Rapa whelks and green sea turtles

Unintended backpackers: bio-fouling of the invasive gastropod Rapana venosa on the green turtle Chelonia mydas in the Rı´o de la Plata Estuary, Uruguay

http://link.springer.com.nuncio.cofc.edu/content/pdf/10.1007%2Fs10530-012-0307-9

In this article Lezama et. al bring to light an important biological issue concerning Chelonia mydas, the green sea turtle. They have documented a previously unknown interaction between adolescent green sea turtles and Rapana venosa or, rapa whelks, wherein which the whelks attach themselves to juvenile green sea turtles during brumation. This research is important to the conservation of Chelonia mydas as well as the invasive capabilities of whelks travelling on the turtles. The researchers use the term “biofouling”, which is a term that refers to instances when (in this case) mollusks have attached themselves to an unusual surface. In some cases, turtles had enough whelks to constitute 20% of their body weight. Many turtles have their shells damaged by the invasive gastropods, though the researchers were not able to determine if the whelks can further harm the turtles.

33 green sea turtles were captured in the study, some were found dead but their data was still used. Turtles were tested for fitness (weight vs length minus weight of whelks). Researchers predicted that the whelks would be detrimental to a turtle’s health, and that those with more whelks would be less fit than others. Surprisingly, the whelks proved to be poor indicators of turtle health. In fact, turtles that had more whelks attached to their carapaces were found to have higher fitness than those without. A strange outcome indeed, but researchers were not so quick to jump to strange conclusions.

Researchers suspected that this increased weight found in infested green sea turtles was due to time spent in foraging areas (in this case, the coast of Uruguay, a home to the rapa whelk), while decreased weight reflected a consumption of stored energy required to travel to foraging areas. The turtles that had been in foraging areas longer would have more time to acquire nutrition to gain weight while at the same time picking up rapa whelks, while those that had just arrived would not have had time to gain weight or pick up whelks.

While it is sure that whelks ravage the carapace of green sea turtles, further, more long-term effects are unknown. Lesama et. al pose a few questions at the end of the article: What are the long term effects of this parasitic relationship? Do the whelks induce greater drag to the sea turtle, making it harder to move in the water/travel long distances?