Semibalanus cariosus (Pallas, 1788)

Common name(s): Thatched barnacle, Rock barnacle, Horse barnacle

Synonyms:  Balanus cariosus
Phylum Arthropoda
Subphylum Crustacea
 Class Maxillipoda
    Subclass Thecostraca
     Infraclass Cirripedia
      Superorder Thoracica
       Order Sessilia
        Suborder Balanomorpha
         Superfamily Balanoidea
          Family Archaeobalanidae
Semibalanus cariosus from a rock near Lopez Island.  Diameter about 1.5 cm.
(Photo by: Dave Cowles, July 2006)
Description:  This sessile barnacle has 6 wall plates.  The rostrum overlaps the wall plates on both sides of it.  The tips of the terga form a slight beak when closed.  The wall plates are composed of vertical tubelike ribs which, especially in the lower half, become downward-pointing fingerlike or thatchlike projections.  The base is not heavily calcified so that, when the barnacle is broken off the rock, the base and some soft tissue remain attached to the rock.  Wall plates white, brownish, gray, or greenish white.  The cirri are almost black.  Up to 6 cm diameter.

How to Distinguish from Similar Species:  This is the only species locally that has the strong thatchlike external appearance.  A bit farther south, Tetraclita rubescens has a similar appearance but it is a pinkish red color and has only 4 plates.

Geographical Range:  Bering Sea to Morro Bay, Central California; Japan

Depth Range:  Mid intertidal to shallow subtidal.  Especially common (even dominant) in the low intertidal, below the densest band of Balanus glandula and near Mytilus trossulus or M. californianus.

Habitat:  Attached to rocks, floats, or pilings.  Not often found around fresh water.  Especially common on steep shores with much current and waves in our area but on the open coast it is found in cracks and protected locations.

Biology/Natural History:  Farther south this species grows individually, but here in the Pacific Northwest aggregations can sometimes be so dense that the thatched appearance is not immediately evident.  The barnacles grow very tall and narrow when densely aggregated.  Competitors for space include Halichondria panicea (crumb of bread sponge), Mytilus trossulus, and Mytilus californianus.  When the barnacles are small they may be bulldozed off the rocks by grazing limpets such as Lottia digitalis.  The large size of adults likely protect them from some predators such as Nucella lamellosa or the seastars Pisaster ochraceous and Pycnopodia helianthoides.  Eggs are brooded in the winter and released as nauplius larvae.  The parent barnacle releases a pheromone which stimulates the larvae to hatch (Clare, 1999).  The nauplii go through several molts, culminating with a cypris.  The cyprid larvae settle in the spring (fall and winter on the open Washington coast).  The larvae preferentially settle near adult barnacle shells.  Lifespan up to 15 years.

These barnacles appear to have been eaten by native tribes in SE Alaska in some coastal locations during an extended time period.  At other times, mussels were a more common food in the same regions.

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Dichotomous Keys:
  Flora and Fairbanks (as Balanus cariosus)
  Kozloff 1987, 1996

General References:
  Carefoot, 1977 (as Balanus cariosus)
  Harbo,  1999
  Johnson and Snook, 1955 (as Balanus cariosus)
  Kozloff, 1993
  McConnaughey and McConnaughey, 1985
  Morris et al., 1980
  O'Clair and O'Clair, 1998
  Ricketts et al., 1985
  Sept, 1999

Scientific Articles:
  Clare, A.S., 1999.  Signal transduction of barnacle egg-hatching pheromone: pharmacological assays indicate a comparatively simple mechansim of eicosanoid action.  Journal of Chemical Ecology 25: pp 673-685
  Moss, Madonna L. and Jon M. Erlandson, 2010. Diversity in North Pacific shellfish assemblages: the barnacles of Kit-n Kaboodle Cave, Alaska. Journal of Archaeological Science 37: pp 3359-3369
  Starr, M., J.H. Himmelman, and J.C. Therriault, 1991.  Coupling of nauplii release in barnacles with phytoplankton blooms: a parallel strategy to that of spawning in urchins and mussels.  Journal of Plankton Research 13: pp 561-571

Web sites:

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

These individuals, on a vertical rock face, are abut 2 cm diameter.  Photo by Dave Cowles July 2007

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