Ugarteburu and McQuaid 1998). This places P. perna at a disadvantage relative to the
alien. However, the external surfaces of M. galloprovincialis shells are heavily eroded
by endolithic lichens and cyanobacteria (Zardi et al. 2009), making them brittle and
fragile compared with the shells of the endemic Choromytilus meridionalis.
In 2009 a second alien mussel, S. algosus, was recorded in Elands Bay on the west
coast (de Greef et al. 2013). This invader has subsequently spread south and around
Cape Point, and now occurs throughout False Bay (TB Robinson pers. obs). Within
the intertidal zone, this South American mussel is dominant in the low-shore, especially under exposed conditions (Skein et al. 2018a). It does not extend as high on the
shore as M. galloprovincialis because of its relative intolerance of desiccation (Zeeman 2016). Also in contrast to M. galloprovincialis, which is virtually absent from the
subtidal zone, S. algosus also occurs in large numbers in this habitat (Skein et al.
2018a). It appears to owe much of its success and rapid rates of spread to an
exceptionally high recruitment rate (Reaugh-Flower et al. 2011; Zeeman et al.
2018). Many of the impacts associated with S. algosus are similar to those of
M. galloprovincialis, as both species dominate previously open rocky surfaces. Notably, both species elevate the structural complexity of invaded rocky shores
(Sadchatheeswaran et al. 2015), ultimately elevating diversity and altering community
structure (Robinson et al. 2007b; Sadchatheeswaran et al. 2018). As they also form an
abundant prey resource, these alien mussels have also altered the foraging landscape of
native predators. While some, such as the whelk Trochia cingulata (Alexander et al.
2015) have incorporated the alien mussels into their diet, others such as the West Coast
Rock Lobster Jasus lalandii and the starfish Marthasterias africana (Skein et al.
2018b), have not. These findings have highlighted that native predators may not
necessarily regulate invasive prey, even when predators are known to be generalist
feeders. In fact, when predators preferentially feed on native species and avoid alien
prey, they may facilitate the invasion by removing native comparators that might have
offered resistance via inter-specific competition. It remains to be investigated if this
process will play out in relation to mussel invasions in South Africa.
Although first recognised as an invasive species in South Africa in 2007 (SimonBlecher et al. 2008), the barnacle B. glandula is likely to have been present along the
west coast since the mid-1990s (Laird and Griffiths 2008). Since its introduction, it
has become the dominant intertidal barnacle on the west coast at the expense of the
native barnacle Chthamalus dentatus (Laird and Griffiths 2008; Robinson et al.
2015). Balanus glandula now occurs on the south coast as far as Cape Hangklip (TB
Robinson pers. obs). Although not to the same extent as the invasive mussels, this
barnacle also elevates structural complexity on invaded shores (Sadchatheeswaran
et al. 2015). In particular the high-shore gastropod Afrolittorina knysnaensis benefits
from the presence of B. glandula (Laird and Griffiths 2008). The abundance of this
native species is raised by more than an order of magnitude as individuals nestle
between the barnacles, presumably gaining protection from wave action
(Sadchatheeswaran et al. 2015).
Together, these three alien intertidal species now dominate west coast rocky
shores. While M. galloprovincialis appears to have reached its maximum range on
the south coast (Assis et al. 2015), S. algosus and B. glandula have only recently
242
T. B. Robinson et al.
alien. However, the external surfaces of M. galloprovincialis shells are heavily eroded
by endolithic lichens and cyanobacteria (Zardi et al. 2009), making them brittle and
fragile compared with the shells of the endemic Choromytilus meridionalis.
In 2009 a second alien mussel, S. algosus, was recorded in Elands Bay on the west
coast (de Greef et al. 2013). This invader has subsequently spread south and around
Cape Point, and now occurs throughout False Bay (TB Robinson pers. obs). Within
the intertidal zone, this South American mussel is dominant in the low-shore, especially under exposed conditions (Skein et al. 2018a). It does not extend as high on the
shore as M. galloprovincialis because of its relative intolerance of desiccation (Zeeman 2016). Also in contrast to M. galloprovincialis, which is virtually absent from the
subtidal zone, S. algosus also occurs in large numbers in this habitat (Skein et al.
2018a). It appears to owe much of its success and rapid rates of spread to an
exceptionally high recruitment rate (Reaugh-Flower et al. 2011; Zeeman et al.
2018). Many of the impacts associated with S. algosus are similar to those of
M. galloprovincialis, as both species dominate previously open rocky surfaces. Notably, both species elevate the structural complexity of invaded rocky shores
(Sadchatheeswaran et al. 2015), ultimately elevating diversity and altering community
structure (Robinson et al. 2007b; Sadchatheeswaran et al. 2018). As they also form an
abundant prey resource, these alien mussels have also altered the foraging landscape of
native predators. While some, such as the whelk Trochia cingulata (Alexander et al.
2015) have incorporated the alien mussels into their diet, others such as the West Coast
Rock Lobster Jasus lalandii and the starfish Marthasterias africana (Skein et al.
2018b), have not. These findings have highlighted that native predators may not
necessarily regulate invasive prey, even when predators are known to be generalist
feeders. In fact, when predators preferentially feed on native species and avoid alien
prey, they may facilitate the invasion by removing native comparators that might have
offered resistance via inter-specific competition. It remains to be investigated if this
process will play out in relation to mussel invasions in South Africa.
Although first recognised as an invasive species in South Africa in 2007 (SimonBlecher et al. 2008), the barnacle B. glandula is likely to have been present along the
west coast since the mid-1990s (Laird and Griffiths 2008). Since its introduction, it
has become the dominant intertidal barnacle on the west coast at the expense of the
native barnacle Chthamalus dentatus (Laird and Griffiths 2008; Robinson et al.
2015). Balanus glandula now occurs on the south coast as far as Cape Hangklip (TB
Robinson pers. obs). Although not to the same extent as the invasive mussels, this
barnacle also elevates structural complexity on invaded shores (Sadchatheeswaran
et al. 2015). In particular the high-shore gastropod Afrolittorina knysnaensis benefits
from the presence of B. glandula (Laird and Griffiths 2008). The abundance of this
native species is raised by more than an order of magnitude as individuals nestle
between the barnacles, presumably gaining protection from wave action
(Sadchatheeswaran et al. 2015).
Together, these three alien intertidal species now dominate west coast rocky
shores. While M. galloprovincialis appears to have reached its maximum range on
the south coast (Assis et al. 2015), S. algosus and B. glandula have only recently
242
T. B. Robinson et al.