Paper: Sammarco, P.W. et al. Population expansion of a new invasive coral species, Tubastraea micranthus, in the northern Gulf of Mexico. Marine Ecology Progress Series (2014) 495:161-173. Doi: 10.3354/meps10576
Marine and freshwater species are constantly being introduced to different parts of the world via a number of different vectors. Some species attach themselves to the hulls of ships, carrying them half way across the world, while other species hitch a ride on oil and gas platforms that are being towed to different locations. Species have even been found in the ballast water on container ships and even in other instances, aquarium hobbyists have released invasive species into the wild simply because they didn’t want them in their homes anymore. Once introduced, species distribution can become rapid and widespread.
The sun coral, Tubastraea coccinea, was introduced in Puerto Rico in 1943 (nobody is sure how, but chances are it was due a ship or barge transporting goods from Indo-Pacific) and is presently found all the way in the northern Gulf of Mexico. While teams were out doing survey counts on this specific coral, they found a new invasive coral species, the black sun coral Tubastraea micranthus, on gas and oil platforms. Black sun corals can thrive on highly exposed habitats (aka gas and oil platforms) competing with other benthic epifauna for space. This poses a potential threat if expansion grows dramatically. For these reasons scientists set out to estimate the current population of this new invasive coral and determine if their numbers are expanding and spreading to new locations.
Researchers chose 14 gas platforms in the Gulf (including the one with initial coral sighting) based on the location and age of the structures. Over a two year time frame the scientists were able to perform population surveys using ROVs (remote operated vehicles) that could reach depths up to 170 meters. Equipped with lights, color cameras, and laser beams to provide spatial reference, the researchers were able to survey approximately one platform a day. The ROVs filmed continuously down several vertical and horizontal transects on each platform and the imagery was later processed on image analysis software. Population densities and colony sizes of both the sun coral and the black sun coral were determined for comparison purposes.
What the researchers found
Video analysis from the ROVs revealed black sun coral populations were distributed on other platforms besides the initial one. In fact, this species was found on over half of the platforms surveyed (9 out of 14). The GI-93-C platform (initial sighting location) had the highest density and distribution was seen to decrease as you moved further from this platform (Fig. 1).
Two major ship fairways are found at the mouth of the southeast Mississippi River, which, coincidentally, is where the highest densities of the black sun coral were found. It would make sense that ships and barges traveling through the Mississippi fairway initiated the introduction of this species.
Densities of the sun coral were, as expected, much higher than those of the black sun coral. Distribution between the two species was similar, however. Population densities peaked around the platforms near the mouth of the Mississippi, and faded out the further you moved from the river’s mouth (Fig. 2).
The highest densities of the black sun coral were found on the initial GI-93-C platform, indicating this platform in particular had a major influx of larvae from a population either on nearby platforms or on the ships coming through the channel. While greatest total density of black sun corals was found on the initial platform GI-93-C, the largest colonies were found on a nearby platform (MC-311A). This leads to the suggestion that the MC-311A platform was the form of initial colonization.
Past research has shown rapidly expanding populations are comprised of pre-reproductive organisms, followed by the reproductive organisms. This would explain why the initial sighting platform GI-93-C had the highest density, but not the highest colony size. Total density of corals is clearly biased toward smaller colony sizes.
The researchers have determined that the black sun coral has indeed successfully invaded the northern Gulf of Mexico and expansion to other regions is a definite probability. Given the fact that sun corals have expanded their population to include a huge geographic range, scientists are concerned that the black sun coral is also on its way to major expansion. If the introduced black sun coral is left unchecked for too long, the species will become integrated into its new-found community and a new community structure will take effect, changing the equilibrium of species interactions. So, watch out for the newly invasive black sun coral; it is out to find its target home!