Balanus nubilus Darwin, 1854

Common name(s): Giant barnacle, Giant acorn barnacle, Horse barnacle

Phylum Arthropoda
Subphylum Crustacea
 Class Maxillipoda
    Subclass Thecostraca
     Infraclass Cirripedia
      Superorder Thoracica
       Order Sessilia
        Suborder Balanomorpha
         Superfamily Balanoidea
          Family Balanidae
Balanus nubilus from about 10 m depth near Rosario.  Note how several individuals are attached to each other.
(Photo by: Dave Cowles, July 2006)
Description:  This large, mostly subtidal barnacle has 6 wall plates and the rostrum overlaps the plates on both sides of it.  The tips of the terga are drawn out into a beak (see photo above and below).  The base of the shell is calcified.  The exterior of the barnacle may be heavily eroded (these individuals may be little eroded because they were almost completely overgrown by a sponge).  Young individuals may have low ridges on the plates but the ridges are usually eroded away in older individuals.  The scuta have no longitudinal striations.  Often 5 or more cm in diameter, and may exceed 8 cm (up to 15 cm?).  This species also shows bright yellow or orange color when it opens up (photo).

How to Distinguish from Similar Species:  This is the largest barnacle likely to be encountered in this area, and perhaps in the world.  Most other species here are less than 5 cm diameter.  Of barnacles with beaklike terga, Semibalanus cariosus has thatchlike projections on the wall plates (unless eroded).  The subtidal Balanus balanus and B. rostratus are rarely greater than 3 cm diameter and the walls of large individuals are much less eroded. B. balanus has septate tubes in the wall plates and B. rostratus has shiny overlaps between the plates.  In California it can be distinguished from the large Balanus aquila because it has beaked terga and no longitudinal striations on its scuta.

Geographical Range:  Southern Alaska to San Qintin, Baja California, Mexico.

Depth Range:  Low intertidal to 90 m

Habitat:  Mostly subtidal rocks; some very low intertidal or may be found on pilings.  Common in areas of current or waves.

Biology/Natural History:  The plates of this species may be heavily eroded by the boring sponge Cliona.  Often grow on one another, forming clusters (druses).  Barnacles are hermaphroditic, and fertilization is internal.  They brood their eggs for several weeks before releasing them as nauplii.  The nauplius molts to a cypris, which feeds in the plankton then settles and metamorphoses into an adult.  Predators of this species include Pisaster ochraceous.  The empty shells are refuges for Cancer oregonensis and Octopus rubescens.  This species contains very large muscle fibers which have been used in the study of muscles.

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

General References:
  Gotshall, 1994
  Gotshall and Laurent, 1979
  Harbo, 1999
  Kozloff, 1993
  McConnaughey and McConnaughey, 1985
  Niesen, 1997
  O'Clair and O'Clair, 1998
  Ricketts et al., 1985
  Sept, 1999

Scientific Articles:

Web sites:

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

This species is very common subtidally around Northwest Island, and also common on Sares Head.

The beaked terga (one of them in this individual is broken) and the orange-yellow orifice in a partly opened animal are visible here.

Here is a photo of an unbroken individual.  Since the movable plates are entirely closed the yellow tissue inside cannot be seen.

Balanus nubilus feeding.  Underwater photo by Kirt Onthank July 2007.  The snail to the left is Calliostoma ligatum.

Authors and Editors of Page:
Dave Cowles (2006):  Created original page