I want to share this obscure paper with you:
J. E. Moriarty, J. A. Sachs and K. Jones. 2008. Directional Locomotion in a Turtle barnacle, Chelonibia testudinaria, on Green Turtles, Chelonia mydas. Marine Turtle Newsletter 119.
I’m going to give you the punchline first: They found evidence of barnacles moving!
Now that I’ve got your attention, lets start with some basic introduction. Chelonibia testudinaria is a species of barnacle that lives specifically on the backs of sea turtles. It is part of an entire superfamily of barnacles (Coronuloidea) that are obligate epibionts, meaning they only live on other living organisms. Members of this group live on everything from whales, manatees, crabs, molluscs, and turtles. This is how these barnacles get to travel the world!
To date, no one has shown any significant harm these barnacles do to their hosts (nor benefit), and this is considered a commensal, not parasitic relationship. In contrast, there is evidence that they may preferentially settle in wounds (ouch!). I imagine this might relate to chemosensory settlement cues that would be strongest at an open wound. It is thought the main advantages to the barnacles are dispersal, and predator evasion (what sea star or snail will climb on the turtle just to eat them?)
Moving sessile organisms
In general, sessile organisms (like barnacles), are considered sessile for a reason. They don’t move as adults. The larvae settle out of the plankton based on certain cues (many are poorly understood at best) on what they hope is a good spot, and then metamorphose, binding themselves to a location for their entire life. This extends to coral, barnacles, sponges, mussels, tunicates, tube worms, bryozoans, and anemones.
This isn’t 100%. Life is full of exceptions. A big one is anemones. Some of the swim (Stomphia), but even less spectacular anemones have the ability to crawl slowly.
This would be of great advantage to a sessile organism. I was with a tour group last week, and someone asked why all the anemones were in pits and crevasses. At first I thought it might just be differential survival, but no, I’m pretty sure those anemones would move to the more sheltered locations all by themselves.
I have not heard of coral, sponges, tunicates, or tube worms moving once settled, but mussels (at least when they are small) can move about as well. They use byssal threads to attach to the substrate, and can move by progressively moving where their byssal threads attach. This is separate from their ability to crawl using their foot.
Barnacles is new, and very exciting (to me at least) as an addition to the list of sessile organisms that are not so sessile!
So what about this paper then?
It is known that filter feeding organisms do best in areas of high flow: more units of water pass by, carrying more food. On a turtle, this would be the front edge or dorsal ridge on the shell. It was previously known that barnacles are more common on these regions of a turtle carapace, but this was thought to be due to cyprid (the barnacle larva stage that settles and metamorphoses) movement and differential survival.
This paper was a casual case of observation. The authors were studying Green Turtles, Chelonia mydas in florida, and noticed that the barnacles were in different locations on the same individual. It doesn’t say so in the paper, but I imagine the reaction was more along the lines of: “What? That can’t be right, check again!”, in the paper this is stated as: “Casual inspection of the photographs indicated that the barnacles were moving on the carapace of these turtles over a period of months. To investigate this, we assembled a series of photographs for each of the three turtles.”
All in all they had 8 barnacles on turtles that moved. Most moved anteriorly and medially (towards the middle and front), as you would expect based on where the highest flow is.
How fast? Average rates varied from 1.4 mm – 0.27 mm/day! Seems kinda slow, I know, but definetly faster than 0. They even leave a trail on the shell, so you could see part of their track.
The authors raise an interesting point: the ideal settling location is in high flow, and its difficult to settle there for the same reason. If you are capable of moving after you have settled, that gets the best of both worlds. A low flow settling location to get properly attached, followed by a migration to an ideal feeding spot.
Unlike many barnacles, C. testudinaria does not have a base plate, making moving a bit easier. The authors suggest they move by creating tension in the leading edge of the shell, causing it to move forward with growth increments of the shell.
To be fair, some barnacles are capable of rotating in place, or getting pushed by growing neighbours, and the settling cyprid larvae can migrate, but this is the first evidence of large scale adult barnacle motion.
Also, there are only three turtles, and less than a dozen barnacles in this study, so some caution is warranted, but I believe the pictures speak for themselves. This paper is available free online here, so take a look for yourself.
Extra bonus paper
I love phylogenies, I admit it. So there’s a cool side note to this story. Zardus et al. 2014 just threw a big wrench into our understanding of these turtle barnacles. Previously there were 5 species of Chelonibia, each with a specific range of hosts (different sea turtle species, as well as some crustaceans and manatees), and associated morphologies. I won’t go into the nitty gritty of this paper, but the conclusion is that there are only two species of Chelonibia, and all that host specific morphology is simply phenotypic plasticity! One is Chelonibia caretta which is specific to a few types of sea turtle, but all the others are being grouped together as morpho-types of C. testudinaria. (Why can’t I hear you whooping in excitement?).
Putting these papers together, does this mean the barnacles on manatees can move too? What about whale barnacles? The soft tissue embedding seems to me unlikely to allow migration, but who knows? I’m pretty sure there’d be no reason barnacles on crabs couldn’t wander about…
There are so many cool things out there just waiting for you to notice them! Take your time, and don’t brush off anomalies.
J. E. Moriarty, J. A. Sachs and K. Jones. 2008. Directional Locomotion in a Turtle barnacle, Chelonibia testudinaria, on Green Turtles, Chelonia mydas. Marine Turtle Newsletter 119:1-4
J. D. Zardus, D. T. Lake, M. G. Frick and P. D. Rawson. 2014. Deconstructing an assemblage of “turtle” barnacles: species assignments and fickle fidelity in Chelonibia. Marine Biology 161:45-59