Pacific Rocky Intertidal Monitoring: Trends and Synthesis

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Semibalanus (Thatched Barnacle)

Semibalanus cariosus (Pallas 1788)

Phylum Arthropoda, class Maxillopoda, order Sessilia, family Archaeobalanidae

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Description

Sessile barnacle with a diameter up to 60 mm comprised of 6 white wall plates that may be brownish, gray, or greenish. The wall plates are composed of vertical tube-like ribs, which, especially in the lower half, become downward-pointing fingerlike or “thatchlike” projections. 

Habitat and Geographic Range

Attached to rocks, floats, or pilings along exposed shores, mid intertidal to shallow subtidal.  Common in the low intertidal zone, below the densest band of Balanus glandula and near Mytilus californianus; Bering Sea to Morro Bay, Central California; Japan (Morris et al. 1980).

Synonyms

Balanus cariosus

Similar species

Tetraclita rubescens has a similar thatched appearance but it is a pinkish red color and has only 4 plates.

Natural History

Eggs of Semibalanus cariosus are brooded in the winter and the planktonic cyprid larvae settle in the spring (fall and winter on the open Washington coast). The larvae preferentially settle near adult barnacle shells. Lifespan is up to 15 years (Morris et al. 1980). Within the Salish Sea, S. cariosus prefers steep shores with strong currents and waves but on the open coast it is found in deep cracks, overhanging ledges and protected locations (Ricketts et al. 1985). In central California this species grows individually, but in the Pacific Northwest colonies can sometimes be so dense that the thatched appearance is not immediately evident. These barnacles grow very tall and narrow when densely aggregated. These dense patches of Semibalanus can significantly reduce survivorship and recruitment of conspecific as well as other barnacle species through direct predation of cyprids (Navarrete and Wieters 2000). When Semibalanus are small they may be bulldozed off the rocks by grazing limpets such as Lottia digitalis. The large size of adults likely protects them from major predators such as Nucella (whelks) and the ochre seastar, Pisaster ochraceus.

A disturbance study in Kachemak Bay AK, done as part of the Exxon Valdez oil spill assessment, concluded that S. cariosus requires more than 2.5 years to fully recover from scraping disturbance. Semibalanus had not recovered to control levels in cleared plots after the three year study was completed (Highsmith et al. 2001).

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