Can Barnacles Reproduce on the Human Body?

To begin with, barnacles are not shellfish. They may look like small shelled mollusks, but taxonomically they are crustaceans, closer to crabs and shrimp.

It is hard to imagine from the adults stuck to rocks, but barnacles are not immobile from birth. Individuals that hatch from eggs drift through the sea as nauplius larvae, feeding on phytoplankton and molting repeatedly. Eventually they become cyprid larvae, which are specialized for attachment. Because they cannot move once attached, the video explains, choosing where to live during the larval stage is a decision that shapes a barnacle's entire life.

Cyprid larvae use their antennae to walk around, probing surfaces such as rocks. The tips of the antennae have sensory organs that are thought to detect water flow, chemical information, and surface conditions. The video's view is that barnacles carefully check whether a spot is habitable.

An important cue is the settlement-inducing protein complex (SIPC). Barnacles are thought to recognize that others of their kind are nearby based on the SIPC those individuals secrete.

Furthermore, recent research has reportedly shown that larvae do not seem to decide on an attachment site all at once. First, they use settlement pheromones spreading through the seawater as a cue to approach, sensing that "there seem to be others around here." Then, they use the amount of SIPC on the substrate surface to judge whether "this is really the right place." The video states that barnacles do not simply attach wherever they happen to end up, but decide on an attachment site by reading, in stages, the signals from their own kind drifting in the environment.

Signals from other barnacles are not the only factor, however. The condition of the surface is also an important factor, and the video points to attachment on ship hulls as an easy-to-understand example.

Ship hulls are hard and stable and spend long periods in the sea. This gives larvae enough time to drift near the surface, attach, and grow. In addition, ions, organic molecules, bacteria, diatoms, and the like collect on the surface of submerged substrates and form a biofilm, which also serves as a cue for deciding whether to attach. For barnacles, a ship's hull can be described as a giant artificial rocky shore that stays submerged in the sea. Once firmly attached, they do not easily come off even when constantly battered by waves. The video describes this as the result of barnacles honing the ability to stay put in exchange for giving up movement.

Barnacles that attach to whales, on the other hand, have chosen a way of life riding on the surface of a living creature rather than an artificial object. Unlike rocks or ship hulls, whale skin constantly replaces its cells through metabolic turnover. Since barnacles are nonetheless found on whales, the video points out that attachment to whales needs to be understood in the context of the relationship with the host.

What ships and whales have in common is that water flows over their surfaces, so they can become feeding grounds where plankton is easily delivered. Barnacles extend their legs from inside their shells to sweep the water and catch plankton and organic particles that drift by. Staying still and letting the sea bring them food is one of the barnacle's survival strategies. The video states that what matters to barnacles is not attaching to something in itself, but obtaining food there, continuing to live, and expanding their range while reproducing.

Now, back to the main question: can barnacles reproduce on the human body? The video's conclusion is that, in reality, this is very unlikely to work.

The following reasons are given. Human skin is not a physically stable surface like rock or a ship's hull, but is protected by a dynamic barrier that is regularly replaced with new cells. Nor can humans stay in the sea permanently like whales. Moreover, ordinary barnacles need seawater and sufficient oxygen to grow. For these reasons, the video says that even if a barnacle somehow entered the body through a wound or similar opening, it would be difficult for it to survive there.

But the story does not end there. Among barnacles, there are relatives that, rather than living attached to surfaces, have evolved to invade the host's body. These are the rhizocephalans.

After a rhizocephalan larva attaches to the surface of a host, it goes through a developmental stage specialized for parasitism and injects a mass of its own cells into the host's body. It then spreads a root-like network called the "interna" throughout the host's body. The video explains that for rhizocephalans, attachment is merely the entry point, and the goal is to get inside the host's body.

The effects rhizocephalans have on their hosts are dramatic. Not only do they grow by stealing nutrients from hosts such as crabs and hermit crabs, but they have also been reported to take over the host's reproductive function, inhibit molting, and, in males, change the body shape to resemble that of a female. They can even alter behavior.

A representative example is Sacculina. In a crab parasitized by Sacculina, the main body of the parasite is spread throughout the crab's body, and only a sac-like organ appears externally on the abdomen. The crab lives on protecting it as if it were its own eggs, and in the end even helps release the parasite's larvae into the sea.

As for the mechanism behind these effects, existing knowledge suggests that Sacculina exerts its influence by disrupting the host's nervous and endocrine systems. One piece of supporting evidence is that a specially shaped part of the interna has been found near the host's nervous tissue, and this is thought to function as a sort of command center that controls the host.

In recent research, the genes expressed and the compounds secreted in this part have reportedly been studied, and candidates for the causative substances have been narrowed down. However, the video states that how these candidate substances affect the host's physiology and behavior is still not fully understood.

So, could rhizocephalans ever infect humans? The video says that since the mechanism is not clearly understood, it cannot be stated that this would absolutely never happen. This is because, unlike ordinary barnacles, rhizocephalans are adapted to the internal environment of other animals.

That said, such parasitic relationships are built over a very long period of time, and rhizocephalans are specialized for crustaceans, whose body structure differs greatly from that of humans. Therefore, the video's view is that it is reasonable to assume they are not equipped to parasitize humans. In fact, no cases of parasitic barnacles infecting humans have been reported to date.

Finally, the video touches on how ordinary barnacles reproduce. Since they are stuck to rocks and cannot move, you might expect them to make do with external fertilization, but most barnacles are hermaphrodites that mate with nearby partners and reproduce in a way close to internal fertilization. What evolved for this purpose is a mating organ that is extraordinarily long relative to their bodies. The video speculates that, in order to mix genes with other individuals without moving, barnacles likely evolved a reproductive system premised on living in close proximity to others of their kind.

In addition, barnacles living in the intertidal zone must also withstand drying out at low tide. They close their shells to retain moisture, endure the waves, catch food drifting in the sea, and during the breeding season mate with individuals right next to them. The video describes barnacles as creatures that arrived at the extreme answer of living in one place without moving, and states that their strategy makes use of everything from ocean currents to the presence of surrounding organisms, and is built with an eye not just to survival but also to reproduction and dispersal.

It concludes by saying that, at least for now, barnacles themselves seem to have judged the human body a poor place to live.