A Study on the Effect of Barnacle Attachment to Loggerhead Turtle Fossils

Bone Modification Features Resulting from Barnacles Attachment on the Bones of Loggerhead Sea Turtles (Caratta caretta), Cumberland Island, Georgia, USA: Implications for the Paleoecological, and Taphonomic Analyses of Fossil Sea Turtles

J-P Zonneveld, Z.E.E. Zonneveld, W.S. Bartels, M.K. Gingras, and J.J. Head 

Summarized by Jackson Asbrand, a current undergraduate at the University of South Florida’s School of Geosciences.

Data being used: Recent Loggerhead sea turtle skeletal material washed up off the Atlantic coast from Virginia to Florida, USA were the subjects of this study, along with any barnacles that were attached to the turtle skeletons.

The point of this paper: The purpose of the paper is to investigate the relationship between bone modification on sea turtles, such as pits (circular holes) and divots, and barnacles that attached to the bones before the turtle died (Figure 1).

Methods: Scientists gathered the skeletal material at various beaches along the east coast.  The skeletons were measured, described, and photographed, with osteological (bone) elements such as depressions or pits being noted. Some of the barnacle pits were recreated in clay to better study the specific shape of the trace left behind. Finally, all of the osteological features were plotted on a digital master sketch of the entire turtle skeleton in order to compare common types of pits on different bones and across skeletons. Each barnacle was identified on the master sketch using a gray circle. The circle would become darker depending on how many barnacles were found in that specific spot.  

Results: The barnacles leave pits on turtles by using either mechanical abrasion (physically wearing the shell down) or excreting a substance that allowed the animal to permanently attach itself to an object. After attaching to the shell, the barnacle causes bioclaustration, or a biological reaction by the host organism in response to an injury or infection by a parasitic organism (in this case, the barnacle). This leads to the bone holes and pits being created, contributed both by the secretion and the bioclaustration. However, this secretion must be renewed to continue being attached to the turtle, so most of the barnacles fall off after death, leaving only the bone pits remaining. There were six types of bone pits that scientists identified. Type 1 is a shallow, but smooth hole. Type 2 is deeper than type 1 and have a smooth, but still angled bottom, Type 3 is similar to 2, but with a flat bottom. Type 4 is a deep pit with many smaller pits on the bottom. Type 5 is a tube-shaped hole that runs even deeper into the bone. The last type, 6, is a ring-shaped indent on the surface of the bone. Broad bone pits were common on most of the skeletons, some digging deeper into the bone than others; in head bones, these pits were generally shallower. There is also a large range of how many barnacles were actually on the head, ranging from zero to even 70 individual bone pits on one unfortunate turtle. The results are similar for the top part of the turtles’ shells, which had a variety in both the depth and the number of bone pits, although they were slightly more common on the front half of the shell than the back half. On the bottom part of the shell, there is little to no relationship between the modification of the skeletons and the barnacles, as they leave no evidence of it occurring. Type 1 pits were seen on the head bones and both sides of the shell. Type 3, 4, and 5 were only seen on the shell. Type 6 was far less common than the rest of the types and were also only seen on the shell. Types 1-4 were all preserved between the barnacle and the bone, meaning that the barnacles did not use physical force to cause the bone pits, but rather dissolve them using secretions Type 5s used both physical and chemical force, as they penetrated through the skin straight to the bone. Type 6 rings were also caused solely by chemical reactions.

Six pictures of turtles are shown in the figure. One returning to the ocean from the beach, one in the ocean with dozens of barnacles on the back of its shell, a third with smaller barnacles on the side of the shell, a fourth with fewer ones atop the edge of its shell, and a fifth and sixth image are zoomed in to highlight the third and fourth turtles’ barnacles’ locations.
Several loggerhead turtles with barnacles in various spots on their shells, in which some will remain on the bone after the turtle dies. A particularly dense cluster of barnacles can be seen in image C, which all are permanently attached via secretion. The types of pits, like those identified in this study, aren’t specified here, since the pits are classified after the death of the organisms.

Why is this study important?: We can use this data to identify patterns in how barnacles not only attach to Loggerhead turtles and dig deeper into how their relationship works, but also other species of turtles, or even other marine animals with which barnacles could also share a similar parasitic relationship. 

Broader Implications beyond this study: This study creates a template to look further at bone modification on sea turtles other than loggerheads from the Cenozoic and Mesozoic Eras, or in other words, up to 252 million years ago. This study also provides insight into how a symbiotic relationship between two species could be permanently preserved in the fossil record, as interactions such as these are not as often preserved.  

Citation: Zonneveld, J.-P., Zonneveld, Z. E. E., Bartels, W. S., Gingras, M. K., and Head, J. J. (2022). Bone modification features resulting from barnacle attachment on the bones of loggerhead sea turtles (caretta caretta), Cumberland Island, Georgia, USA: Implications for the paleoecological, and taphonomic analyses of Fossil Sea Turtles. PALAIOS, 37(11), 650–670. https://doi.org/10.2110/palo.2022.021 

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