Goose barnacles of the genus Lepas live exclusively on debris floating in the open ocean. Like other barnacles, their larvae spend the early part of their life swimming freely and then, in a final larval phase called the cyprid stage, search out a floating object on which to settle. Once they find a suitable object, says marine biologist Hank Carson, “cyprids in general do do a fair bit of exploration for that cementation spot” upon it, and with good reason: they’ll spend the rest of their life there. Among the criteria they assess is how crowded a spot is, what the underlying substrate consists of, and how deep it is. Once satisfied, they glue their heads in place.

In general Lepas barnacles like to spread out, and prefer a spot in the shade; they grow best away from the top of the water column. The reason is that close to the waterline, the rising and falling of waves periodically exposes the animals to the air, which interferes with their feeding. It’s unhealthy for them in other ways, too. “The uppermost centimeters of water are normally a quite harsh environment with strongly changing ecological parameters, like water temperature, salinity (heavy rains or intense evaporation in tropical areas). Moreover they are subjected to intensive UV radiation,” says Hans-Georg Herbig of the Institut für Geologie und Mineralogie in Cologne, Germany. “From several organism groups it is known that they avoid the uppermost centimeters of the water column.”

Given a healthful environment, Lepas barnacles are notoriously fast-growing. The animals evolved to live on floating organic debris which after a time will break apart and sink, so time is of the essence. Whereas a species of goose barnacle that lives attached to a rock might take five years to reach sexual maturity,1 Lepas can do it in mere weeks. Japanese researcher Yoichi Yusa and his colleagues raised L. anserifera barnacles on tethered debris in a bay in Japan and found that “individuals on the average grew from 3 mm to more than 12 mm in capitulum length within 15 days and some were brooding.” Thus, in less than a month after settling onto a piece of debris, Lepas can begin producing new generations to further their colonization.2

As a result Lepas-settled flotsam can become extremely crowded in short order, with individuals crammed onto every available surface right up to the uppermost limit of what they can survive. Pictured above in Figure 1 is a Japanese skiff that was swept to sea after the Tohoku tsunami in March, 2011, and made landfall on a beach in Washington state in June of the following year, meaning that it floated capsized for about 15 months. If you think it’s remarkable that the barnacles could have grown so huge in so little time, think again. “They grow really fast,” says Cynthia Venn, a professor of oceanography and geology at Bloomsburg University in Pennsylvania. “That boat could get covered like that in six months, even.”

Venn has studied the genus Lepas intensively for more than twenty years. For ten of them, she collected specimens from NOAA’s Tropical Ocean and Atmosphere array of research buoys dotted across the central Pacific Ocean, carefully preserving material that the maintenance crews considered pesky marine fouling. “It was basically a 3-D time series of barnacle settlement,” she says. “I couldn’t find anyone to take the project so I just did it myself. I was able to go two cruises, for the rest I sent my studentsand they then shipped the barnacles back to me so I could work on them. I’ve got hundreds of thousands of barnacles in my garage.”

Looking at the skiff more closely, we see that the upper part of the hull is ringed with a very well-defined boundary below which the Lepas are cheek-by-jowl (orange line in Fig. 2, below). Above that lies an intermediary zone, extending to the waterline (green line), where algae predominate. While some barnacles are visible, they are small and few in number. “They get a better shot at what they’re going to eat if they’re a little bit below that,” says Venn. “I don’t know if it’s too much UV or just they don’t like the temperature changes, or what.”

A Lepas line is also easily seen in the picture below (Figure 3), which shows meteorological research buoys before (“a”) and after (“b”) a 26-month deployment in the North Pacific. “The waterline is at the center (max diameter) of the buoy, where there is a seam in the hull,” says Jim Thomson, a scientist at the Scripps Institution of Oceanography who studies the buoys.3 “The barnacles appear to start about 10 cm below that line.”

Here’s another piece of tsunami debris, this one a refrigerator that made landfall in Hawaii in October, 2012, meaning that it was in the water for just over a year and a half. Both the Lepas line and the algae zone are clearly visible. The waterline, Venn says, would lie about where the green algae shades into black:

You may have noticed that while the hard part, called the capitulum, is of similar sizes in all these pictures, the fleshy, goose-neck part (called the peduncle) is dramatically smaller on the Hawaii debris. Like other fleshy appendages, peduncles can change in size fairly dramatically, especially when they’ve been pulled from the sea. “How long they are kind of depends on how long they’ve had to dry out,” says Venn. So when scientists talk about the growth rate of barnacles, they usually talk about the length of the capitulum.

