Mid-Atlantic Ridge: Biodiversity Around Seamounts

A species is an identifiable taxonomic unit, a lineage per say, that persists through time, gives rise to other species and finally becomes extinct. Fossil records include extinct and extant species that can be studied mainly on the basis of morphological traits. The phylogenetic data, however, typically represents only extant species and can be identified through phenotypic and/or genetic data.

According to the theory of insularity of MacArthur and Wilson, the number of species on an island reflects an equilibrium condition, where the processes of immigration and emigration determine the biodiversity on an island. As the number of species on an island increases, the rate of immigration decreases. For instance, when an island is ‘young,’ the immigrants find and fill the empty niches. But with time, only a few species can find the remaining empty space and fit in. In addition, the more distanced an island is, the lower will be the rate of immigration/colonization. The maximum diversity also depends on the dimensions of an island, meaning that a bigger island will have a higher biodiversity; on abiotic (i.e. climate-dependent) and biotic factors (e.g. competition, etc.). In parallel, the extinction velocity depends on the number of species present, where competition is lower between just a few species. Again, the bigger the island, the lower the extinction rate.

The intersection of these two curves represents the island equilibrium. This theory is often applied in the designation of natural reserves.
Credit: Marcus Lapeyrolerie

Although the Azores archipelago is very diverse and has many endemisms (endemic species are animals or plants typical of a certain area), terrestrial and marine, their presence is not only due to the islands’ formation but because numerous seamounts are present, too. These conical shaped underwater mountains are characterised by accelerating currents that enhance the flux of prey organisms flowing over them. From the scheme below, it is possible to observe how the upwelling cold and nutrient rich water rises up to the apex of the seamount and supplies zooplanktonic communities with food. Furthermore, the zooplankton is limited to remain in the upper part due to the vortex-like currents, or eddies.

Credit: Mullineaux et al., 1997

Plankton preys are thus influenced by these water mass movements on a daily basis and intercepted by bottom fish predators. Along with stationary prey organisms, larvae of seamount fish communities are also retained by the seamount-generating flows.

Credit: Oceanic Seamounts: An Integrated Study by Susan Gubbay

Lastly, much less is known about the ‘visiting’ animals over seamounts. In some of these locations, fisherman reported the presence of host populations such as that of bigeye tuna (Thunnus obesus), albacore (Thunnus alalunga), skipjack (Katsuwonus pelamis), pelagic sharks, marine mammals (common dolphin, spotted dolphin, bottlenose dolphin, sperm whale), loggerhead (Caretta caretta) and so on. Not to mention seabirds like the black-footed albatross (Diomedea nigripes), Madeiran storm petrel (Oceanodroma castro) and many others, which seem to feed on zooplankton, small fish and cephalopods. Interestingly, the visitors have been observed more abundantly within the first 20-30 km from the peak of the seamounts as a result of more food sources.

What will follow next are the specific cases of animals observed around these formations and, why and how they manage to efficiently stay for longer periods of time. Now that we have got a zoomed picture of what biodiversity is and why seamounts could possibly present a ‘stepping stone’ area (which can sustain rare but crucial species’ dispersal events), we can only continue to get deeper into the knowledge about the Azores archipelago.

  • Silvestro, D., Warnock, R.C., Gavryushkina, A. and Stadler, T., 2018. Closing the gap between palaeontological and neontological speciation and extinction rate estimates. Nature communications, 9(1), pp.1-14.
  • Freitas, R., Romeiras, M., Silva, L., Cordeiro, R., Madeira, P., González, J.A., Wirtz, P., Falcón, J.M., Brito, A., Floeter, S.R. and Afonso, P., 2019. Restructuring of the ‘Macaronesia’ biogeographic unit: A marine multi-taxon biogeographical approach. Scientific reports, 9(1), pp.1-18.
  • Mullineaux, L.S. and S.W. Mills 1997 “A test of the larval retention hypothesis in seamount-generated flows,” Deep-Sea Research 44: 745-770, a study on currents and larval settlement at Fieberling Guyot, a Pacific seamount.
  • Oceanic Seamounts: An Integrated Study by Susan Gubbay; OASIS, Hamburg & WWF Germany, Frankfurt am Main, November 2003
  • Morato, T., Varkey, D.A., Damaso, C., Machete, M., Santos, M., Prieto, R., Santos, R.S. and Pitcher, T.J., 2008. Evidence of a seamount effect on aggregating visitors. Marine Ecology Progress Series, 357, pp.23-32.

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