The Barrier Between Atlantic & Pacific: The Speciation Process Of Ophioblennius spp.

Credit: Wikipedia
Ophioblennius steindachneri

SPECIATION: evolutionary diversification and reproductive discontinuity between previously inter-fertile populations. The first step that triggers speciation for cladogenesis is the reduction of the gene flow between the adjacent populations.

For Darwin, this process sees the species born with continuous, small and gradual evolutionary changes. In this way, the perception of the end of one species and the beginning of another is lost. For this reason we speak of anagenesis, that is, progressive and extensive changes with which a species evolves directly into a new species.

On the other hand, the process of speciation by cladogenesis proves to be more realistic: from one mother species, which continues to exist, one or more new species branch out.

We will deal with the speciation process involving a family of bone fish commonly called Blennidae, on both sides of the Isthmus of Panama. The species taken into consideration are: Ophioblennius atlanticus and Ophioblennius steindachneri.

Ophioblennius atlanticus:

DISTRIBUTION: central-western Atlantic ocean.
HABITAT: shallow coral reefs with crystal clear waters, dults can be found up to 10-12 m deep.
ATTITUDE: territorial.
DEPOSITION: benthic.
LENGTH: can reach 20 cm.

Ophioblennius steindachneri:

DISTRIBUTION: Pacific ocean.
HABITAT: slit in the rock in shallow water.
ATTITUDE: territorial.
DEPOSITION: benthic.
LENGTH: can reach 18 cm.

Obviously to understand the appearance of these 2 species within the same family we have to resort to paleogeography. They began to diverge from each other since the closure of the Isthmus of Panama, which occurred approximately 60 million years ago and ended 3.5 million years ago (during the Pliocene). This closure occurred due to 2 plates that push one onto the other: the Pacific one and the Caribbean one.

Considering biogeography, the biogeographical provinces at sea are delineated on the basis of faunal dissimilarity and / or percentage endemism. If the fauna differences are due to oceanographic barriers, dispersion (rather than habitat requirements, competition or other ecological considerations), they should be coherently reflected in the geographical divisions between the provinces.

Caribbean, Brazilian, Ascension-Saint Helena, West Africa and Lusitanic biogeographical provinces are identified for Atlantic. While 3 have been described for the Pacific: Sea of Cortez, Galapagos Islands and Panamanian.

In the Pacific, the island of Clipperton is not assigned to any of the provinces listed above but we will see that it has a particular determination in the context of the pacific species of Ophioblennidae.

The differences in the degree of speciation between these 2 Blennidae can be found in 2 main causes:

Habitat and geography
Oceanography and paleoceanography

HABITAT AND GEOGRAPHY

In the tropical Pacific, the coral reef extends continuously from the Sea of Cortez to Ecuador along the coast of central and southern America. The only exception is therefore the island of Clipperton. In this case, probably the oceanic stretch between the mainland and this island proves to be a partial barrier to gene flow. But then someone might wonder why the same thing does not happen with the Galapagos Islands. In this case the open waters between the mainland and Galapagos do not seem to act as a barrier to gene flow, probably because of El Niño which periodically pushes the water masses towards this stretch of open ocean.

The Atlantic, on the other hand, has highly discontinuous barriers, for example, between Brazil and the Caribbean, there are no coral reefs for about 2300 km. The Mid Atlantic Ridge of 3000-5000 km separates the western Atlantic reefs from the eastern ones. In the Atlantic, river flows such as those of the Amazon River (west coast) and that of the Niger and Volta rivers (east coast) also act as barriers. So river outflows are likely to be an obstacle to larval dispersion.

OCEANOGRAPHY AND PALEOCEANOGRAPHY

In the Pacific, currents are not very stable. For example, the Peruvian current flows north along the coast of South America, veering west near the equator and the Galapagos to become the southern equatorial current. In the north, there is the california current that flows south and then veers west to form the equatorial current of the north. Among the currents mentioned, there is also the equatorial countercurrent. In addition, the Pacific presents variable upwelling, and episodic events of ENSO. The Paleoceanography in fact reveals that the oceanographic models also for the temperatures have changed several times since the Pleistocene. This may have facilitated gene flow in O. steindachneri.

In the Atlantic we have various oceanographic systems that may have contributed to the dispersal of organisms of the O. atlanticus species. Among the possible choices we have: the North Atlantic Gyre, the equatorial bottom current, the Southern Gyre. In this case the current that determined the transport of the Blennidae larvae from one side of the Atlantic to the other is the equatorial bottom current which travels at a speed of 1.8-5.4 km per hour with a T of 20 ° C (temperature and travel time more suitable for larval transport).

Following the closure of the Panama isthmus, the Atlantic suffered a hydrological imbalance. This generated instability of the systems we know today, or rather they were already present but with a higher energy. This energy probably served to allow these species to cross the mid-Atlantic barrier. However, once the equilibrium condition was restored, the equatorial bottom current no longer allowed the crossing of the Mid Altlantic Ridge.

This explains the enormous diversity of Atlantic species compared to Pacific ones.

What I want to point out, in addition to this experimental evidence, is that we are facing 2 species which, thanks to seven million years of evolution, could have become independent of each other. But will it really be so? Because we must consider that the biological concept of species is not verifiable under allopatric conditions (species that occupy completely separate areas). If these 2 species find themselves coexisting again, they can merge into one species again and all the gene mutations that have emerged, during this long time, may be lost.

Diversity is beauty, diversity in nature has been the only step towards wealth. Let’s not make everything uniform, to appear the same. Because nature always makes us different, even if in our eyes this is imperceptible.

Maria Bruno