Oh, Barnacles! The Journey from Free-Swimming to Rock Pimple

By Noah Bressman

If you have ever been to a rocky shore or pier and looked down at the hard bumps encrusted on the rocks or pilings, you have probably thought:

this is a load of barnacles - Roblox
From Spongebob Squarepants.

Barnacles, or rock pimples as I call them, are actually living organisms. While they may be firmly attached to rocks and other hard surfaces, they are actually a type of crustacean, a group known for very mobile animals like crabs and shrimps. Barnacles are not always glued to rocks, though. They start their lives as mobile larvae called nauplii, which swim around and drift through the ocean before finding a nice spot to settle down permanently, at which point they become what you typically think of as barnacles. Some nauplii are planktotrophic, meaning they are able to feed by eating other plankton. Others are lecithotrophic and are unable to feed as nauplii. Instead, these larval barnacles subsist off the nutrients that their parents gave them until they settle down and transform into adults.

You would think being able to feed would help you survive, as you do not have to worry about starving to death, right? Well, this was something of which Jin Yung Wong of Academia Sinica (Taiwan) was skeptical. If being able to feed is better, then why are there both planktotropic and lecithotropic barnacle nauplii? Using two types of barnacle nauplii, the planktotrophic Tetraclita japonica and the lecithotrophic Polyascus planus, Wong and a team led by Dr. Karen Chan at Swarthmore College, used high-speed videos and performed particle image velocimetry (PIV) experiments to assess their performance. PIV is a method used to visualize flow by having tiny, neutrally-buoyancy particles suspended in water that move with water flow. They also compared the swimming speed and kinematics – or the pattern of limb motion – between the two species.

They found that the rounder Polyascus were more hydrodynamic than the Tetraclita nauplii, meaning they could move more easily through the water, and they were able to swim more quickly. Polyascus also used more synchronous limb movements that moved less water than Tetraclita, making them essentially quieter and less likely to be detected by a predator. However, by moving their limbs out of synch, Tetraclita are able to direct water flow toward their mouthparts, helping them capture food.

The planktotrophic Tetraclita nauplii (A and B) move more water (indicated by paths of the particles in B, which go from yellow to blue) than the rounder, lecithotrophic Polyascus nauplii (C and D). Figure from Wong et al., 2020.

Therefore, there are trade-offs for barnacle nauplii. Those that can feed may be able to survive longer and disperse farther before settling by being able to capture their own food, storing energy to increase their survival chances when they finally settle. The downside of this strategy is that predators can more easily eat these larvae. While lecithotrophic nauplii may not be able to feed and survive as long adrift, they are less likely to be eaten before settling. However, this strategy requires greater parental investment (i.e., more egg yolk). Natural selection may favor planktotrophy when food is bountiful, or lecithotrophy when predators are plentiful, but the dynamic nature of the oceans allows both types of barnacles to persist. The next step for Wong and their team is to determine which larval life history strategy Barnacle Boy uses.

SpongeBob on Twitter: "We are sad to learn Tim Conway, the ...
Barnacle Boy from Spongebob Squarepants.

Dr. Noah Bressman is a Postdoctoral Fellow at Chapman University, studying fish biology, biomechanics, biomaterials and behavior. You can find more at NoahBressman.wixsite.com/Noah or @NoahwithFish, or contact him at NoahBressman@gmail.com.

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