Potential effects of the invasive colonial ascidian (Didemnum vexillum Kott, 2002) on pebble-cobble bottom habitats in Long Island Sound, USA.

The colonial ascidian, Didemnum vexillum, is a relatively recent invader to the east and west coasts of North America and since its appearance it has often become a dominant member of a variety of coastal communities in these regions. The species has the unique ability to colonize cobble-pebble substrates and form extensive mats in this habitat type. The mats essentially ‘glue’ the small pebbles and cobbles together and alter habitat complexity of the seafloor from a more three-dimensional system to a more two-dimensional one. We examined the potential impacts of the mat-forming activities on benthic macro-invertebrate population and community structure by comparing sets of samples collected inside and outside of the mats. Sampling was conducted at a site located in eastern Long Island Sound, USA, over a period of approximately one year using a corer and a suction sampler. Contrary to our prediction that the presence of the ascidian mats would reduce benthic species richness and abundance we found that these parameters either were not different or were significantly higher in samples taken inside Didemnum mats compared to samples collected immediately outside the mats. The presence of the mats did result in subtle shifts in benthic community structure and functional group dominance with greater numbers of infauna and deposit-feeders residing inside the mats compared to samples collected adjacent to the mats.