There’s a system of swampy red mangroves, deep in the rainforests of the Yucatan peninsula, that’s a long, long way from home.
The nearest seashore lies 170 kilometers away (105 miles), and yet these salt-loving shrubs didn’t just get up and walk away from the coastline for a fresh drink on the banks of the San Pedro river.
Combined genetic, geologic, and botanical research has now confirmed what many locals and scientists have suspected: this was once an ancient saltwater mangrove ecosystem left stranded during the last ice age, when the oceans receded.
What we see today is thus the freshwater relic of a coastal lagoon ecosystem some 125,000 years old.
“This discovery is extraordinary,” says biologist Felipe Zapata from the University of California Los Angeles.
“Not only are the red mangroves here with their origins printed in their DNA, but the whole coastal lagoon ecosystem of the last interglacial has found refuge here.”
Red mangroves (Rhizophora mangle) usually grow in brackish or salty tidal waters of the tropics, but in rare instances they are sometimes found in freshwater too.
When calcium deposits are rich enough, the mangroves can establish themselves without any need for nutrients from the sea.
Several other studies along the Mexican Caribbean coast have found possible ‘fossil lagoons’ that have also been separated from the sea, although they are not nearly as far away as the one found on the limestone banks of the San Pedro.
Analyzing the genomes of 79 trees at 11 sites around the Yucatan, researchers found river populations of red mangroves were distinct from coastline populations of red mangroves.
The freshwater trees were most closely related to a mangrove found at Términos Lagoon on the Gulf of Mexico side of the Yucatan, which suggests these two mangroves are sisters and share a common ancestor derived from the northside of the peninsula.
Other inland lagoons in Mexico, on the other hand, appear to be connected to Caribbean mangroves, which suggests there are two distinct lineages of R. mangle: one from north and one from south.
“The population genetic analysis confirms that San Pedro River mangroves are a relict of a coastal ecosystem that colonized the river’s tufa lakes, possibly during the Last Interglacial, and stayed behind along the riverbanks after the oceans receded during the Wisconsin glaciation,” the authors surmise.
Running models on sea level data for the southern Gulf of Mexico, researchers identified a low coastal plain that could easily turn into a bathtub if sea level rose even a little bit.
Before the last glaciation event, all the polar ice caps had melted and sea levels were roughly 6 to 9 meters (20 to 30 feet) higher than what we see today.