Giants and titans: first records of the invasive acorn barnacles Megabalanus tintinnabulum (Linnaeus, 1758) and Megabalanus coccopoma (Darwin, 1854) on intertidal rocky shores of South Africa

During intertidal rocky shore surveys on the east coast of South Africa in 2018, the non-indigenous giant purple barnacle Megabalanus tintinnabulum (Linnaeus, 1758), a well-known fouling and globally-invasive species, was discovered. This motivated a survey of the entire South African east coast at 31 rocky shore sites, which confirmed that breeding populations of this barnacle have been established in most wave-exposed low-shore intertidal habitats between the Mozambique border and Mkambati Nature Reserve and that its current South African distribution spans 725 km of coastline. Another non-indigenous and common fouling species, the titan acorn barnacle M. coccopoma (Darwin, 1854), was discovered at three of the sites, its local distribution spanning 370 km of coastline. While currently uncommon, this species is known to reach high densities in other non-native regions. Both Megabalanus species are large and conspicuous and were not found during extensive surveys in the 1990s and early 2000s, suggesting that their introduction and spread occurred within the past two decades. Their establishment on subtidal reefs remains to be confirmed but is likely, based on frequent encounters of empty shells washed up on beaches or on buoys. Prior to this, no non-indigenous invasive species have been reported to occur on intertidal rocky shores of the subtropical South African east coast, while the country's south and west coasts have experienced severe ecological impacts from invasive mussels and barnacles since the 1970s. Two alternative processes have likely led to the introduction and spread of the two species: (i) their dispersal from ship fouling communities and spread along the coast from focal points, such as local ports and harbours; or (ii) their arrival by southward range expansions of tropical populations concurrent with recent climate-mediated thermal shifts in the region. The supporting evidence of both are discussed, as well as management implications.