5.27 Ascidian fauna south of the Sub-Tropical Front

• Main Authors: Primo, C, Vazquez, E
• Format: Book Part
• Language: English
• Published: Scientific Committee on Antarctic Research, Scott Polar Research Institute 2014
• Subjects: Biological Sciences; Evolutionary Biology; Biogeography and Phylogeography; Antarctic; Bouvet; Bouvet Island; Indian; New Zealand; Pacific; Ramos; Southern Ocean; The Antarctic; Varela; Antarc*; Iceberg*; Polar Research
• Online Access: http://atlas.biodiversity.aq/; http://ecite.utas.edu.au/94381

Ascidians are a group of exclusively marine animals (both colonial and solitary)belonging to Class Ascidiacea (Subphylum Tunicata, Phylum Chordata). Theadults are benthic and sessile, occurring both on hard and soft substrates, fromintertidal to abyssal depths, ranging from tropical to polar seas. In Antarcticwaters, ascidians are known to be one of the main sessile benthic groups interms of number and biomass (e.g. Arnaud et al. 1998, Griffiths et al. 2008)and to play a relevant role in the structure of suspension-feeding communities(e.g. Gili et al. 2001, Gutt 2007). Most ascidians produce eggs that develop into pelagic lecithotrophiclarvae, whilst about a dozen species are characterised by a direct development(Jeffery & Swalla 1990). Indirect developers can be oviparous (producing eggsthat hatch in the water) or ovoviviparous (eggs are brooded within the parentsbody and develop into a larva that is released). Hence, larvae representthe only life stage where active dispersal occurs in ascidians. Nonetheless,the larval stage in ascidians is relatively short, varying from a few minutesin tropical seas (Monniot et al. 1991) to 8 days or more in cold regions(Strathmann et al. 2006). Hence, active dispersal of ascidians is quite limitedand most species have a restricted geographical distribution characterisedby specific ecological conditions. On the other hand, passive dispersal canoccur by transport of eggs or fragments of colonies by currents, or by thedisplacement of solitary/colonial ascidians attached to other invertebrates orto natural marine debris. However, it has to be noted that no debris carryingfauna has been observed beyond 60 of latitude (Barnes 2002). Dispersalof Antarctic ascidians can also be linked with the phenomenon of icebergscouring (Monniot et al. 2011). Indeed, icebergs abrading the bottom cancarry rocks to deeper environments, which represents an alternative methodfor passive dispersal (Monniot pers. comm.). In addition, ascidians representa common component of the fouling communities on the hulls of ships, andfragments of colonies can also be transported with ballast water (Carlton1989, Lambert 2007). Hence, anthropogenic vectors might be responsible forthe widespread distribution observed in some species. To present, 245 species of ascidians (excluding dubious identifications)have been recorded below the Sub-Tropical Front (STF) from the intertidalzone to abyssal depths (Primo & Vzquez 2007b, Varela & Ramos Espl2008, Monniot 2011, Monniot et al. 2011), presenting distinct distributionalpatterns (Appendix 4, at the end of volume). The area below the Sub-Tropical Front (considered by a number ofoceanographers as the Southern Ocean) comprises the Antarctic continent,Scotia Arc islands (South Orkney, South Sandwich and South Georgiaislands), Bouvet Island, the sub-Antarctic islands (including those belongingto New Zealand), and the southernmost part of South America (from ChiloIsland on the west coast to Valds Peninsula on the east, as well as FalklandIslands). This area is characterised by a number of major oceanic currentsand fronts. The Antarctic Divergence (a region of rapid transition locatedapproximately at 65S) corresponds to the boundary between the AntarcticCoastal Current (flowing westward parallel to the Antarctic continent) and theAntarctic Circumpolar Current (ACC, flowing in the opposite direction). ThePolar Front is a circumpolar area within the ACC where the cold superficialwater sinks below warmer waters from northern latitudes, leading to a rapidchange of temperature within a very small area. Finally, the Sub-TropicalFront limits the ACC and separates its eastward flow from the anticlockwisecirculation of the Indian, Atlantic and Pacific oceans.