Common Acorn Barnacles on Call
Semibalanus balanoides, more commonly known as the common acorn barnacle or the northern acorn barnacle, is a widespread species of barnacle found all along the Pacific and Atlantic coasts of North America and the intertidal zone of Europe-- though it is notably absent from the Bay of Biscay. Its range is limited by the temperature of the water in which it breeds; in the winter, they require water as cold as 7.2 °C (45.0 °F), but throughout the rest of the year they can tolerate much warmer climates.
Like many barnacle species, S. balanoides is hermaphroditic, meaning it has both male and female reproductive organs. Once the water reaches the optimal temperature, typically in November or December, the barnacles begin to breed. An individual will reach out with a penis that can be up to 7.5 centimetres (3.0 in), and inseminate a nearby barnacle; this process may be repeated up to eight times between a single pair. After insemination, the penis disintegrates to be regrown the following year.
In the meantime, the fertilized barnacle will produce up to 10,000 eggs that are stored in a special sac within the shell cavity. The common acorn barnacle releases its eggs between February and April, in concert with warming waters and the spring algal bloom. The eggs then hatch into miniscule larvae, which spend several weeks feeding on plankton in the water column and going through a series of molts. Following the sixth molt, the larvae stops feeding and seeks out a suitable place on which to anchor itself. Once it has found a surface, the larval barnacle 'crawls' along the surface until it encounters one of its own species. At this point, they will attach themselves with a cement-like glue which is capable of supporting up to 3,200 kg (7,000 lbs) Here, a single northern acorn barnacle may live anywhere from one to seven years.
Adult S. balanoides are identified by the white, six-sided limestone casing with which they protect themselves. However, one unique feature of this species is the way in which they attach themselves to their chosen surfaces; unlike other barnacles, which have a hard calcified shell base, the northern acorn barnacle has a membranous base. Within their shells, they have rather soft bodies, including a set of feathered appendages called cirri with which they use to breathe and collect food.
The common acorn barnacle is a filter feeder, consuming microscopic plankton and bits of detritus from the water column. Though their hard shells deter most predators, adults are vulnerable to other predatory invertebrates like dogwelks, nudibranches, and sea stars, as well as fish like the shanny that are specialist feeders of barnacles.
Conservation status: The common acorn barnacle has not been assessed by the IUCN. The species is extremely common and very resilient, and in fact has been in existence since the Jurassic period. However, rising ocean temperatures may negatively affect their breeding period.
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Photos
Neil DeMaster
Oregon State Parks and Recreation
Kim Hanson via Wikimedia commons
