Biotic Interactions

In all marine habitats, species naturally interact with other species throughout a series of different possible relationships, usually grouped under the broad category of ‘biotic interactions’, i.e. the interactions between living organisms. In the ecological literature, these relationships are explained under a plethora of conceptual schemes, which differ according to the adopted criterion of classification. For example, these can be categorised according to the position of the guest (i.e. the ‘symbiont’) with respect to the host (topological criterion), according to the balance of benefits between the two partners (energetic criterion), or according to other points of view (see Morton 1988, Schiaparelli et al. 2011a). Generally speaking, it is really difficult (if not impossible) to find a synthetic and unique classification scheme, none of these criteria being exhaustive or capable to capture all nuances and exceptions. Ideally, biotic interactions are comprised between two extreme situations, ranging from predation (exerted by carnivorous species, herbivores, grazers, etc.), to the partnerships usually referred to as symbiotic interactions or symbioses (e.g. commensalism, amensalism, etc.), which are less cruel and more finely tuned. It is worth remembering that the use of the term ‘symbiosis’ has changed through time, shifting from the original de Bary’s definition (de Bary 1878), where it just indicated the close living together of two different species, including parasitism, to a modern one where it is generally restricted only to those interactions that are mutually beneficial (Wilkinson 2001, Lincoln et al. 2003). Within biotic interactions, predation is considered one of the main drivers in determining the structure of the community, both in qualitative and quantitative terms. In Antarctica, predation on benthic organisms is, however, greatly reduced. This is due partly to the extinction of selected clades of predators, elsewhere ubiquitous, and partly to dietary shifts occurred in the clades that have survived the climatic cooling phases. Most abundant predators are now slow moving organisms such as asteroids, ophiuroids, echinoids and polychaetes (Aronson & Blake 2001), which exert an important role in determining the structure of Antarctic marine benthic communities (e.g. Dayton et al. 1974, Bowden et al. 2011). Differently from predation, symbiotic interactions have often been regarded as oddities or naturalistic curiosities, with no or negligible impact on marine communities. However, in recent years, their ecological role has been fully re-evaluated and considered to be, in some cases, of the same magnitude of predation or physical disturbance (Hay et al. 2004). Symbiotic interactions also represent important ecological and evolutionary drivers, often promoting speciation through host shift (e.g. Faucci et al. 2007). In Antarctica, for a long time, these kinds of relationships have been considered to be absent.