More often cursed than celebrated, barnacles are so ubiquitous that we sometimes view them as an extension of the rocky shoreline. That’s a mistake, because barnacles are actually fascinating little creatures.
Giant Acorn Barnacle. Photo © Karolle Wall, sourced from: https://inaturalist.ca/taxa/49127-Balanus-nubilus
Barnacles are crustaceans, meaning that they surprisingly share many traits with crabs, including a calcareous shell. But did you know that unlike crabs, barnacles spend most of their lives standing on their head? Barnacles actually begin their lives as a free swimming larvae, and pass through several stages before choosing to settle down for good. Just like humans, barnacles are a little picky about selecting their “forever home”. Hard surfaces are preferable over soft, as is a location near barnacle friends, but any surface will do in a pinch, even a humpback whale! Once a barnacle selects a home, it quite literally glues its head to the surface by excreting a glue through its tentacles. This is where things get interesting, because barnacles are able to produce the strongest natural glue
known, with a tensile strength of 5000lb per square inch. Scientists continue to study barnacle glue, but have yet to fully unlock the secret to creating such a strong bond underwater.
Once settled, a barnacle works to build the surrounding ring of pointy plates that we’re all familiar with, and most develop a set of doors called an “operculum” to seal themselves up tight at low tide. The barnacle won’t ever move again, but it’s far from quiet. Anytime the tide is high, barnacles feed continuously by waving their feathered “legs” (also known as “cirre”) rhythmically through the water to collect plankton. If you’ve ever been on a night paddle, you may have noticed the shoreline sparkle. That’s the movement of the cirre stimulating bioluminescence in the water with beautiful effect. It’s not just you enjoying that sight either; barnacles actually have eyes that can sense light, dark, and shadows, so they’re probably watching the sparkles as well.
Typical barnacle life cycle. Sourced from: https://www.nyharbornature.com/blog/barnacles-are-reproducing
It’s no secret that barnacles are well hung. Many people know that they have the largest penis to body size ratio of any creature in the animal kingdom, but most don’t actually know why. The majority of sessile organisms in the ocean either reproduce asexually or use broadcast fertilization, meaning they send out a lot of sperm and eggs to drift and hopefully connect in the ocean. Barnacles don’t do that. They only reproduce sexually, and must make physical contact with another barnacle to reproduce, hence a “penis” that can be 8 times the length of their body!
We’re all most familiar with the acorn barnacle, which is the most common species in the world, but there are more than 1000 different species worldwide, and at least a dozen commonly seen here on the coast. Notable among these is the giant acorn barnacle which can grow to a diameter of 15cm and height of 30cm (don’t fall on these!). Another unique species is the stalked goose barnacle, which lacks cirre for feeding and relies instead on wave action to deliver its food. Strange as they may be, there are none weirder or creepier than those of the parasitic superorder Rhizocephala. Whereas most barnacles settle on rocks, boats, and logs, rhizocephala barnacles attach to, and work their way inside male crabs. Once there, they grow a fibrous mass throughout the crab, eventually taking partial control and modifying the crab’s behaviour to suit their lifecycle. Most disturbingly, they essentially turn a male crab into a female, causing it to “give birth” to and actually care for the barnacles’ own reproductive organs. Yes this is real, and it occurs right here in the Discovery Islands. No, it does not affect humans, so you’re safe to swim as much as you like!
So what’s the point? In addition to being able to wow our friends with odd bits of trivia, our perspective on barnacles may also reveal some insights to our relationship with nature. Barnacles are a remarkable living organism, yet we squish dozens without a thought each time we walk up the beach. How many other species do we inadvertently damage en masse in our pursuit of recreation? If you’re like me, you may walk a little more gingerly up the beach on your next paddle.