Department of Physics, Chemistry and Biology (IFM)

Despite the fact jellyfish are part of C. caretta's diet, the tested turtles at oceanografic didn't show a high interest for this prey during our experiments. Different possible explanations for this phenomenon:

1. The species used (Aurelia aurita) was different from the one eaten by C. caretta in the wild
2. The jellyfish, due to the lack of current in the tank, where staying at the surface instead of swimming in the water column. Because the turtles were rarely swimming near the surface they might have missed the jellyfish
3. A lack of contrast in the tank (completly white) and the translucidity of the jellyfish might have make them hard to see for the turtles. However, since we colored the jellyfish in orange with plankton, and it didn't increase our success rate, it is unlikely to be the case
4. Jellyfish might have a too poor nutritive value for a well fed turtle kept in captivity for few weeks.

The reasons why we observed significant differences in approach and feeding phase between the two kind of prey are relatively simple to imagine (which doesn't mean they're correct though):

1. Crabs were given alive to the turtles which usually had to chase them in order to eat them. That is obviously not the case with clams
2. Clams shell were sometimes hard to break, especially for the younger turtles, which would explain the difference in feeding phase duration
3. The interest of the turtles for eating the remaining pieces of the shell that fell on the floor is also an explanation

It is uncomon to imagine that an animal who spend the same amount of energy trying to catch a prey with a lower value than an other one for which they would spend the same amount of energy in order to get it. It is then usual to guess they might have another interest in this than pure nutritional value. Here is a list of the possible answers to the above question (3): 

1. Despite being poorer in calories, clams are an easy prey to catch for a turtle since they're not really moving from the substrate their fixed on. But regarding the energy expenditure, this argument isn't really interesting except if, because fed regularly by the vet, the calorific value and energy expenditure were not as important factor as it is in the wild
2. Clams’ shell are rich in calcium which is known for being important against metabolic acidosis. But it is difficult to imagine this as being a good explanation concerning sea turtles, since all the studies done on the subject and turtles, were specifically done with fresh water turtles, which have a quite different metabolism than sea turtles
3. Being kept in a simple tank, with no distraction (compare to their wild environment) the turtles might simply have been bored and were considering our experiment as a simple distraction and they just ate crabs/clams because of pure boredom or even maybe by gluttony (Species like clams and crabs are belived to be way more tasty for a turtle than a jellyfish)