In “On the origin of Species” of Charles Darwin (1872) he listed some “impassable” barriers from the dispersion process. In addition to the continents, Darwin also included a marine barrier among these and was the first to understand its zoogeographic importance:

“Westward of the shores of America, a wide space of open ocean extends, with not an island as a halting-place for emigrants; here we have a barrier of another kind, and as soon as this is passed we meet in the eastern islands of the Pacific with another and totally distinct fauna”.

It is a barrier ranging from 5000 to 8000 km with deep waters capable of separating the Eastern Tropical Pacific biota from the Indo-Western Pacific region. The long distance between the lower coastal waters blocks the migration between the thermophilic fauna of the East Pacific and the fauna of the Western Pacific province. Only 4% of coral species and 2% of echinoderm species present in the Western Pacific are also present on the other side.

The biogeographical zone of the Eastern Tropical Pacific extends from the Sea of ​​Cortez to the northern Pacific coast of Peru and isolated from the Caribbean about 3 Mya with the closure of the Isthmus of Panama.

But in light of the new studies, can we really speak of an impassable barrier? Are corals, despite the sharing of some species on the two sides of the Pacific, really separated by this “formidable” barrier?

The coral fauna of the Eastern Pacific was initially thought to be a relict of the Western Atlantic (Caribbean) species that were interconnected right from the Isthmus of Panama. After its closure, the Eastern Pacific communities were modified by extinctions and evolutionary changes, based on the climatic, oceanographic and geographical factor as we saw in the previous article.

According to some studies, however, the Eastern Pacific corals are more recent due to the dispersion, precisely through the Eastern Pacific Barrier. But how was this possible? Thanks to the low sea level of the Pleistocene period with the motion generated by the North Equatorial Counter Current. These conclusions are based on the taxonomic affinities of the corals that build reefs inhabiting the Eastern Pacific and on their dispersion potential, inferred from the larval life span, by the rafting capacity and the study of the Pacific currents.

Consider the species Porites lobata. It is an ecosystem engineer who builds the structure of coral reefs throughout the Pacific. Porites spp. can reach 7 meters in height and 41 m in circumference and an age of about 1000 years.

P. lobata produces eggs that already contain symbiotic algae by direct transmission, which develop into planktonic larvae. Therefore, the larvae can obtain nourishment during their planktonic life, thus extending their dispersion potential.

Coral populations in the Eastern Tropical Pacific survive at very difficult conditions. In fact, the habitat is limited, characterized by shallow waters, with frequent environmental disturbances and the rise of cold waters (for upwelling) with low pH, which limit the production of the calcium carbonate skeleton. This environmental setting makes the reefs of the Eastern Tropical Pacific a particular case study, both as marginal and vulnerable populations and as a reference for world coral reefs under the influence of climate change.

But today these genetic exchanges, as the origin of coral reef fauna, are still possible as in the Pleistocene period through the Eastern Pacific Barrier? Does this eventual passage contribute to the resistance and resilience of the species present in the Eastern Pacific? Do currents and climatic conditions still allow this passage?

We will know more in the next article.

Maria Bruno