Depending on their choice of homestead, this works out pretty well for them; the castles they have built for themselves are roomy, the walls strong and easily locked down in case of enemies. But, opting for security in place of freedom, they have sacrificed choice.* If the area becomes polluted, or if food is scarce, they have to put up with it. If they have chosen a mobile base, such as a crab shell or a ship, they are at the mercy of the whims of their transport, or condemned to its fate, if it dies.
Some of the barnacles in my aquarium were lucky, sort of. They attached themselves to a crab, which died and was cast up on the beach. Many of their companions died, too, exposed to the chill, dry air. But I rescued a few and added them to my nice, nutrient-rich tank. Ahhh! Except that the tank also houses three trophon snails, hungry barnacle eaters. And a few shore crabs, which also appreciate a nibble of barnacle flesh, and even more often, flip the chunks of crab shell upside down, leaving the barnacles out of the current, combing through sand like a snail.
Every day, when I feed my critters, I flip upside-down barnacles over with a chopstick and they begin fanning again.
|Crenate barnacle, feeding, while hermit crab looks on.|
There's another aspect to this problem; finding mates. They can't go in search of one, and the mate is likewise cemented to a rock. Maybe it's close by; barnacles usually live in tightly-packed communities. But often they are separated by the movement of their base, or by predation.
There are a variety of strategies to overcome this; some sessile (non-moving) animals just dump sperm out into the water, in the hopes that it will find a receptive female. (Corals, for example.) Others simply clone themselves; the aggregating anemone is one of these, at least sometimes.
The barnacle has another approach; he has the longest penis for body size of any member of the entire animal kingdom. Depending on the currents, it may reach to 8 (or 10) times his body length. The tip is prehensile; when he finds another barnacle, he inserts it and fertilizes her. Since barnacles are hermaphroditic, with both male and female organs, he need not look further than the closest receptive neighbour.
This is what I saw in the early hours of Christmas Eve; one of the barnacles was looking for a mate. For about 10 minutes, I watched as he extended his penis parallel to the sand, waving it about slowly, exploring. At the farthest reach, he was searching about 4 times his body length away.
|A bad location for a lovelorn barnacle.|
In this photo, the penis is barely visible, as a smooth tube. The feeding cirri are feathered and clustered around the mouth of the shell. There is a clearer photo, and more information, at BogLeech. NewScientist has an interesting article about the effect of current on penis length. Wikipedia contributes information on predators and competition. The life cycle is described on MESA. (And my previously-posted photo is here.)
*This is not a parable; it's the way things work.