As an Alaskan child, visiting Florida for the first time when I was 17, mangroves were a fuzzy concept, loosely associated with swamps, alligators, and primordial ooze, and definitely NOT somewhere that I was looking forward to snorkeling. But, once I plunged off the side of the boat and under the two-foot thick opaque layer of vivid green algae, I was entranced by the fairy-tale scene of colorful sponges, tunicates, bryozoans and fishes. The trip was quite educational- I learned that mangroves do not , in fact, produce mangos, and informed that these plants are not one thing, but actually many different (and sometimes unrelated) species. A ‘mangrove’ is like a ‘tree’; it is plant with a special lifestyle, a lifestyle involving lots of salty water and a nice hot climate.
Fast forward ten years, and I am now learning that mangroves are a favored friend of biogeographers. For one, miniature mangrove islands played host to one of the founding experiments in the field of island biogeography. But, more directly, the bull’s-eye of mangrove diversity hovering over southeast Asia was a long-time poster child for the notion that the location where a group of species originates is the location where they eventually reach their highest diversity. How scientists eventually pieced together the evidence to show that the global distribution of mangrove diversity did not result from dispersion out of an east-Asian center of origin, is a classic saga of vicariance versus dispersal (read about it here).
Which finally brings me to today’s mangrove mystery- a story of vicariance and dispersal played out on a smaller stage, around the peninsula formed by Myanmar, Thailand, and Malaysia. During the most recent glaciation, when lots of water was locked up in polar icecaps, the Malay Peninsula was much larger and divided many mangrove species into two separate populations on its eastern and western sides. Individuals descended from each of these population centers now have different sets of genes (this is vicariance). Since we don’t have a historical record from 18,000 years ago, scientists have to think backwards- from the observation that mangroves growing on the east and west sides of the Malay Peninsula have two different sets of genes- to the inference that these two groups were once separated.
A group of scientists who study mangrove genetics set out to document whether a particular species of mangrove (Rhizophora mucronata) also showed evidence of having been historically divided. After sampling trees from all over the peninsula and nearby Sumatra, they did find two distinct groups, but these groups weren’t located where the scientists expected them to be. Instead of eastern and western populations being different, mangroves living on both the eastern and western sides of the peninsula were similar and only a few populations in the west around the Andaman Sea comprised a second group. Even more oddly, this distribution did not adhere to another proposed hypothesis predicting that individuals living closer together should have more similar genes.
What could be going on? To find the answer, the researchers had to look at a map of how water flowed around the peninsula. They discovered that ocean currents prevent water from the Andaman Sea from mixing with strait and South China Sea. Mangroves live on land, but their biology and dispersal are influenced by the ocean, especially for this particular species whose large seeds can probably float much farther than other species. While the currents could transport seeds from the eastern group all around the South China Sea and into the Malacca Strait, the movement of the water also prevented these seeds from making it into the Andaman Sea. So, the working hypothesis is that at some point in the past (maybe during the last glaciation) two groups of this mangrove species were separated and the genetic differences they developed are still maintained because ocean currents keep the two groups from mixing. Yet, the mystery of the Malaysian mangroves isn’t completely solved. The scientist still can’t explain exactly how the two groups arose in the first place.
What we learn from this research is that geography affects species’ genetics. Given enough time and the right circumstances these differences can result in the evolution of new species. Biogeographers typically think that distance and barriers are the two main features of geography that affect evolution and tend to view geological features as their chief determinants. Ocean currents add another layer of complexity, and beg the question ‘what other processes that operate over a shorter time-scale can influence species genetics?’ Where in the terrestrial realm are the analog of ocean currents?
You can find this article at:
Wee A.K.S., Takayama K., Asakawa T., Thompson B., Onrizal , Sungkaew S., Tung N.X., Nazre M., Soe K.K. & Tan H.T.W. & (2014). Oceanic currents, not land masses, maintain the genetic structure of the mangrove Rhizophora mucronata Lam (Rhizophoraceae) in Southeast Asia , Journal of Biogeography, DOI: 10.1111/jbi.12263