How Composite Objects Float

According to reader Gavin Grimmer, The upper and lower surfaces of 777 flaperon are “made of honeycombed composite – presumably carbon fiber” while “the leading edge is mainly made from high tensile aluminum (2024-T3) apart from the fibreglass doubler.”4 As a general rule, things made of composite material exhibit excellent buoyancy. The honeycomb materials which makes up most of the volume of the composite skin weighs only about 5 percent as much as water.5 Composite aircraft parts, therefore, tend to float fairly high in the water, like this:

Mike Exner, one of the leading members of the Independent Group, conducted his own study of how the flaperon must have floated, building a model out of plastic poster board. After the interior compartment was flooded it settled into the water like this:

Another example of a composite floating object is this motor boat, which capsized in a storm off the northwestern coast of Australia and then was carried for eight months by waves and currents across the Indian Ocean to the island of Mayotte, near Madagascar — a very similar route that the MH370 presumably took on its journey from the 7th arc. Though the resolution is too low to discern the Lepas line from the algae zone, you can clearly see which part was above the water and which part was below:

Now let’s turn our attention to the 777 flaperon that washed up on a rocky beach on Reunion Island. More than two months later, the French authorities still haven’t released a report detailing what they’ve learned about the piece, which now resides at a facility near Toulouse. Fortunately journalists took photographs of the flaperon from every angle shortly after it was discovered so that just by gathering publicly available images from the web we can assess the whole surface.

As a general observation, we should note that the general shape of the flaperon is plank-like: rectangular when seen from above, with an airfoil cross section. In referring to the part, I will use the nomenclature shown in Fig. 8, below.

Note that the geometry of the piece is essentially planar, by which I mean that the faces do not bulge outwards. As a result, if one point on the edge of an end-cap is underwater, and the corresponding point on the edge of the far end-cap is under water, then the surface between them will be immersed, too. (You can get a sense of this “flatness” in Figures 10 and 14, below.)

To begin with, let’s look at the outboard end cap. Barnacles, either individual or in clumps, are circled in green. I have not necessarily circled all of them, but at least those necessary to show the range of distribution. (To see the full-resolution version of this and all subsequent images, click on the link in the caption.)

Given that the end-cap is rimmed in barnacles, it must have all floated below the waterline. One could argue that a small portion of the strip marked with the red line could emerge from the water, but to my eye it lies between the outer edges of the barnacle clusters marked “A” and “B,” which would not grow up out of the water.

Moving on to the leading edge, we see in Figure 10 (below) that there is a substantial accumulation of barnacles on the outboard end of it, as well as some growth on the inboard side. Though there is little or no growth between these areas, that portion must have been submerged by virtue of lying between those two submerged areas:

This view offers more detail of the inboard end of the leading edge. Growth is quite heavy, though only the tips of barnacle clusters extend outward beyond the plane of the leading edge:

It’s fairly self-evident that the top surface was immersed:

As well as the trailing edge, where the flaperon was evidently severed along the line of a transverse spar. Here we see the top edge, along with some of the bottom:

Here’s the rest of the bottom part of the trailing edge:

Now let’s look at the inboard end cap.

Onward to the object’s final face, the bottom surface. It does not exhibit the same degree of encrustation as we see on the top side. In Figure 16, below, we see the underside of the flaperon with the trailing edge at top. We’ve already noted the presence of barnacles on the bottom of the trailing edge and the bottom of the inboard end cap. We haven’t seen as much yet of the bottom of the outboard end cap, so I’ll focus on that area in this image:

Barnacle growth is much less profuse on the bottom than it is on the trailing edge, but there are enough individuals present on this portion to suggest that the entire bottom edge of the outboard end cap must have been submerged. So, therefore, must have the entire underside. Note that the numbers “1,” “2,” and “3” correspond to the clusters of barnacles marked likewise in Figure 9.

How did the Reunion flaperon float?

The contrast between the Reunion flaperon and other floating debris we’ve looked at is quiet stark. The piece that came off MH370 does not have a Lepas line. There is no significant area that could have protruded above the waterline. The entire surface resembles the deeply submerged areas seen on the other flotsam.

This fact evidently did not escape the French investigators who took custody of the piece. On August 21, the French news outlet La Depeche reported in August that “According to a Toulouse aeronautics expert who requested anonymity, the element of the wing would not have floated for several months at the water’s surface but would have drifted underwater a few meters deep.” Similarly, an article that ran in Le Monde on September 3, 2015, stated that “Les études de flottabilité du flaperon ont quant à elles confirmé que le débris flottait légèrement en dessous de la surface de la mer.”: “Studies of the flaperon’s flotation have… confirmed that the debris floated slightly below the surface of the ocean.”

This seems a reasonable assessment to Venn, based on the distribution of barnacles visible in photographs of the flaperon. “I think it was probably floating just barely subsurface,” she says.

