Barnacles are so strange that they were misidentified as molluscs by taxonomists until 1830. After all, they had a shell and live a sessile life, so surely they were related to snails and clams? Their larvae finally gave them away. Barnacle larvae are free-living in the plankton and have jointed appendages and other arthropod characteristics, which reveal their Crustacean natures. But these are no ordinary prawn or shrimp larvae, destined to grow into active crawling or swimming lifestyles. Instead barnacle larvae leave their wandering ways behind, settle on their heads on some hard surface and turn into wannabe molluscs. The eyes and brain disappear. Shells are formed around the body and the former swimming antennules do the work of tentacles, waving about in the water, picking up microscopic food particles and feeding them to the mouth.
This may seem like a strange life choice but it has been successful for the barnacle clan. Clad in solid plates, adult barnacle are safe from predators and can concentrate on the two things they do best: eating and reproducing. After that, lifesyles are mostly related to substrate choice. Some barnacles settle on inanimate surfaces such as rocks, or on human-made surfaces such as boat hulls and piers. If they can't find a lifeless surface, they settle on other organisms such as oysters, and often smother them, creating the desired lifeless hard substrate that the barnacles need to mature. At this point barnacles develop into one of two basic forms: those with a stalk or peduncle and those without. Stalked barnacles get some advantages in height and maneuverability but may be a little more vulnerable to predation without the harder shells of the stolid, stalkless forms.
Having a stalk is useful for sex also. Barnacles are monoecious, with both sexes are found in a single individual. This enables them to self-fertilize if there are no other barnacles in reach. If there are, (as there usually are because barnacles quickly fill up available hard spaces), then orgies occur when the sexually mature hermaphrodites take turns fertilising each other. Barnacles have penises which they insert into the ovaries of neighbouring barnacles. This internal fertilisation technique is useful because it increases fertilisation rates considerably as well as probably being more fun for the barnacles in what otherwise would be a rather boring existence.
The female half of the barnacle then keeps the eggs safe until they hatch into nauplius larvae. These youngsters, identical to the nauplii of more 'normal' crustaceans such as prawns, have strong swimming appendages and so are suited for dispersal in the plankton. Nauplii are also equipped with an eye and a brain and they are 'positively phototropic', or attracted to light. Since in the ocean, light comes from above, while the depths below are dark, this trophism takes the nauplii up into the food-rich plankton zones and away from their parents, who usually don't have room around them to accomodate their offspring.
As the nauplius swims, feeds and grows, it moults regularly until it becomes a cypris larva, which is remarkably similar in shape to an ostracod. Ostracods resemble bivalve molluscs, having two shells joined at a hinge, but ostracods are also crustaceans because of their jointed legs, nauplius larval stages and other arthropod characteristics. Perhaps this is another clue to barnacle evolution and barnacles are in fact descended from ostracod ancestors.
The cyprid larvae continue to swim with their jointed antennules, feeding on smaller planktonic organisms, growing and moulting. Gradually they reache a size and heaviness where further changes occur. They tend to sink rather than swim, and they lose the desire to swim towards light, becoming negatively phototropic instead. The cypris larva develops adhesive glands and grows its penduncle or stalk from a part of the head near the antennules . At this point it needs a substrate and that is the limiting factor in the barnacle lifestyle. Many larvae must never find what they are looking for, eventually settling in mud and smothering or being eaten before they can finish their quest.
For this reason barnacles have branched out into parasitism to find new substrates. Some are commensal ectoparasites, just hitching a ride on whales, sea turtles and other marine animals, while others have become true parasites, dependent on their hosts for food as well as a hard surface. On whales and other moving hosts, barnacles can become even lazier than usualj. If the larva orients itself well on the body, food just comes naturally by in the currents created by the host so the barnacle doesn't even have to wiggle its antennules much to get a feed.
Humans have inadvertently created a lot of potential barnacle habitat in the form of bridges, piers, docks, wharfs and of course ships and boats. Billions of dollars have been spent on antifouling methods, which are now being linked to significant marine pollution. So barnacles have had a significant economic impact on many human marine endeavours. Nonetheless they are interesting little animals which have developed a highly successful strategy for survival.