Seamounts comprise a unique deep-sea environment, characterized by substantially enhanced currents and a fauna that is dominated by suspension feeders, such as corals1,2,3,4. The potential importance of these steep-sided undersea mountains, which are generally of volcanic origin, to ocean biogeography and diversity was recognized over 40 years ago5, but this environment has remained very poorly explored. A review3 of seamount biota and biogeography reported a total of 597 invertebrate species recorded from seamounts worldwide since the Challenger expedition of 1872. Most reports, based on a single taxonomic group, were extremely limited: 5 seamounts of the estimated more than 30,000 seamounts in the world's oceans4,6 accounted for 72% of the species recorded. Only 15% of the species occurring on seamounts were considered potential seamount endemics. Here we report the discovery of more than 850 macro- and megafaunal species from seamounts in the Tasman Sea and southeast Coral Sea, of which 29–34% are new to science and potential seamount endemics. Low species overlap between seamounts in different portions of the region indicates that the seamounts in clusters or along ridge systems function as ‘island groups’ or ‘chains,’ leading to highly localized species distributions and apparent speciation between groups or ridge systems that is exceptional for the deep sea. These results have substantial implications for the conservation of this fauna, which is threatened by fishing activity7.
Whereas previous studies of the seamount fauna have often focused on particular taxa, the present study sought to describe the benthic community as a whole and enlisted broad taxonomic support (see Acknowledgements). In all, 516 species of fish and macro-invertebrates were obtained from 6 seamounts along the Norfolk Ridge, 108 from 4 seamounts on the Lord Howe Rise, and 297 from 14 seamounts south of Tasmania (Table 1, Fig. 1 ). Thirty-six per cent of species from the Norfolk Ridge seamounts were new to science and not known from sampling of the open seafloor and are therefore potential endemic species, along with 31% of species from the Lord Howe seamounts and between 16 and 33% from the Tasmanian seamounts.
Over the range of sampling carried out on Norfolk and Lord Howe Ridges, we found a linear relationship between number of species recorded from each seamount and the number of samples obtained there (Fig. 2). This relationship implies that the greater number of species obtained from seamounts on Norfolk Ridge is owing to greater sampling effort there rather than greater species richness. The lack of an asymptote in the relationship implies that the number of species present on these seamounts is far greater than the number currently recorded. Species richness on the Tasmanian seamounts appears to be significantly greater, but it is unclear whether this is due to greater diversity or is an artifact of sampling over a greater number of seamounts, albeit within a limited area. Although sampling on the Tasmanian seamounts extended deeper than on Lord Howe Ridge and the Norfolk Rise, sampling in the northern Tasman Sea extended into shallower depths ( Table 1). Most species on the Tasmanian seamounts exhibited a broad depth distribution and few were found only at the greatest depths sampled7. High species diversity has been previously noted off southeastern Australia8,9.
The coefficient of community (C) between seamounts decreased significantly from a median value of 0.21 (range 0.08–0.49) for adjacent seamounts on the same ridge to a median of 0.04 (range 0–0.17) for seamounts on different ridge systems within the northern sector of the Tasman Sea and southeast Coral Sea (that is, the Norfolk Ridge and Lord Howe Rise) (analysis of variance (ANOVA), F1,43 (C) = 46.56, P < 0.001) (Fig. 3). Just four species from the Tasmanian seamounts are known from the seafloor around New Caledonia (the bryozoan Haswelliporina multiaviculata, the bivalve Acesta sp., the barnacle Hexelasma sp., and the hydroid Symplectoscyphus commensalis), but none of these was found on seamounts in that region (C = 0). Thus there are significant differences in species composition of the seamount communities between ridge systems at a similar latitude and separated by ∼1,000 km across the northern Tasman Sea and southeast Coral Sea, and a complete changeover of species between the northern and southern sectors of the Tasman Sea. C did not decline significantly with distance on smaller scales (tens to several hundred kilometres) (Fig. 3).
Although the region around southern Australia and New Caledonia is noted for its relatively isolated faunas10,11, the complete lack of affinity between the seamount communities across this area appears remarkable. For comparison, about 60% of the 93 species of non-mesopelagic decapod crustaceans (Anomura, Caridea, Palinura, Brachyura and Astacidea) recorded from depths greater than 200 m on the continental slope off southeastern Australia (south of ∼35° S latitude) are recorded from the Indo-West Pacific region as well (G.C.B.P., unpublished data). This mirrors the general pattern for decapods on the continental shelf in this region, although isopod affinities are mostly with Antarctic and deep-sea faunas9. Thus, the isolation of the seamount faunas between the southern and northern Tasman Sea contrasts markedly with the generally strong affinity displayed by the soft-sediment slope fauna between these two regions.
It was also striking to find such little overlap in community composition in the deep sea between sites sharing the same habitat type, at similar latitude and depth, and only ∼1,000 kilometres apart. This distribution pattern is consistent with Johannesson's hypothesis that many taxa adapted to seamount conditions limit their dispersal to maintain their populations, because of the generally small size of seamounts, the considerable distance between them and their unique oceanographic environment12,13. Circulation cells in the vicinity of seamounts and along seamount chains have been shown to facilitate recruitment to seamounts within relatively close proximity of each other and along ridge systems14,15.
