Abundance and distribution of the invasive polychaete Ficopomatus enigmaticus in three South African estuaries

Abstract

Ficopomatus enigmaticus (Fauvel 1923) is a reef-building serpulid polychaete that is found in warm and temperate estuaries across the globe. This ecosystem engineer causes ecological and socio-economic impacts in invaded systems through efficient filtering of the water column and the formation of large calcareous reefs. As such it is important to monitor F. enigmaticus invasions and track changes in distribution and abundance. Although recent genetic studies have highlighted cryptic sympatric species within invasions previously ascribed to F. enigmaticus the genetic status in South Africa remains unresolved. This study provides an assessment of the invasion by this tube worm in three estuaries of conservation importance in the Western Cape, South Africa. Through the use of historical data, the trajectories of these invasions were considered. Zandvlei Estuary supported the largest standing stock (29 194.22 kg) while Berg River Estuary supported only 3.39 kg. It was notable that in both estuaries the worm was almost totally confined to artificial substrata. Despite the presence of large intact reefs in Milnerton Lagoon, no live worms were recorded in 2019. Comparisons with historical data revealed that populations have declined in all three estuaries. These results highlight the need for routine monitoring of Ficopomatus populations. It is suggested that episodic removal of the small reefs from Berg River Estuary could minimise spread within the estuary and prevent the negative impacts known from elsewhere. Strategic removal from Zandvlei Estuary, and Milnerton Lagoon should the worm be detected again, could limit its negative impacts in these systems while retaining the benefit of enhanced water quality associated with this prolific filter feeder.