Data from: Distribution and localised effects of the invasive ascidian Didemnum perlucidum (Monniot 1983) in an urban estuary

Didemnid ascidians are notorious marine invaders, fouling infrastructure
in many ecosystems globally. However, there have been few reports of
direct interactions with native species in their natural environment. The
invasive colonial ascidian Didemnum perlucidum was discovered in the Swan
River estuary (Western Australia) growing on the native seagrass Halophila
ovalis. Given the known effects of other related Didemnum species it was
expected that D. perlucidum could adversely affect the seagrass, with
possible flow on effects to the rest of the ecosystem. This study aimed to
document the distribution and abundance of D. perlucidum in the estuary,
and to determine whether this species had a negative impact on H. ovalis
or associated flora and fauna. D. perlucidum was largely present near
areas of infrastructure, particularly mooring buoys, suggesting these were
the source of D. perlucidum recruits on the seagrasses. It showed a clear
seasonal pattern in abundance, with highly variable cover and colony size.
D. perlucidum had a measurable effect on H. ovalis, with colonies
enveloping all plant tissue, likely restricting the photosynthetic ability
of individual leaves and total plant biomass. There were also
significantly less seagrass-associated mud snails (Batillaria australis)
where D. perlucidum cover was high. These results demonstrate the ability
of invasive ascidians to colonise and affect native seagrasses and
associated biota. Seagrasses are pivotal to the ecological function of
many urban estuaries world-wide. Biodiversity in these systems is already
vulnerable to multiple stressors from human activities but the potential
stress of fouling ascidians may pose an additional and increasing threat
in the future.