This sessile barnacle has
6 wall plates. The rostrum
overlaps the wall plates on both sides of it. The tips of the
form a slight beak when closed. The wall plates are composed
tubelike ribs which, especially in the lower half, become
fingerlike or thatchlike projections. The base is not heavily
so that, when the barnacle is broken off the rock, the base and some
tissue remain attached to the rock. Wall plates white,
gray, or greenish white. The cirri
are almost black. Up to 6 cm diameter.
Semibalanus cariosus (Pallas, 1788)
Common name(s): Thatched barnacle,
Rock barnacle, Horse barnacle
|Semibalanus cariosus from a rock near Lopez
about 1.5 cm.
|(Photo by: Dave Cowles,
How to Distinguish from
This is the only species locally that has the strong thatchlike
appearance. A bit farther south, Tetraclita
a similar appearance but it is a pinkish red color and has only 4
Bering Sea to
Morro Bay, Central California; Japan
Mid intertidal to shallow
subtidal. Especially common (even dominant) in the low
below the densest band of Balanus
and near Mytilus
trossulus or M.
Attached to rocks, floats,
or pilings. Not often found around fresh water.
common on steep shores with much current and waves in our area but on
open coast it is found in cracks and protected locations.
History: Farther south
this species grows individually, but here in the Pacific Northwest
can sometimes be so dense that the thatched appearance is not
evident. The barnacles grow very tall and narrow when densely
Competitors for space include Halichondria panicea
(crumb of bread
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
The larvae preferentially settle near adult barnacle shells.
up to 15 years.
These barnacles appear to have been eaten by native
SE Alaska in some coastal locations during an extended time period.
At other times, mussels were a more common food in the same
and Fairbanks (as Balanus cariosus)
1977 (as Balanus cariosus)
and Snook, 1955 (as Balanus cariosus)
and McConnaughey, 1985
et al., 1980
and O'Clair, 1998
et al., 1985
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
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
General Notes and
Observations: Locations, abundances,
These individuals, on a vertical rock face, are abut 2 cm
Photo by Dave Cowles July 2007
Authors and Editors of Page:
Dave Cowles (2006): Created original page