5.27 Ascidian fauna south of the Sub-Tropical Front

Ascidians are a group of exclusively marine animals (both colonial and solitary) belonging to Class Ascidiacea (Subphylum Tunicata, Phylum Chordata). The adults are benthic and sessile, occurring both on hard and soft substrates, from intertidal to abyssal depths, ranging from tropical to polar seas. In Antarctic waters, ascidians are known to be one of the main sessile benthic groups in terms 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 lecithotrophic larvae, whilst about a dozen species are characterised by a direct development (Jeffery & Swalla 1990). Indirect developers can be oviparous (producing eggs that hatch in the water) or ovoviviparous (eggs are brooded within the parent’s body and develop into a larva that is released). Hence, larvae represent the only life stage where active dispersal occurs in ascidians. Nonetheless, the larval stage in ascidians is relatively short, varying from a few minutes in 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 limited and most species have a restricted geographical distribution characterised by specific ecological conditions. On the other hand, passive dispersal can occur by transport of eggs or fragments of colonies by currents, or by the displacement of solitary/colonial ascidians attached to other invertebrates or to natural marine debris. However, it has to be noted that no debris carrying fauna has been observed beyond 60° of latitude (Barnes 2002). Dispersal of Antarctic ascidians can also be linked with the phenomenon of iceberg scouring (Monniot et al. 2011). Indeed, icebergs abrading the bottom can carry rocks to deeper environments, which represents an alternative method for passive dispersal (Monniot pers. comm.). In addition, ascidians represent a common component of the fouling communities on the hulls of ships, and fragments of colonies can also be transported with ballast water (Carlton 1989, Lambert 2007). Hence, anthropogenic vectors might be responsible for the 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 intertidal zone to abyssal depths (Primo & Vázquez 2007b, Varela & Ramos Esplá 2008, Monniot 2011, Monniot et al. 2011), presenting distinct distributional patterns (Appendix 4, at the end of volume).

The area below the Sub-Tropical Front (considered by a number of oceanographers as the Southern Ocean) comprises the Antarctic continent, Scotia Arc islands (South Orkney, South Sandwich and South Georgia islands), Bouvet Island, the sub-Antarctic islands (including those belonging to New Zealand), and the southernmost part of South America (from Chiloé Island on the west coast to Valdés Peninsula on the east, as well as Falkland Islands). This area is characterised by a number of major oceanic currents and fronts. The Antarctic Divergence (a region of rapid transition located approximately at 65°S) corresponds to the boundary between the Antarctic Coastal Current (flowing westward parallel to the Antarctic continent) and the Antarctic Circumpolar Current (ACC, flowing in the opposite direction). The Polar Front is a circumpolar area within the ACC where the cold superficial water sinks below warmer waters from northern latitudes, leading to a rapid change of temperature within a very small area. Finally, the Sub-Tropical Front limits the ACC and separates its eastward flow from the anticlockwise circulation of the Indian, Atlantic and Pacific oceans.