emerge. This happened recently when a citizen brought Garden Route National Park rangers an unknown creature found in the estuary. After referring to the Marine Guide booklet (two oceans), it was identified as an Argonauta argo.

Commonly referred to as a paper nautilus or Greater Argonaut, it’s a type of pelagic octopus belonging to the genus Argonauta. Pelagic refers to a certain zone of ocean or in this case, a zone in the estuary.

All argonauts create a paper-thin eggcase that coils around the octopus, similar to molluscs in their shells – hence the name of paper nautilus. According to the Marine Guide Booklet, “the Chinese name for this species translates into ‘white sea-horse’s nest.” Males of this species grow up to eight millimeters whilst females mature at about double the size, although they can grow up to 100 mm. Males do not exceed 20 mm. Its paper-thin shells are treasured by beach-goers.

René de Klerk – SANParks Times

It slipped in like a thief at night, hiding amongst our very own barnacles on the rocky shoreline. With these stealthy skills, nobody took notice of the Pacific acorn barnacle (Balanus glandula). While they first appeared on South Africa’s west coast, speculations are rife about their presence.  It might have been from an action as innocent as a freight ship getting rid of bulge water from the Pacific Ocean. In their larval form, they would go by unnoticed.

The invasion became evident in 2007 and in the last nine years, they have extended their 400km range by a further 150km. These barnacles now occur in both the West Coast National Park and the Table Mountain National Park.

Researchers are now hoping to get a better understanding on why they are thriving in our waters from Lambert’s Bay to St James near Muizenberg. “It is difficult to determine what caused them to spread around Cape Point, as conditions are much different from that at home,” says Dr Tammy Robinson researcher at the Centre for Invasion Biology at Stellenbosch University.  In this case it seems like a lack of predators may have enabled this spread.

Recently, Robinson and her team discovered that the whelks (marine snails) that feed on barnacles mostly prefer the indigenous barnacle Notomegabalanus algicolas. The invader has a thick shell, making it difficult to get to the flesh.  She says that South African barnacles are very unpredictable and numbers differ significantly annually, but there are definitely much less of them around. “We can’t say if numbers are low because of the invader, but it is a cause for concern.” At this stage it is unfortunately too late to do anything. The invasive species can’t be removed from the rocks without damaging our own species. While the functionality of the system is still the same, it differs ecologically because of the different species now present.

In their natural home range, starfish and crabs would eat them, but in our waters, these large predators don’t occur in the intertidal zone where the barnacles are found. Through observations, researchers have realized that our whelks, although they could eat the invader, prefer the indigenous species if they are given the option.

They are difficult to differentiate from our own species, especially if you are not trained. The intruder looks very similar to those already growing along our coast. The easiest method to distinguish the Pacific acorn barnacle is by removing it from the surface. Because of the platelet that stick to objects, they leave a little calcium spot behind. Local species stick their flesh directly to a surface.

Robinson says that through their work they have realized the importance of a combination of field work with field and laboratory experiments. While it remains difficult to determine the impact of the invasion, ongoing monitoring remains the only answer and will hopefully provide more answers in the future.

A barnacle is an arthropod related to crabs and lobsters, although they are easily confused with mollusks because of their outer shell. They can’t move on their own and permanently attach themselves to surfaces. In juvenile form, they are free-floating larvae. As filter feeders, they feed on food particles that they strain out of the water.

© All photographs by Professor Charles Griffiths