Tye Pettay and Todd LaJeunesse at Pennsylvania State University in University Park and others have published new work that does some unique finger-pointing. The genetic lineages of Symbiodinium trenchii from the Indo-Pacific have opportunistically invaded Caribbean species of symbiotic corals. This invasion is a double edged sword that may simultaneously place Caribbean corals infected “X steps forward and Y steps back”.

The S. trenchii zooxanthella, as mentioned above, is an Indo-Pacific species, and it is known for its ability to thrive in coral colonies that live in the margins of their survival ranges. The species is well documented for maintaining photosynthetic rates within its host at temperatures where other zooxanthellae species begin to shut down and lose a grip on population. Of further interest, once conditions recover, S. trenchii is often displaced by the former, native zooxanthellae. The mechanism for this “recovery” is unknown.

With this context in mind: in 2005, significant and widespread bleaching and die-back of stony corals in the Eastern Caribbean coincided with a mass infection of Caribbean corals with the S. trenchii zooxanthella. Tye et al. examine the population genetic diversity of these Caribbean S. trenchii and find, indeed, that they are invaders. It turns out that when West Pacific and Eastern Indian corals are examined, they each tended to have unique strains of S. trenchii, even when only 10 km apart.

The Caribbean told an entirely different story, however. Corals were found to contain very few strains across the entire basin. One single genotype comprised as many as 42% of all samples analyzed, and was found in every location sampled in the Greater Caribbean region, with the exception of the Gulf of Mexico.

So what’s the big deal? Well the good news is that the species does allow photosynthesis to be maintained in specimens that may otherwise be lost to bleaching and other stressor-related mortality. The bad news is that the S. trenchii zooxanthella reduces calcification rates in host organisms dramatically… by as much as 50%.

The population genetics of these invaders mean that they are an isolated, introduced organism. Where in the Pacific, S. trenchii is regarded as a highly important coral species diversifier in the Pacific over the last ~2.5 million years, in the Atlantic, it appears a limited number of strains were “dumped” there as recently as decades ago. The source? Humans. Introduction and survival of S. trenchii from the Pacific to the Caribbean through the Panama Canal is a documented occurrence. Ready compatibility of one genetic strain with Caribbean coral species is all that is necessary to facilitate a rapid spread.

More interestingly, the greatest diversity of S. trenchii in the Caribbean occurred nearest port regions. What the future holds for current carriers of S. trenchii in the Caribbean, and potential future hosts, remains to be seen…

For the original article: http://www.pnas.org/content/early/2015/05/27/1502283112.abstract

And further reading: http://www.udel.edu/udaily/2015/jun/caribbean-algae-060115.html