Hitchhiking to the abyss.

We investigated, for the first time, the hitchhiker-host fidelity of deep-diving whale sharks and Chilean devil rays. We found that two of the most ubiquitous oceanic hitchhikers, the common remora and the pilot fish, are able to follow their hosts to bathypelagic depths, where they are exposed to extreme gradients of light, dissolved oxygen, temperature, and pressure. We documented a deep dive of a large whale shark hosting remoras and pilot fish. Common remora was observed at the deepest section of the dive, at 1460 m, where the water temperature was 3.6°C. A pilot fish was recorded at 900 m, during the ascent phase, with the water temperature of 7.5°C. Although the adaptations that allow these hitchhikers to mitigate the impacts of such extreme environmental conditions remain unknown, we discuss these findings in the framework of the ecophysiology of deep diving and the hitchhiker-host fidelity.