Balanus glandula Darwin 1854

Balanus glandula is an acorn barnacle, occurring in the high-mid intertidal of much of the rocky coastline of western North America, where it can be one of the most abundant solitary animals, reaching densities of more than 70,000 individuals per square meter (Morris et al., 1980; Ricketts et al., 1985).

“The sinuous line of contact between the terga and scuta distinguish this species from most other intertidal barnacles. Semibalanus balanoides has a sinuous line of contact but also has no centripetal ridges at the interior base of the shell. Chthamalus dalli has a straight, crosslike junction between the terga and scuta and the rostrum is overlapped by the rostrolaterals.” (Cowles et al., 2002)

Aleutian Islands south to Bahía de San Quintín in Baja California (Morris et al., 1980).

B. glandula has recently established in Japan and Argentine, likely introduced via shipping ballast water (larvae) or hull-fouling (adults). In Japan it competes with B. albicostatus (Kado, 2003).

B. glandula occurs in the high and mid-intertidal, attached to rock or other hard surfaces -- artificial (pilings, floats) or biological (other organisms). They’re common in the exposed outer coast, as well as in bays and estuaries. (Morris et al., 1980)

Mothers brood eggs, which are released into the water as nauplius larvae. These planktonic larvae undergo five molts to become a non-feeding cypris with six pairs of legs. Cyprids attach to suitable substrate (avoiding potential predators or competitors) and metamorphose into the adult form. They reach adult size in about two years. The entire lifespan is about 10 years. (Cowles et al., 2002)

Balanus glandula is similar in size and general shell morphology to Balanus crenatus (Morris et al., 1980).

On juveniles & adults: include Nucella lamellosa (snail); Lottia digitalis (limpet); Pisaster ochraceus, Pycnopodia helianthoides, Evasterias troschelli, Leptasterias hexactis (sea stars); Emplectonema gracile (nemertian worm); Onchidoris bilamellata (nudibranch), gulls, goldeneye ducks.

On larvae: include juvenile rockfish

(Cowles et al., 2002)

Shell to 22mm diameter (Morris et al., 1980)

Filters food (plankton and edible detritus) from the water during high tide, using it’s cirri (6 pairs of thoracic appendages).

Mainly on intertidal rocks. Open ocean and protected waters. Also common on pilings and on floats.

Biology/Natural History: Often the most abundant barnacle in the upper half of the intertidal zone, and the most nearly ubiquitous barnacle species on the Pacific coast. It can obtain oxygen both from the air and underwater. Barnacles are hermaphrodites which fertilize one another internally by means of a long penis. The eggs are brooded by the parent, and released as nauplius larvae. Up to 6 broods of 1000-30,000 young may be produced per year from this species. After 5 molts the larva becomes a nonfeeding cypris with 6 pairs of legs. The cypris attaches itself to a substrate by an antennal gland and metamorphoses into the adult form. Cyprids avoid rocks which Nucella lamellosa, an important predator, has recently crawled across, and also rocks with the red alga Petrocelis middendorffii. Adult size is reached in 2 years and lifespan is about 10 years, Barnacle molts are frequently seen debris in marine habitats. Predators include oysterdrill snails such as Nucella lamellosa, the ribbed limpet Lottia digitalis, seastars such as Pisaster ochraceous, Pycnopodia helianthoides, Evasterias troschelii, and Leptasterias hexactis, goldeneye ducks, gulls, and even the nemertean worm Emplectonema gracile and the barnacle nudibranch Onchidoris bilamellata. Juvenile rockfish feed on the larvae swimming through kelp beds. Competitors include Semibalanus cariosus and the mussels Mytilus trossulus and M. californiensis, to the shells of which it often attaches. In many areas of the upper intertidal juveniles of this species compete with Chthamalus dalli juveniles by pushing them off the rock. Chthamalus is usually common only in the zone above where Balanus glandula thrives. Less common in estuaries. While it has no eyes, this species is sensitive to light and will rapidly withdraw if a shadow passes over it. Balanus glandula has recently become established in Japan and in Argentina. In Japan it is directly competing with B. albicostatus. The morphology of the feeding legs and penis of this species is highly plastic. Individuals transplanted to high flow areas grew legs which were 50% shorter and 25% wider than those in low flow and their penises were 25% shorter and 50% wider. These differences were not seen between barnacles in high-density vs low-density sites, even though barnacles can only breed with individuals that they can reach with their penis.

A sessile barnacle in which the rostrum overlaps the rostrolateral wall plates on either side of it, the tips of the terga are not drawn out into a beak, the lines of contact between the terga and scuta are sinuous, and the interior of the base of the shell has numerous centripetal ridges. The scuta have a diagonal line of pits. The inner surface of the scuta and terga is black, and especially in small individuals this shows through to the outside as a dark patch in the center of the scutum where the inner surface is excavated into a deep pit and the scutum is thin. The six wall plates are solid, not filled with hollow longitudinal tubes, and do not have fingerlike projecting spines pointing downward. The base is calcified and leaves a calcium deposit on the rock when removed An extremely common intertidal species, especially in the upper half. Diameter to 2.2 cm, but usually 1.5 cm or smaller. The height is usually about equal to the diameter, unless the barnacles are so crowded that they grow very tall and thin. Walls usually white or grayish white, may have longitudinal ribs.

Depth Range: Intertidal, mostly in the upper half

Geographical Range: Aleutian Islands, Alaska to Bahia de San Quintin, Baja California; recently introduced at Puerto de Mar del Plata, Argentina.

How to Distinguish from Similar Species: The sinuous line of contact between the terga and scuta distinguish this species from most other intertidal barnacles. Semibalanus balanoides has a sinuous line of contact but also has no centripetal ridges at the interior base of the shell. Chthamalus dalli has a straight, crosslike junction between the terga and scuta and the rostrum is overlapped by the rostrolaterals.

Balanus glandula (commonly known as the North American acorn barnacle or common acorn barnacle) is one of the most common barnacle species on the Pacific coast of North America, distributed from the U.S. state of Alaska to Bahía de San Quintín near San Quintín, Baja California. They are commonly found in the upper intertidal zone on mussels, rocks and pier pilings. They can obtain oxygen from both water and air.

This acorn barnacle is a moderate-sized one with a diameter of up to 22 mm (0.9 in). The shell is formed by overlapping plates and has a calcareous basis. It has more the shape of a cylinder than the shape of a cone. The white operculum has heavily ridged walls. It can live up to ten years.

It has been intensely studied in recent years as a model species for linking physical oceanography and population genetics (or phylogeography) surveys. This species was introduced to the shores of Argentina in the 1960s, and has become an invasive species, displacing other barnacles and mussels.