This presents something of a paradox. “It is very hard to build something that will float slightly below the surface,” wrote David Griffin, an oceanographer with the Commonwealth Scientific and Industrial Research Organisation (CSIRO), in an email. “The probability that an aircraft part does this is miniscule. The only way it can do this is if some of the object breaks the surface. If it does not break the surface AT ALL it must sink.”

One could just about imagine that, by sheer good luck, the flaperon might have wound up taking just enough water to give it an overall density almost exactly that of seawater, so that it floated with perhaps a minuscule portion above the water. But such a situation would not be stable. Objects floating with only very slightly positive buoyancy can be pushed below the surface by the action of large waves, says Sean Kery, a hydronamicist at CSC Defense Group who has extensive experience modeling the impact of waves on floating objects. If storm waves push down an object being held afloat by open air pockets, the increase in depth would cause those pockets to shrink, reducing their buoyancy and causing the object to sink further, a phenomenon well-known by recreational scuba divers, who must learn to keep inflating their BCDs as they descend. Of course, without an active compensation system like a BCD a flaperon that was neutrally buoyant at the surface would become negatively buoyant below it.

What’s more, even if an object did manage to float just barely touching the surface, it would eventually sink lower as marine life accumulated. “Things never stay statically neutral,” says oceanographer Curtis Ebbesmeyer. “It’s a dynamic situation. It has to do with infiltration of water, it has to do with the weight of barnacles growing on it.”

Thus, the distribution of barnacles on the Reunion flaperon is difficult to understand. Because they are found all over its surface, the flaperon must have settled into the ocean with a buoyancy exactly identical to that of seawater. And somehow it remained there, floating in a stable manner. Yet this is close to physically impossible.

How could the flaperon have remained underwater?

Given the seeming impossibility of the flaperon floating free across the ocean while submerged, is there another way it might have arrived in its current barnacle-encrusted condition? Since the piece must have been completely underwater, it might have become colonized on the sea bottom. That explanation, however, is problematic. The 7th arc passes through an area of the southern Indian Ocean that is thousands of feet deep. In order to have become colonized by Lepas on the seabed, it would have had to have floated thousands of miles to shallower water, sunk, then refloated to the surface and almost immediately been washed ashore. Also, while Venn says that while she has collected specimens from as deep as 100 meters, “that was not on the bottom or anywhere close to the bottom. It was simply 100 meters below the surface where the ocean was probably more than 5000 meters deep. I have never heard of Lepas colonizing anything on the sea bottom.”

Another possibility is that the flaperon was positively bouyant but remained beneath the ocean surface because it was tethered to the seabed. As it happens, in the past researchers have successfully managed to raise Lepas on substrates anchored offshore. In Yoichi Yusa’s experiment noted above, he collected Lepas specimens growing on pieces of driftwood and floating plastic and attached them to tethers in a bay in Japan. There he monitored their progress as they grew over the next month and a half.

The view of the flaperon seen in Figure 17, below, might provide evidence of how the tethering was accomplished. On the inboard edge of the upper face one can observe a peculiar strip where the surface appears considerably less weathered than the surrounding area:

When this was first pointed out to me I figured it had to do with the missing piece of rubber gasket along the inboard edge of the top surface, which might have been knocked off by contact with a reef. But now that I look closer I see that it isn’t actually that. I’ve marked the “white area” on a photo of a new flaperon below (image reversed to make a left flaperon look like a right one):

It seems that something was clamped to the “lighter patch” that isn’t normally attached to a flaperon, and which was detached after the part spent some time in the ocean. Since it’s hard to imagine this happening without human agency, perhaps it was part of a tethering/untethering operation. Perhaps an anchor line was attached there.

Duration of immersion

Up until now, it has been assumed that the flaperon was deposited somewhere along the 7th arc soon when MH370 impacted the southern Indian Ocean on March 8, 2014. If it was actively tethered to the seabed, obviously, this timeline is no longer relevant. Instead, we can turn to the barnacles to provide some indication of the likely duration of the flaperon’s immersion.

“Assuming they have enough food, and the temperature is good, barnacles will follow a steady growth progression,” Venn says.

The clock starts running the moment the flaperon hits the water: So long as the water is warm enough, Lepas will begin to colonize an object almost immediately. (Yachtsman who make long oceanic passages report that after spending a few weeks heeled over on a single tack a section of hull that is normally high and dry can pick up a colony of Lepas; Venn says she has seen cyprids attach to material as ephemeral as floating paper bags.) While the precise growth rate depends on water temperature and food availability, a rough notion of these parameters is enough to yield a ball-park figure for how long immersion has continued. Earlier this year, Venn co-authored a paper in which she and her colleagues ascertained that a human body found floating off the cost of Italy must have been in the water at least 65 to 90 days, based on the size of the Lepas barnacles growing on its clothes.6

We can do something similar for the barnacles on the flaperon, using the Mayotte boat as a reference. Since both traveled through a similar stretch of the southern Indian Ocean, their growth rates should be in the same ball park.

