Quitting the wandering life to settle down

by Mark Brazil

Where waves crash across rocky shores there is a narrow region between the uppermost level covered at high tide and the lowest level exposed at low tide, the intertidal zone, that is prolific of life. Living there are seaweeds, shellfish, fish that prefer shallow water and can survive the varying temperatures of tidal pools, a scattering of shorebirds such as heron and egret, crabs that scuttle to avoid them, octopuses that hide by changing color or by squeezing and flowing through tiny rocky gaps, sea urchins, limpets and, for those who go without sandals, the creatures that provide grip on slippery rocks, but which are quick to graze and even tear your skin should you fall — the barnacles.

Humble acorn barnacles cemented to rocks in the intertidal zone have already been through one of nature’s most elaborate metamorphoses.

There are two broad groups of barnacles. Those that are attached directly to the rock are known as acorn barnacles, whereas those that are attached at the end of a stalk are goose barnacles. Barnacles are found from equatorial to polar regions. Those in the intertidal zone trap seawater in their shells so as to remain moist until the tide rises again, but some deep-water species live at depths of 2,000 meters or more.

If you have ever been whale watching and seen humpback whales with their apparently calloused chins and pectoral fins, then you have seen another type of barnacle — one that attaches to a living substrate. Boat owners paint their hulls with potent chemicals to prevent barnacles settling and establishing colonies there. At docks and harbors anywhere astonishing encrustations coat pilings, buoys and even ropes where these are exposed to the waves.

The soft body of the familiar acorn barnacle is protected by surrounding pairs of calcareous plates, cemented to the substrate; it is these that form the hard conical structure that is so painful to scrape against. Another pair of plates seals the opening (the operculum) at the top of the barnacle, through which it feeds.

Barnacles are crustaceans, related to shrimp and crabs, but of a kind that is only free-dwelling when immature. Myriad young drift with the ocean tides as plankton until they find a suitable substrate. Once they find the perfect home, they transform, trading oceanic peregrinations for a settled life.

Theirs is a total change, a metamorphosis. The extent of modification is astonishing; some body parts are lost altogether and the uses of others change. They abandon their ability to float and swim and cement themselves by the tops of their heads to the rocks. The cement they use sets under water and can stick even to extremely smooth surfaces; dental researchers are showing a keen interest in the substance’s properties because of its potential applications in that field.

Now, instead of traveling with the tides, the tides come to the barnacles, carrying planktonic food that they access by filtering the flow.

How do these topsy-turvy creatures feed? They use their feet, of course! No longer necessary for swimming, six pairs of feathery, jointed legs (cirri) extend into the water through the portal of the operculum. They wave these and sieve the water to filter out their microscopic planktonic food.

Imagine, if you will, that throughout your adult life you are cemented by your head to the floor. How can you possibly meet your neighbors, or find a partner and mate? Barnacles are usually hermaphrodites: That is, each individual has both male and female reproductive systems. Neighbors apparently alternate in their male and female roles over time, with those acting as females laying hundreds of eggs inside their own mantle cavity. The “females” then secrete a pheromone that alerts nearby “male” neighbors to their condition.

Those acting as males can’t get up and move around bodily as mollusks do, so instead, barnacles have (in proportion to their body size) the longest penis in the animal kingdom. “Acting males” extend the probosciform penis out from their shells, following the pheromones to their source to fertilize the females. They are able to reach females up to seven shell diameters away, inject their spermatozoa into the mantle cavity and fertilize all the eggs at once.

Adult barnacles may survive for up to 30 years and during that time are able to lay (or fertilize) several batches of eggs each year. The human equivalent hardly bears thinking about, but it has been likened to consenting adults, living several houses apart in the same street, having sex together without either of them leaving their own beds.

The fertilized female barnacle broods her eggs until they hatch as microscopic larvae (nauplii). These are expelled from the mantle cavity and disperse into the ocean plankton. At first the tiny, weakly swimming nauplii merely feed and molt, but after a couple of weeks, they transform into relatively strong-swimming cyprid larvae.

Now their priorities have changed. They are temporarily done with feeding; now they must swim, for in this phase of their life they must find a suitable place to settle down. They have just a few weeks to do this in, or they will die, so finding a good surface to settle on is crucial. It seems likely that they follow some sort of chemical trail that enables them to locate sites where other barnacles have already settled.

Once the swimming cyprid has found a suitable spot, it produces a temporary cement and attaches itself by its antennae, like a ship mooring. Then, finally, a permanent cement is secreted and the larva undergoes further metamorphosis into a juvenile barnacle, capable of feeding again.

The lowly barnacle seems unassumingly sessile, but that crusty exterior belies an astonishing life story.