It’s not every day that you need to talk to one of the world’s leading experts on goose barnacles of the Indian Ocean, but today is one of those days, so I considered myself very fortunate to get in touch with Charles Griffiths, an emeritus professor of marine biology at the University of Cape Town and author of the seminal paper “South African pelagic goose barnacles (Cirripedia, Thoracica): substratum preferences and influence of plastic debris on abundance and distribution.”
I reached out to Dr Griffiths by email and he graciously answered my questions about the sea life found growing on the Reunion flaperon after I sent him a more detailed version of the picture above.
Is it possible to identify the species of barnacle growing on the debris?
In this case it is possible to identify this as being Lepas anserifera striata on the basis of the small row of pits across the shell, which is characteristic of that subspecies.
Can this tell us anything about where the debris might have been floating?
This is not much clue as the species has a wide global distribution in tropical and subtropical seas.
Can you say in very rough terms how long it takes the barnacles to reach this stage of growth?
I cannot accurately gauge the sizes of the largest specimens from the image but goose barnacles grow spectacularly fast e.g. 21 mm head length ( i.e. Without the supporting stalk) in 21 days cited in one paper I have at hand. I have seen very large barnacles (as long as my finger) growing on a cable known to have only been in the water for 6 weeks!
UPDATE: To clarify a point raised by commenters, I asked Dr Griffiths a follow-up question:
Is it true that barnacles can’t survive in the open ocean? Is it possible for a piece of debris floating far out to see be colonized by Lepas anserifera, or would it need to be in a coastal environment?
No, that is not the case. These goose barnacles are in fact characteristically oceanic beasts and only occur in floating objects in the open sea. Reaching the coast is in fact a death warrant for them and any that get washed up die! Interestingly they seem to know whether an object is floating, so for example are common on kelp that is uprooted and floating but never occur on the same kelp when it is attached.
Can you tell whether the barnacles in that picture are alive or dead? If alive, how long can they live after being washed up?
If you find a washed up item that is fresh (same day) the barnacles will still be opening their shells and waving around their cirri (legs) to try to feed. Obviously in a still image cannot see this. However I can see the cirri projecting from some animals. These would rot away and drop off in a few days in a tropical climate, so this wreckage has only been washed up a couple of days at most. Also crabs and other scavengers love to eat goose barnacles and will clean off most within a couple of days. There is no evidence of feeding damage or headless stalks here, so that suggests to me this wreckage was collected and photographed within a day or two of stranding.