Soft corals and sea fans (order Alcyonacea) of the Tamar River estuary, northern Tasmania, with an assessment of the wider rocky reef community structure

The order Alcyonacea Lamouroux, 1816 (Octocorallia: Anthozoa: Cnidaria) is a species-rich group comprising soft corals and sea fans, also known as gorgonians. Alcyonaceans are perennial, long-lived and can inhabit up to 70% of coastal reef habitats. Despite this, alcyonacean taxonomy is poorly resolved, and substantial knowledge gaps remain in regards to the biology, ecology and responses to environmental change of species within this taxon. Alcyonaceans occur in all oceans of the world; however, few studies have examined temperate assemblages. Although alcyonaceans have been observed and collected from the Tamar River estuary, northern Tasmania, virtually nothing is known about these species. This study examined colony and sclerite morphology, and variation in the mitochondrial marker, mtMutS, to describe the alcyonaceans of the Tamar River estuary and their relationships with previously described species. In addition, variation in the distribution and abundance of alcyonaceans and the wider rocky reef community structure was determined among six depth categories and five sites in the lower Tamar River estuary, using photoquadrats. In total, nine alcyonacean species were described from the Tamar River estuary, including two novel genera and six novel species, along with one species (Drifa gaboensis) which represents a first record for Tasmanian waters. Although exhibiting a large range of morphological variation, these novel genera and species can be distinguished from previously described taxa on the basis of colony and sclerite morphology, and, in some cases, mtMutS sequences. The benthic survey revealed the highest abundance and diversity of alcyonaceans at Fish Beacon, the most marine site closest to the estuary mouth, where alcyonaceans comprised up to 14% cover of the benthic assemblage. Moreover, rocky reef community structure varied with depth and longitudinal distance from the mouth of the estuary. This study has established detailed baseline knowledge of the Tamar River estuary alcyonacean assemblage and rocky reef communities that is important for ongoing monitoring and informing future management of this unique estuary