ABSTRACT
Zoanthus and Palythoa are benthic colonial zooxanthellate cnidarians common in shallow hard substrate marine ecosystems in subtropical and tropical regions, where they are an important component of coral and temperate reefs. However, recent studies have discovered areas where zoantharians dominate the benthos, potentially indicating a degraded or shifted reef state. In this review, we collated the available information on areas of high Zoanthus and Palythoa coverage. We found 34 reports from 19 locations and categorized them into three states: (1) “dominance”—dominant Zoanthus and Palythoa with no information on past states; (2) “barrens”—zoantharians have replaced another community, with no information on this state’s persistence over time; and (3) “phase shifts”—zoantharians have replaced another state and have been present for five years or longer. The reports encompass all subtropical/tropical oceans of the world. We confirmed only one zoantharian phase shift and noted eight cases each of barrens and zoantharian dominance. Records spanned from the late 19th century to the current day, and 12 of 19 instances were associated with anthropogenic impacts, primarily decreasing water quality. A temporal shift in zoantharian communities was also apparent, with historical records before the 1990s largely reporting intertidal zoantharian dominance while records that are more recent note subtidal zoantharian barrens. With shallow hard substrate ecosystems undergoing increasing degradation, knowledge regarding different possible states of these ecosystems, including Zoanthus and Palythoa outbreaks, is needed in order to recognize and report such events and track them over time and space.