Endemicity of the benthic fauna on southwest Pacific seamount groups/chains appears to be greater than that of deep-sea vent communities, another specialized deep-sea habitat. For comparison, we calculated C on the basis of available data for vent sites from the Explorer, Juan de Fuca and Gorda Ridges in the northeast Pacific at 41–49° N latitude, several sites along the northern East Pacific Rise (9–13°, 21° and 27° N latitude), the Galapagos Rift on the equator and 17–18° S latitude on the southern East Pacific Rise16. In contrast to the seamounts, there was substantial similarity in community composition, even at distances of 3,000–4,000 km (Fig. 3). There was some indication that similarity might be reduced between disjunct ridge systems (for example, between the Juan de Fuca and East Pacific Rise) but these differences were not significant (analysis of covariance (ANCOVA), F1,12 = 3.02, P = 0.11), and the similarity of vent communities between disjunct ridge systems was still clearly greater than between seamounts in the northern and southern Tasman Sea (Fig. 3). Vents are relatively ephemeral environments, whereas seamounts are typically millions to tens of millions of years in age, so dispersal between vent sites is essential to the continuance of these communities. Cold seeps, on the other hand, appear to have higher levels of endemicity: most species are apparently restricted to single regions17, which has been related to the stability and isolation of these habitats. However, direct comparison with cold seep communities would be premature, as most cold seeps are poorly explored with only a few species recorded per site.
Wilson and Kaufmann3 estimated the endemism of fishes and invertebrates on seamounts as 12 and 15%, respectively, but half of the reports that they cited from reasonably well-studied regions (that is, the Hawaiian Ridge, Mid-Pacific Mountains, Kyushu-Palau Ridge, Vema seamount) indicated levels of endemism of 22–36%, consistent with our results. Citing Hubb's ‘stepping-stone’ hypothesis5, Wilson and Kaufmann3 emphasized the faunistic links among seamount chains extending around the rim of the North Pacific, along the East Pacific Rise into the southeast Pacific and through the Emperor Chain and Hawaiian Ridge into the central North Pacific. The apparently distinct seamount chains of the southwest Pacific, Indian and Southern Oceans and much of the Indo-West Pacific18 were, however, virtually unexplored at that time. Seventeen new genera were obtained from the Norfolk Ridge samples, four from the Lord Howe Ridge and seven or eight from the Tasmanian samples. Some appear to be relicts of groups earlier believed to have disappeared in the Mesozoic19,20,21,22,23. These seamounts thus appear to be isolated marine systems and provide an exceptional opportunity to examine evolution and speciation in the deep sea.
The highly localized distribution of many seamount species has profound implications for their conservation. Seamount communities are extremely vulnerable to the impacts of fishing: their limited fixed habitat, the extreme longevity of many species (of the order of 100 years and more22,23,24), and the apparently limited recruitment between seamounts, all compound the uncertainty of recovery from trawling, which sweeps away the benthic epifaunal community as a bycatch7. The global status of seamount benthic communities is unknown; however, the localized distribution of many benthic seamount species greatly increases the threat of extinction and may require that conservation and protection of seamount communities be undertaken on a local scale.
Our study is based on 23 cruises that the Muséum National d'Histoire naturelle and the Institut François de Recherche Scientifique pour le Développement en Coopération (MUSORSTOM) carried out since 1984 to study the fauna of the seamounts, ridges and adjacent seafloor within the EEZ of New Caledonia at the northern edge of the Tasman Sea, 11 of which focused on seamounts, primarily along the Norfolk and Lord Howe Ridges10; a single cruise carried out by the Commonwealth Scientific and Industrial Research Organization (CSIRO) in 1998 to study the benthic fauna of 14 small seamounts off southern Tasmania at the southern extreme of the Tasman Sea7; and surveys by the Museum of Victoria of the soft-bottom fauna of the continental slope off southeastern Australia based cruises in 1984, 1986 and 1994 (ref. 9) ( Table 1, Fig. 1). ORSTOM carried out 462 trawls and 748 dredge hauls using a cod end with 10 mm mesh22; many results are presented in the 21 volumes of Resultats des campagnes MUSORSTOM26. On the CSIRO cruise, so far as was possible, each seamount was sampled on its summit, slope and base using a dredge with a cod end of 25 mm of stretched mesh; 34 samples were obtained in all.
To compare the similarity in fauna between two seamounts, we used the coefficient of community (variously referred to as Sorensen's, Dice's or the Bray–Curtis similarity statistic for binary data): C = 2a/(b+ c), where a is the number of species in common to both sites and b and c are the total number of species found at each site. C is commonly assumed to vary between 0 and 1 (ref. 25), but in fact the maximum value is sensitive to the relative number of species present at the two sites27 and is conservatively biased when the sampling and number of species at two sites differs significantly. The proportion of species at the less well-sampled site in common to the two sites (that is, C′ = a/b, where b < c) is biased in the opposite direction and was used as an upper limit to the coefficient of community between two sites, where there were large differences of sampling intensity; however, use of C′ did not materially affect any of our conclusions.
Results from the northern Tasman Sea were obtained as part of the MUSORSTOM program, a collaboration between ORSTOM and the Natural History Museum in Paris, and involved some 181 researchers from 92 Institutions and 24 nations. Work around Tasmania was supported by CSIRO and grants from Environment Australia and the Fisheries Research Development Corporation and involved 25 researchers. Their contributions are gratefully acknowledged; in particular, we thank R. Grandperrin, A. Crosnier, P. Bouchet, K. Gowlett-Holmes, M. Lewis, S. Cummings, J.S. Philippe and the crews of the RV Alis and Southern Surveyor. J.A.K. received support from a National Research Council Senior Research Fellowship.
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Biology & Philosophy (2012)