With brusque candour, Louis Agassiz described barnacles as "shrimp-like animals that stand on their heads in limestone houses kicking food into their mouths". Inspired by the older naturalist, Charles Darwin delved into cirripedes as a bridge of formality between his journey on the Beagle and the long fruition of a theory that changed the game of thrones in the natural world. Barnacles were long thought to be molluscs until ontogenic studies revealed the transformation of a sessile filter-feeder from egg to cyclopian nauplius and penultimately, to a cyprid stage that resembles an fat-filled ostracod but does not feed. Before its reserves run out, the 'cypris' must find a suitable substrate on which to stick its front end. Attaching themselves to rock, wood, coral and sponge, the young barnacles erect forts of calcite, miniature mountains of armoured plates with apertures protected by pairs of opercular valves.
One group has undergone reductio in extremis, becoming essentially bundles of nerves and nads nourished by a network of roots that suck the lifeblood (and libido) of the fellow crustaceans they inhabit. Others settle on flotsam and ships, whales and turtles, or shells alive and dead. But the barnacles most familiar to people are those that merely infest the foreshore and threaten the bare feet of beachcombers in this zone of eternal contention between the sea and the soil. Here, the young cirripedes land on a vertical arena of conflict and competition in which life and dearth are determined by how high or low you are in the pecking order of the waves. There's safety in the heights but fewer chances to feed. Thus, the upper limits of their class are occupied by hardy little chthalamids, while the larger balanids and volcano barnacles dominate the wetter parts and sheltered boulders. There are also tiny cirripedes living in close-fitting valves on benthic debris or rubble, which never cease to grasp at straws of nourishment from their shallow but seldom-raised beds.
In the lower reaches, the rewards of regular inundation come with the risks of displacement or smothering by other settlers – mussels, algae, limpets as well as fellow barnacles that force each other to grow upwards instead outwards, thus forming dense hummocks with thin grips and perilous holds prone to mutually assured destruction should an individual get dislodged. Drills, melongenas, sea stars and crabs take their toll, leaving cavities that provide homes for blennies, snails, shrimp and even littoral spiders. Tiny isopods, too, dive into the hollows of dead cirripedes on the pilings of Sembawang Beach, where the splash zone is a battlefield between the stationary crustaceans and sacs of stinging cells. Their mature kin merely use restless appendages to molest the opercula of water retentive plates, causing the animals within to shift slightly in place, as if in anticipation of an existential assault on their state of being. Neither care, though, that they last met in deep time when forms strange and bodies fantastic floated through ancient seas and toyed with modes of life that sometimes led to the loss of meaningless segments and a headlong descent into hardened ends.