By comparing features on the flaperon to reference objects of a known size (e.g., the rear door of a Gendarmerie Land Rover Defender in Figure 16) we can estimate the capitulum lengths of the largest barnicles on the flaperon. They turn out to be approximately 2.3 cm.

Applying the same technique to the Mayotte barnacles yields capitulum lengths of about 3.5 cm.

Yusa’s paper on Lepas growth rates states that “Individuals <5 mm long (mean ± SE = 3.09 ± 0.19 mm) grew rapidly, reaching 12.45 ± 0.54 mm on day 15 (Fig. 2). After that, their growth slowed and finally reached 16.26 ± 0.49 mm on day 42.”

The Lepas anserifera that Yusa studied are somewhat smaller than the Lepas anatifera that predominate on the flaperon, but if we use Yusa’s growth rate as a conservative lower bound, and suppose that the largest flaperon barnacles were 16.3 mm at day 42 and grew at 0.1 mm/day thereafter, that means it would take them another 67 days to reach 2.3 cm, for a total growth time of 109 days, or about four months.

If they proceeded to grow at 0.1 mm for the following four months, that would take them to 3.5 cm, which is what the Mayotte barnacles achieved.

Interestingly, when I asked Yusa via email how long it seemed to him that the colony had been growing on the Reunion Island flaperon, based on photographs I sent, Yusa answered: “I would guess that they had been there for a short time (between 2 weeks and a few months).”

Venn’s seat-of-the-pants estimate was “less than six months.”

Conclusion

Photographs of barnacles living on the MH370 flaperon discovered on Reunion Island, combined with expert insight into the lifecycle and habit preferences of the genus Lepas, suggest that the object did not float there from the plane’s presumed impact point, but spent approximately four months tethered below the surface.

UPDATE 10/10/15: Could the distribution of barnacles be explained by continual flipping?

Since I posted this piece yesterday evening, a number of people have suggested that perhaps the flaperon flipped over every few hours, allowing barnacles to survive on both sides. Such a scenario might also explain why the density of Lepas is rather low compared to that seen on other objects. It faces two difficulties, however.

First, the flaperon is broad and flat, and once its inner cavities were filled with water it would weigh thousands of pounds. With only a few inches of freeboard in even the most optimistic scenarios, it would be very resistant to being flipped — much more so than, say, the fridge, which nonetheless clearly floated in a stable manner. Even if it were fairly easy to invert, high waves and wind would be required to do so, which would mean that flaperon would have had to have spent a year or more in constant storm conditions. Yet tranquil conditions are actually more normal. “Calm seas are actually pretty common in the stable high pressure cells that more-or-less permanently inhabit the center of ocean basins,” says Hank Carson, who has traveled across the Pacific gathering floating debris. It’s hard to envisage anyhing flipping over a day like this.

Second, the reason that the Lepas line exists is that these animals don’t like to be exposed, even for a few seconds. They can survive close to the waterline, where they are risk being exposed and immersed with every wave cycle, but only a few small outliers attempt it. They are simply not adapted to frequent long-duration exposure, like their relatives who live attached to rocks in the intertidal zone. “I do not think they can survive more than one day above the water,” Yoichi Yusa told me, while Venn says she has seen them live as long as three days. Apart from the physiological stress of being exposed to what to them is a toxic environment, the animals would spend half their time unable to feed. So even if we imagine the essentially impossible scenario in which the flaperon keeps flipping back and forth every few hours, we would not expect to see dense aggregations of mature individuals.

The implications of low settlement density

While we can learn a lot about how long an object has been afloat by the length of Lepas capitula, it’s harder to draw conclusions based on the density with which they settle. Barnacles do not land randomly, like plant seeds, but actively sniff out an object’s surface in the cyprid stage before settling down in the spot they like best. While they prefer living in the shade, they even more prefer cracks and crevices, and dislike a smooth surface. You can see several places on the top of the flaperon where they’ve preferentially settled down into dings and divots. Most of the broad expanse of the upper and lower surfaces they have avoided, most likely because it’s just too smooth and exposed. They especially seem to like the exposed broken honeycomb on the trailing edge, which presumably offers a nice rough surface for holding fast to. Here they are living in quite high density, with some actually growing on top of one another:

By way of comparison, here’s a shot of the barnacles on the Mayotte motorboat. Their distribution is much more uniform on every surface — here Lepas seem to like everything equally well:

Therefore, I wouldn’t necessarily say that Lepas density on the flaperon is low, but rather that the suitability of the substrate is very heterogeneous.