The case of the black turtle

How many species of sea turtle are there? Depending on who you ask, you might get a different answer. Some might say eight, others will say seven. The debated eighth species of sea turtle is the black turtle.

To trace back the history of the black turtle, one important event stands out: sometime between 3 and 20 million years ago Central America rose up out of the sea. The creation of the Isthmus of Panama was an important paleozoogeographic event that allowed for the migration of terrestrial species between the two American continents. On the other hand, this created a land barrier between the Atlantic and the Pacific, isolating two populations of green turtles from each other. One population that is found in the area from southern California to Chile and around the Galapagos islands looks different from all other green turtles: individuals are smaller in size, their carapace is oval shaped and tapered towards the tail, and they are darker in colour. Because of this last trait, they were given the common name ‘black turtle’, but it is also known by some as the Eastern Pacific green or the Galapagos green turtle.

A juvenile black sea turtle (right) next to a juvenile green sea turtle (photo taken by Javier Rodríguez-Zuluaga, courtesy of PLoS One)

A juvenile black sea turtle (right) next to a juvenile green sea turtle (left)
(photo taken by Javier Rodríguez-Zuluaga, courtesy of PLoS One)

But is the black turtle a unique species? Studies of skull anatomy suggest that black turtles are indeed different to green turtle. On the other hand, DNA analyses reveal no genetic distinction between the two turtle populations. There is still ongoing scientific debate as to whether or not the black turtle should be considered a stand-alone species, or possibly a subspecies. Officially, neither the species Chelonia agassizii nor the subspecies Chelonia mydas agassizii are recognized but both continue to be widely used by scientists.

I contacted Dr Jeffrey Seminoff from NOAA who wrote his dissertation on black turtles in 2002. “Taxonomy is not a static thing and there will always be disagreement among scientific colleagues on the topic,” he tells me. “Today it is not an argument that I prefer to spend energy on. Instead, I am focusing on the fantastic population recovery of black (i.e. green) turtles in the eastern Pacific. These dark green turtles are doing very well currently, but we need to keep our focus on efforts to preserve this recovery!”

Regardless of their taxonomic status, it is great to hear that devoted scientists are successfully preserving these mysterious turtles!