Crab Discovered Living Stuck On Turtle

Scientists who rescued a turtle discovered after lifting its tail that it had a tiny crab living on its backside and eating its faeces in what appears to be a symbiotic relationship.

The little crab was found stowed away under the shell of the loggerhead sea turtle, where it spends its days cleaning and eating the turtle’s poop.

Marine biologist Gloria Fernandez told Real Press: “Turtles often transport a lot of organisms on their backs. These animals will often stick to the turtle’s body over the course of a phase of their lives, before they drop off.”

“This crab lives in the area of the cloaca, in a hole between the shell and the body of the turtle, which is the reproductive organ and also the place where the turtle pees and poos”, said Fernandez, who works at the Foundation Palma Aquarium, based on the Balearic island of Majorca, in eastern Spain.

The loggerhead sea turtle (Caretta caretta) has specifically evolved to accommodate the pelagic crab (Planes cyaneus) under its shell, giving it pride of place over any other epibionts – organisms that live on the surface of another living organism in a non-parasitic way.

Along with the faeces, the crab, which is harmless to the turtle, also eats algae and molluscs that become stuck to the turtle’s shell and body.

Fernandez said that turtles are like mobile ecosystems, and often host invertebrates such as sponges, worms, and leeches that are specific to the type of turtle and where it has been.

“They have a special relationship, called “commensalism”, which is like a host for dinner or lunch”, Fernandez said.

The relationship between epibionts and turtles is generally a positive win-win situation, but when there are a high number of them it can be a key indicator that the turtle is weakened as injured or diseased turtles are slower, making it easier for the epibionts to set up shop.

Ms Fernandez reported that 76 turtles have been found stranded in Balearic Island waters so far this year, almost double the 42 that were found in 2019 in the same area. It is currently unclear why, but Fernandez said it was a record.

Of the 76 found, only 46 were still alive and had to be treated at the islands’ recovery centres.

This was what the scientists found in the case of Lyn, a turtle that is currently recovering at the foundation after she was found stuck in a “ghost net” in Mediterranean waters.

“Lyn arrived on 8th October after sailors picked it up as it was stuck in a net that was surrounding its neck, causing it deep injuries in the neck”, Fernandez explained.

“Lyn’s is a very serious case. It must have been adrift for weeks, which means that it was an easy place for epibionts to latch on.”

So far, all of the live-rescue turtles have survived, but rescuers are watching Lyn very anxiously as “its life is in danger” and she still has to react to treatment.

Another turtle that is also recovering at the foundation centre was found completely covered in barnacles, which rescuers had to get rid of before it could recover.