Eastern Pacific Barrier: Exchange Area? (Part 2)

El Niño Southern Oscillation (ENSO) establishes climate variability in the Tropical Pacific. The warm phases of El Niño or the cold phases of La Niña, every 2-7 years, regulate the ecology of this strange stretch of ocean. In particular, the high temperatures associated with El Niño induce “coral bleaching” and in severe cases can also lead to death. The extreme events of El Niño of 1982-1983 and 1997-1998 were considered the most intense of the last century as they caused widespread bleaching, followed by mortality and also by local extinctions of some species of the Eastern Pacific. From these events, coral reefs have been characterized by a slow recovery of normal growth conditions, and some have never been recovered.

El Niño events have been proposed to improve eastward dispersion across the vast expanse of the Eastern Pacific Barrier, thereby reducing the isolation of reefs in the Eastern Pacific. Following the phenomenon dating back to 1982-1983 El Niño, were found a number of Indo-Pacific molluscs, echinoderms, fish and corals which colonized the Tropical Eastern Pacific. This phenomenon was attributed initially, and erroneously, to the process of dispersion from the Central Pacific through the North Equatiorial Counter Current (by crossing the Eastern Pacific Barrier).

In fact, what can be seen from various studies is that genetically speaking, the coral populations of the tropical Eastern Pacific were isolated from those of the Central Pacific before the El Niño event, whether it is from the 80s or 90s. This makes us understand that the actual Eastern populations derives from survivors of these phenomenon. For example, it has been demonstrated the presence of a dense coral reef in the Galapagos consisting of Pocillopora, entirely made up of a single propagule reproduced asexually following El Niño in the 1990s.

The supposed conditions of transport for the larve by El Niño, among other things, see high temperatures, at which these must be able to survive. Therefore, even if after the transport, the larvae don’t die, the conditions in which they find themselves see immediate and long-term reductions in the fertility of the corals due to stress such as the total change of environmental conditions compared to those of origin. For this reason the larvae and their possible settlement are seriously threatened, and they are difficult to survive.

However, this does not exclude that in the future, especially in light of the new and unpredictable climate changes, the frequency, intensity and dynamics of El Niño Southern Oscillation will change and perhaps have implications for the transport, connection, resistance, resilience of the coral reefs of the Eastern Pacific.

Furthermore, it is also probable the presence of other currents that can act in today’s possible connection and that have not yet been investigated.

In any case, what we can say today is that the large stretch of deep waters between the Central and Eastern Pacific, act as a real “impassable” barrier as Darwin said. If the whole Eastern Pacific seems to be isolated from the Central one since the Pleistocene, we have once again a possible exception.

The Island of Clipperton has also proven to be an exception in the Blennidae speciation process. In fact, although this island is part of the Eastern Pacific, for P. lobata proves to be genetically similar to the Central Pacific. But in reality what triggered this genetic affinity is still to be determined and new studies need to be done in this regard.

Credit: Oceanlight

We have always thought of the barrier as a real present, visible obstacle, which stands vertically and which does not allow us to overcome it, except through possible adaptations. Today, however, we have understood that in reality long distances and very deep waters are equally an insurmountable barrier for marine inhabitants. Man has built railways, cars, ships and planes to cross continents. At sea, however, those who live there simply respect the laws of nature.

Maria Bruno

  • “No gene flow across the Eastern Pacific Barrier in the reef-building coral Porites lobata” Iliana B. Baums, Jennifer N. Boulay, Nicholas R. Polato And Michael E. Hellberg. Molecular Ecology (2012) 21, 5418–5433.
  • “El Niño and coral larval dispersal across the eastern Pacific marine barrier” S. Wood, I.B. Baums, C.B. Paris, A. Ridgwell, W.S. Kessler & E.J. Hendy. NATURE COMMUNICATIONS | 7:12571 |, 2016.

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