Seamounts: identity crisis or split personality?

Less than 100 years after Mosely’s statement, Hubbs (1959) contemplated the ‘scientific interests, particularly in respect to zoogeography and speciation’ of recently discovered submerged mountains in the Pacific Ocean. Approximately 14,000 seamounts, undersea mountains peaking below sea level (Wessel, 2001; Kitchingman & Lai, 2004), rise up from the ocean bottom. Like terrestrial mountains (Brown, 1971), seamounts seem an obvious system for the application of island biogeography and, to a large extent, the discourse on seamounts addresses issues of isolation, larval retention and endemism. For example, seamount literature often contains quotes such as ‘...the seamounts in clusters or along ridge systems function as ‘‘island groups’’ or ‘‘chains’’ leading to highly localized species distributions and apparent speciation...’ (De Forges et al., 2000); or ‘Seamounts represent biological islands in the deep sea and often feature characteristic faunas that are quite different from those in the surrounding soft sediment and abyssal habitats’ (Moore et al., 2001). Conservation and policy also place seamounts in an island context. For example, in their contribution to the 2003 Defying Ocean’s End Conference, Stone et al. (2003) claim that ‘seamounts are rich and unusual deep-sea biological communities that support highly unique and endemic faunas’. In ‘Toward a strategy for high seas marine protected areas’, Gjerde & Breide (2003) notes that ‘Seamounts are areas of high endemic biodiversity with little overlap in community composition between seamount clusters’. Alternatively, others suggest that seamounts are unique habitats for reasons not related to their ‘islandness’. Seamounts may represent ‘oases’, where high carbon input increases standing benthic biomass, which in turn yields increased species richness (Samadi et al., 2006). Biodiversity may also be elevated on seamounts because they possess dense coral and sponge meadows, providing habitat for a variety of organisms (Roberts, 2002). The distinctiveness of seamounts may also reflect the observation that they regularly contain hard-substrate habitats, for example rock outcrops, contrasting with the background soft-sediment that dominates a majority of the deep sea (Rogers, 1994). Yet many seamounts lack coral/ sponge meadows or rock outcrops, possessing simply shallower soft-sediment communities than the deeper abyssal plain. Certainly, seamounts are often idiosyncratic habitats that differ from the background deep-sea ecosystem. Here, I ask whether seamounts function ecologically or evolutionarily as islands, oases, biodiversity hotspots or reefs, or whether they are something wholly different. Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039, USA