Study on tube-dwelling anemones suggests that South America had an "inland sea"

A study of the evolutionary process of diversification of marine organisms reinforces the geological theory that 10 million years ago an ocean extended across the continent from the Caribbean to Uruguay, covering the entire Amazon basin.

By Fábio de Castro

Agência FAPESP – After studying the evolutionary process of diversification of a group of tube-dwelling anemones in the South Atlantic Ocean for four years, a group of researchers from Universidade de São Paulo (USP) obtained an unexpected result: the biological investigation ultimately contributed results that reinforced the geological theory that roughly 10 million years ago, the Amazon basin was occupied by an inland sea that linked the Caribbean to Uruguay.

The initial objective of the study published in PLoS One was to use genetic and molecular analysis to identify the precise evolutionary moment of differentiation of the two tube-dwelling Atlantic Ocean species of Isarachnanthus anemones, a genus belonging to the taxonomic group Ceriantharia. 

The results showed, however, that the most probable scenario for the diversification of the two species – and a third found in the Pacific Ocean – was consistent with the theory of the so-called “Middle Miocene Amazon Seaway.”

According to this theory, an inland sea connected the Caribbean to the current region of the coast of Uruguay between 9 million and 11 million years ago, cutting across the continent. In this period, most of the area of modern-day Brazil would have been an island separated from the rest of South America by an inland sea.

The article was prepared by researchers of the departments of Zoology and Genetics and Evolutionary Biology at the USP’s Biosciences Institute (IB-USP), the Caribbean Research and Management of Biodiversity (CARMABI) in Curacao (Dutch Antilles) and the Institute for Biodiversity and Ecosystem Dynamics (IBED) at the University of Amsterdam.

FAPESP funded the study through the project “Sistematics, life cycle and reproductive patterns of jellyfishes,” conducted under the auspices of BIOTA-FAPESP and coordinated by André Morandini, also a co-author of the article and professor at IB-USP.

According to the first author of the article, Sérgio Stampar, who is pursuing post-doctoral studies at the Zoology Department at IB-USP under the supervision of Morandini, there have not been new studies on tube-dwelling Isarachnanthus anemones in Brazil for 50 years because of the difficulty of collecting specimens of this group, which is only found at night on marine substrates.

“Our idea when we began this study was to resume studies on this forgotten group of cnidaria. We collected samples in several regions of the Atlantic Ocean and obtained large numbers of these organisms. In addition to routine morphological studies, we began to conduct unprecedented genetic analyses on these tube-dwelling anemones,” Stampar comments in an interview with Agência FAPESP.

According to Stampar, the phylogenetic analyses indicated that only one type of tube-dwelling anemone occurred 16 million years ago. This organism, the ancestor of all Isarachnanthus included in the study, inhabited the North Atlantic Ocean, most likely near the latitude of the outlet to the Mediterranean Sea. It is probable that this species crossed the ocean and made its way to the Caribbean.

“We discovered that from a genetic standpoint, the species in Brazil, Isarachnanthus nocturnus, was more closely related to the species found in the Pacific, Isarachnanthus bandanensis, than to that in the North Atlantic, Isarachnanthus maderensis. This finding surprised us because we thought the two species in the Atlantic would be closer,” said Stampar.

In principle, the South Atlantic species, having changed more recently, could have reached more southerly regions along the coast of South America as a result of transport by ocean currents. However, this dispersal scenario is not possible because geological studies show that at that time, the currents flowed as they do today, from south to north. Therefore, the organisms would have had to travel by another route.

”It is virtually impossible for the South American tube-dwelling anemones to have come directly from the Atlantic. Nevertheless, the molecular and DNA analysis that we conducted allows us to estimate that these organisms reached the South Atlantic some 8 million to 9 million years ago. This date coincides with geological speculations on the existence of an inland sea that cut across South America. It is very probable that this was the route taken by anemones,” explains Stampar.

Other organisms

The seaway would have connected the region that is today the Caribbean, on the coast of Venezuela, to modern-day Uruguay, extending throughout the South American continent, covering the regions that are today Mato Grosso, Mato Grosso do Sul, Amazonas and Acre. When this inland sea closed, the anemones that had reached the South Atlantic would have been isolated, initiating the process of diversification into another species.

“The geological process would explain the isolation of these anemones, which would allow for diversification of the nocturnus species when the inland sea closed and the population was segregated in the South Atlantic,” says Stampar.

“Later, the bandenensis species could have emerged through a similar process: after reaching the Caribbean, the nocturnus tube-dwelling anemones from the South Atlantic would have passed to the Pacific because there was not a barrier between the oceans. After the Isthmus of Panama closed 4 million years ago, they were segregated and began differentiating from the Pacific species,” he said.

Although the study reinforced the theory of the Amazon Seaway, the conclusions are still speculative, according to Stampar, because the study was conducted with only a single group of organisms.

“Still, the study showed that pursuing other organisms that follow the same pattern may be worthwhile. My doctorate is largely related to these studies – mainly studying other cnidaria such as jellyfish, which are the specialty of our group, and, in theory, have different dispersal characteristics,” he stated.

The article Evolutionary Diversification of Banded Tube-Dwelling Anemones (Cnidaria; Ceriantharia; Isarachnanthus) in the Atlantic Ocean, by Sergio Stampar and others, can be read in PLoS One at www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0041091.