Charles Darwin, famously, had a scientific passion for barnacles, but there is nothing in his research to suggest he ever thought of using them to better understand turtles.
Two pieces of Australian research, however, are now heading in exactly that direction – and may represent the best chance around for protecting the endangered loggerhead (Caretta caretta) and others pecies.
The turtles, being essentially underwater platforms that move through clouds of plankton, make very attractive homes for barnacles. Indeed, researchers at the Wider Carribean Sea Turtle Conservation Network have noted 29 different species living on the animals.
Opinions among marine biologists differ regarding the effect of barnacles on turtles.
It has been noted that large encrustations create extra drag, making it more strenuous for the reptiles to swim around. Some barnacle species drill into the carapace, causing infection.
On the other hand, thus, perhaps, has it ever been. Biologists have often suggested that the relationship between turtles and barnacles is an example of co-evolution, with each species continually adapting to changes in the other.
The cost-benefit balance between the turtles and their passengers is distinctly one-sided. The barnacles get transport and exposure to food, but the turtle gets only a bit of added weight and drag.
Now, however, a researcher at Griffith University in Queensland, Australia, has uncovered a way to even things up a bit – by using the barnacles as devices with which to map turtle migration, and thus better identify areas crucial for the conservation of the species.
Marine biologist Ryan Pearson extracts isotopes – variants of particular chemical elements – from turtle-borne barnacles and uses them to identify where and when the reptiles have been.
It is potentially a comparatively simple and cost effective way to solve a difficult problem: turtles cover vast distances and keeping track of them is both challenging and, using current approaches, very expensive.
Past research has revealed that loggerhead turtles travel up to 2600 kilometres to reach nesting sites.
“We know roughly where they can go but we don’t have a great understanding of the links between feeding and nesting areas and the proportions of turtles that use each location,” Pearson explains.
“That knowledge would point to critical areas we need to protect. This new technique should give us that level of understanding.”
The practice of using isotopes extracted from the soft tissue of the turtles themselves is already well established. Different concentrations of different isotopes can be matched to specific geographical areas, offering a retrospective map, but he distances involved and the difficulties of even finding some nesting sites has resulted in a bias within the literature.
In a paper published in the journal Marine Ecology Progress Series, Pearson reviews previous studies using this technique and reports that overwhelmingly the specimens used come from populations that are considered safe and well managed.
These populations, he finds, have been studied at a rate 13 times higher than ones thought to be under threat. Loggerhead and green turtles (Chelonia mydas) are the most commonly studied species, while flatback (Natator depressus) and Kemp’s Ridley (Lepidochelys kempii) turtles have been largely ignored.
These factors, he suggests, means that there is a mismatch between sub-populations and species studied and those that are high priority for conservation management.
His work using isotopes recovered from barnacles to improve the mapping of turtle movement between feeding, mating and nesting sites has yet to be made public, but it has been assisted greatly by recently published findings regarding the age and lifespan of the barnacles themselves.
Another group of biologists at Griffith University, led by Jason van de Merwe, pinpointed one of the key problems in the use of barnacles as information repositories – uncertainty about the lifespan of the arthropods.
To resolve this, van de Merwe and his colleagues made multiple, successive measurements of 78 turtle-barnacles (Chelonibia testudinariai) living on the backs of loggerhead turtles that had come ashore to lay eggs at an Australian site called Mon Repos.
The barnacles were re-measured when the loggerheads were caught (and released) between 12 and 56 days after the first encounter.
The initial measurements revealed the barnacles were between 3.7 and 62.9 millimetres wide. Recording growth at subsequent measurements and applying some clever mathematics allowed the researchers to estimate that the largest one found was 642 days old.
This means that turtle hitchhikers can contain at least two years of isotopic data – a boon for biologists such as Pearson seeking detailed understanding of their unwitting hosts.
They may also make life much easier for the scientists in another way. The cost of satellite tracking a turtle averages $8000. Barnacle biopsies can be done for just $20.