London Lad,
I have seen some other turtles shifting back and forth under a ledge and figured one reason might be to scrape their carapace but this session that I observed of Friendly is the first time I have seen a turtle work its whole shell. My friend Bob had told me beforehand that he has seen and photographed a Hawksbill out at the same spot over a week ago and it too had been doing a shell scrub and he watched it rotate around and get both sides.
There are fish cleaning stations where usually a pair of Hawaiian Cleaner Wrasses will stake out and take on any fish or animal that comes in for a cleaning. I believe they target tiny parasites. I have seen them go at a turtle that comes by. Below is a shot of one working the underside of a Hawksbill that happened to be feeding in the wrasse's jurisdiction:
I have also visited a couple turtle cleaning stations where turtles will come to be attended to by any number of fish; often a school of convict tangs. I have yet to get a good shot of this because the visibility has not been good and the tangs split before I can get close enough. You can see some of the fish who hadn't vacated yet in the image below:
It's a terrible example because before spooked away, the turtle has a full cover of fish all over its shell! I don't know if the fish go for the algae or just parasites on the turtles. I also don't know if the turtles are sensitive to tiny critters and organisms hitching a ride on their shell or not. I don't know if they feel "itches" that need to be scratched or not. If you watch fish getting cleaned by the cleaner wrasses, it looks like they are enjoying the procedure based on their bright colors, open drooling mouths and 600 yard stares. Perhaps I am anthropomorphizing here a bit but if you ever see a fish getting cleaned, you will understand where I am coming from!
Back to the turtles, I found some shots of Hawksbill turtles in other parts of the world and their shells were beautiful and clear of any algae. Our reefs are significantly impacted by algae over here and much of the coral has been taken over. This is attributed to runoff from golfcourses, general landscaping and storm drains and injection wells which pump nutrient rich, treated water into the ground, well below sea level. My suspicion is that the sea turtles here have a greater issue with algae and perhaps other shell infestations because of an altered habitat.
I have seen turtles embrace other turtles and climb over each other and figure it is something beyond a sex based encounter; especially when it is a pair of females doing this. Perhaps they both benefit from rubbing against each other. :shrug:
I have often been tempted to go out there with a 3M scrub pad and see if a turtle would be game for an algae cleaning. The laws against turtle harassing though don't make this a good idea let alone my lack of understanding. I know from a number of years cleaning sail boat bottoms for racers that an algae film can create significant drag and these turtles would be swifter in the water, without the growth. If nothing else, they look so much better and are more photogenic when all clean and scrubbed up! I suspect Friendly knows how pretty she is and takes pride in her appearance!
There is a constant battle for real estate and surfaces in the ocean and the shells of these turtles are no doubt exempt from exploitation of would be squatters.
I would be willing to bet that if someone was to fabricate a framework that was host to a series of stiff brushes that in time some turtles would avail themselves of such a self serve scrubbing station! For all I know this turtle in the image below was hanging around and rubbing against my wave ski mooring line with the intent of some scrub work. :shrug: :thinking:
As I understand it, appearance is a significant consideration in many animal species when it come to selecting a mate and I have read where sea turtles can recognize each other based on their unique patterns in the scutes (scales) and such. If they are covered in growth. that might not be so cool. The female below has algae covering most of her shell and recognition might be difficult: