Balanus glandula Darwin, 1854

Common name(s): Acorn barnacle, white buckshot barnacle (when small)

Phylum Arthropoda
 Subphylum Crustacea
  Class Maxillopoda
   Subclass Thecostraca
    Infraclass Cirripedia
     Superorder Thoracica
      Order Sessilia
       Suborder Balanomorpha
        Superfamily Balanoidea
         Family Balanidae
Balanus glandula attached to a cobble at Ala Spit, Whidbey Island.  Diameter approximately 1.5-2 cm
(Photo by: Dave Cowles, September 2005)
Description:  A sessile (acorn) barnacle in which the rostrum overlaps the adjacent wall plates (called carinolaterals in Balanus species) 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 (Photo).  The scuta have a diagonal line of pits. The inner surfaces of the scuta and terga are black, and especially in small individuals this shows through to the outside as a dark patch near 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 (except in some young individuals), and do not have fingerlike projecting spines pointing downward.  The base is calcified and leaves a calcium deposit on the rock when removed (photo)  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 (photo).  Walls usually white or grayish white, may have longitudinal ribs (which are often eroded away except in small individuals).

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. The dark patch visible through the scutum is also more prominent in B. glandulaChthamalus dalli has a straight, crosslike junction between the terga and scuta and the rostrum is overlapped by the adjacent wall plates, which are rostrolaterals.

Geographical Range:  Aleutian Islands, Alaska to Bahia de San Quintin, Baja California (most common north of San Francisco); recently introduced at Puerto de Mar del Plata, Argentina.

Depth Range:  Intertidal, mostly in the upper half

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

Biology/Natural History:  Often the most abundant barnacles in the upper half of the intertidal zone (can reach densities of up to 70,000 per square meter), and the most nearly ubiquitous barnacle species on the Pacific coast.  It can obtain oxygen both from the air and underwater.  However, at warm temperatures in air the species experiences lowered aerobic metabolism and the production of lactate, especially in northern populations (Rangaswami et al., 2020). Barnacles are hermaphrodites which fertilize one another internally by means of a long penis.  The eggs are brooded by the parent, and released as a nonfeeding nauplius larvae.  Up to 6 broods of 1000-30,000 young may be produced per year from this species.  After 5 molts and feeding, 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 oysterd rill whelk snails such as Nucella lamellosa and Nucella ostrina, the ribbed limpet Lottia digitalis (bulldozes and feeds on juveniles; barnacles 6.7 mm diameter or greater have a refuge in size), the Oregon cancer crab Glebocarcinus oregonensis, 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. californianus, 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 as an adult, this species is sensitive to light and will rapidly withdraw if a shadow passes over it.

Balanus glandula has become established in Japan and in Argentina.  In Japan it is directly competing with B. albicostatus. Kerckhof et al. (2018) discovered it on the coast of Belgium in Europe.

The morphology of the feeding legs and penis of this species is highly plastic (Neufeld and Rankine, 2012).  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 student report (A. H. Wang) cited in Branscomb et al., (2014) states that barnacles of this species can fertilize each other up to a distance of approximately 6 cm.

In the San Juan Island area, this species becomes sexually mature at a diameter of approximately 10-15 mm.  Adults hatch at least about 2.5 to 3 broods per year (range 0-6), although broods which were released immediately upon maturity may have been missed (Branscomb et al., 2014).  The eggs are brooded by the parent in individual envelopes, aggregated into masses called lamellae, each of which has its own lamellar envelope.  These lamellae are held within the parent's 'mantle cavity' enclosed by the external plates.  The immature eggs are relatively clear (or yellow or tan) but the nauplii, the stage at which the larvae are released, have eyes and are dark-colored (brown), allowing the maturity of the offspring to be determined by transmitted light if the barnacles are growing on glass plates.  The broods which develop during the winter do so slowly and development stops when the nauplius stage is reached, often in February and March.  These larvae remain within the parent until April, at which time they are released.  Broods carried later in the spring and summer are released more quickly, without a long waiting period after reaching the nauplius stage.  Disturbance of the lamellae at any time after maturity of the nauplii, such as occurs during predation by Glebocarcinus oregonensis, can also trigger rapid release of the larvae.  Some nauplii from yellow or tan lamellae (presumably not fully mature) were red, while the eyes of nauplii in brown lamellae (presumably ready to hatch) were dark red to almost black.  In contrast to crushing by the crab, drilling by the whelk Nucella ostrina did not disturb the lamellae enough to trigger rapid release of nauplii; and those that were released were often trapped by the parent's opercular plates, which remained closed.

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Dichotomous Keys:
  Flora and Fairbanks, 1966
  Kozloff 1987, 1996
  Smith and Carlton, 1975

General References:
  Brandon and Rokop, 1985
  Carefoot, 1977
  Harbo, 1999
  Hinton, 1987
  Johnson and Snook, 1955
  Kozloff, 1993
  Morris et al., 1980
  Niesen, 1994
  Niesen, 1997
  O'Clair and O'Clair, 1998
  Ricketts et al., 1985

Scientific Articles:
Branscomb, E. Sanford, Karen Vedder, and Richard R. Strathmann, 2014. Risks for larvae mediated by plasticity in hatching. Invertebrate Biology 133:2 pp. 158-169

Emlet, Richard B. and Steven S. Sadro, 2006.  Linking stages of life history:  how larval quality translates into juvenile performance for an intertidal barnacle (Balanus glandula).  Integrative and Comparative Biology 46:3 pp 334-346

Gilman, Sarah E., Jennifer W.H. Wong, and Shelly Chen, 2013.  Oxygen consumption in relation to body size, wave exposure, and cirral beat behavior in the barnacle Balanus glandula.  Journal of Crustacean Biology 33:3 pp 317-322

Kado, Ryusuke, 2003.  Invasion of Japanese shores by the NE Pacific barnacle Balanus glandula and its ecological and biogeographical impact.  Marine Ecology Progress Series 249: 199-206

Kerckhof, Francis, Ilse De Mesel, and Steven Degraer, 2018. First European record of the invasive barnacle Balanus glandula Darwin, 1854. BioInvasions Records 7:1 pp 21-31.

Neufeld, Christopher J. and Cassidy Rankine, 2012.  Cuticle and muscle variation underlying phenotypic plasticity in barnacle feeding leg and penis form.  Invertebrate Biology 131:2 pp. 96-109.

Pilsbry, Henry A., 1916. The sessile barnacles (Cirripedia) contained in the collections of the U.S. National Museum, including a monograph of the American Species. Smithsonian Institution United States National Museum Bulletin 93. (available through Google Scholar)

Rangaswami, Xenia L., Gordon T. Ober, and Sarah E. Gilman, 2020. D-lactate production in the acorn barnacle Balanus glandula (Darwin, 1854) (Cirripedia: Balanidae) under emersion stress. Journal of Crustacean Biology 40:6 pp. 739-745.

Web sites:

General Notes and Observations:  Locations, abundances, unusual behaviors:

This group is clustered on a cobble at Ala Spit.  Photo by Dave Cowles, September 2005

When individuals become too crowded, as on this rock, they grow in a tall, thin columnar formation.  Photo by Dave Cowles, September 2005

When individuals are dislodged from the rock they leave a well-developed calcified scar behind.  Note also that the scar has a series of centripetal ridges especially around the margin. These two features, plus the lack of a prominent beak on the tergum and the dark patch caused by the thin spot on the scutum, are useful in distinguishing B. glandula from Semibalanus cariosus, another species common in the Pacific Northwest intertidal. Photo by Dave Cowles, September 2005

Unlike Chthamalus dalli, Balanus glandula frequently grow on other substrates such as the wood of this stump.  Photo by Dave Cowles, September 2005.

Balanus glandula is usually white or gray-white, but these individuals growing on a stump are mostly brownish, probably due to overgrowth by diatoms or other algae.

Authors and Editors of Page:
Dave Cowles (2005):  Created original page