Chthamalus dalli Pilsbry, 1916

Common name(s): Brown buckshot barnacle, Little brown barnacle

Synonyms: Chthamalus dalli
Phylum Arthropoda 
Subphylum Crustacea 
Class Maxillopoda 
Subclass Thecostraca 
Superorder Thoracica 
Order Sessilia 
Superfamiy Chthamaloidea 
Family Chthamalidae 
Chthamalus dalli, about 0.5 cm diameter, on a high intertidal rock at Sares Head. Rostrum and tergal plates are at the top right, carina and scutal plates are at bottom left. Note that the rostrolateral plate adjacent to the rostrum overlaps the rostrum.
(Photo by:  Dave Cowles, July 2020)

Description:  This very small acorn barnacle is found almost exclusively in the very high intertidal. It is the only local barnacle in which the plate adjacent to the rostrum overlaps the rostrum (for this reason that plate is called a rostrolateral instead of carinolateral 1). Since the carina is also overlapped by the adjacent plate (the carinolateral plate), both the rostrum and the carina are overlapped by the adjacent plate. The suture between the terga and the scuta is perpendicular to the suture between the two terga and the suture between the two scuta, so that the intersection looks like a cross (see photo above).  Further to the side, however, the suture becomes sinuous (see photo above). Diameter not more than 1 cm, usually 0.5 cm or less. Color often brownish, sometimes gray.

How to Distinguish from Similar Species:  This is the only local acorn barnacle species in which the lateral plates overlap both the rostrum and the carina. Also, its small size and high intertidal location are shared only by Balanus glandulaChthamalus dalli can be distinguished from high-living Balanus glandula because B. glandula's rostrum overlaps the adjacent (carinolateral 1) plate, Balanus glandula often grows to larger than 1 cm diameter,  and because although both species have a sinuous line in the interface between the terga and scuta, the intersection is perpendicular (cross-like) in Chamalus dalli but it is not in Balanus glandula (see diagram below). Another Chamalus species, C. fissus, lives south of San Francisco and must be dissected to distinguish it from C. dalli.

C dalli vs B glandula

Geographical Range:  Unalaska Island, Alaska to San Diego California

Depth Range:  Very high intertidal

Habitat:  Attached to rocks, sometimes to crustaceans

Biology/Natural History:  This species can withstand desiccation better than most other barnacle species, so it is found higher in the intertidal zone than nearly any other barnacle species. However, on our coast Balanus glandula can often be found virtually as high, so the two species may be mixed together at the very highest intertidal zones. At slightly lower intertidal elevations Balanus glandula quickly outcompetes and pushes Chamalus dalli off the rocks. C. dalli may be parasitized by the epicaridean isopod Cryptothir balani, which prevents the individual from reproducing. Since barnacles have internal fertilization in which sperm is transferred by a long penis, individuals living over about 5 cm from another individual of the same species probably cannot reach them to cross-fertilize but may fertilize themselves. However, Wares and Castaneda (2005) reported that this species appears to be panmictic (freely interbreeding) throughout its range, which is not often expected for a species such as this which can only breed with adjacent individuals. Perhaps it has a long larval life so the larvae can disperse broadly up and down the coast.  Unlike Balanus glandula, this species does not crowd together so closely that it grows into a "high rise" form.



Dichotomous Keys:
  Carlton, 2007
  Kozloff, 1987, 1996

General References:
  Kozloff, 1993
  Morris et al., 1980
  Ricketts et al., 1985
  Sept, 1999

Scientific Articles:
Wares, J.P, and A.E. Castaneda, 2005. Geographic range in Chthamalus along the west coast of North America. Journal of the Marine Biological Association of the United Kingdom 85:2 pp. 327-331. doi

Web sites:

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

Authors and Editors of Page:
Dave Cowles (2018):  Created original page
CSS coding for page developed by Jonathan Cowles

Salish Sea Invertebrates web site provided courtesy of Walla Walla University