I titled this photograph from the location where I took it, Gig Harbor, but on second thought I probably should have titled it ‘Barnacles’ or something similar. There often overlooked except when you’re walking on them barefooted, or you hear your hull scrapping across them which just sends shivers up your spine as you imagine the damage they’re doing to the gelcoat, but they are really quite fascinating creatures. They were the main focus of Charles Darwin’s studies long before, during and after his work on ‘On the Origins of Species’.
Barnacles are encrusters, attaching themselves permanently to a hard substrate. The most common, “acorn barnacles” are sessile, growing their shells directly onto the substrate. The order Pedunculata (“goose barnacles” and others) attach themselves by means of a stalk.
No naturalist has devoted more painstaking attention to the structure of the barnacles than Mr. Darwin. – Richard Owen
Free-living barnacles are attached to the substratum by cement glands that form the base of the first pair of antennae; in effect, the animal is fixed upside down by means of its forehead. In some barnacles, the cement glands are fixed to a long, muscular stalk, but in most they are part of a flat membrane or calcified plate. A ring of plates surrounds the body, homologous with the carapace of other crustaceans. These consist of the rostrum, two lateral plates, two carinolaterals, and a carina. In sessile barnacles, the apex of the ring of plates is covered by an operculum, which may be recessed into the carapace. The plates are held together by various means, depending on species, in some cases being solidly fused.
Inside the carapace, the animal lies on its back, with its limbs projecting upwards. Segmentation is usually indistinct, and the body is more or less evenly divided between the head and thorax, with little, if any, abdomen. Adult barnacles have few appendages on their heads, with only a single, vestigial pair of antennae, attached to the cement gland. The six pairs of thoracic limbs are referred to as “cirri”, which are feathery and very long, being used to filter food from the water and move it towards the mouth.
Barnacles have no true heart, although a sinus close to the oesophagus performs similar function, with blood being pumped through it by a series of muscles. The blood vascular system is minimal. Similarly, they have no gills, absorbing oxygen from the water through their limbs and the inner membrane of their carapaces. The excretory organs of barnacles are maxillary glands.
The main sense of barnacles appears to be touch, with the hairs on the limbs being especially sensitive. The adult also has a single eye, although this is probably only capable of sensing the difference between light and dark. This eye is derived from the primary naupliar eye.
So what does this have to do with Gig Harbor…well, there are a lot of barnacles in the harbor??