Acmaea mitra 

White-Cap Limpet, Dunce-cap limpet, Bishop's cap limpet, Chinese hat limpet

Synonyms: none
Phylum Mollusca 
 Class Gastropoda
  Subclass Prosobranchia
   Order Patellogastropoda
    Family Acmaeidae
Acmaea mitra shell found at Rosario.  The true shell color (white) shows through, and the shell of this individual is partly eroded by a burrowing species.
Photo by: Ryan Lunsford 2002
Description:  Acmaea mitra is a sea snail, specifically a limpet.  Its body has a soft, muscular foot, which secretes a hard, cone-shaped shell.  This is an entirely white limpet, but it is often encrusted by a coralline red alga.  The shell is thick, taller than other local limpets, and the apex is only slightly anterior to the center.  The surface may be  sculptured with fine concentric growth lines and radial striations.  Length to 3.5 cm, height to 3 cm.  This limpet is easily identified by its color and height.  Note:  Formerly nearly all our intertidal limpets were classified in family Acmaeidae.  Now most of the others have been moved to family Lottiidae, and this is the only species left in family Acmaeidae.

How to Distinguish from Similar Species:  Acmaea mitra is the only all white limpet, and is much taller and conical like a miter or short dunce's cap than are other limpets.

Geographical Range:  Aleutian Islands, Alaska to Baja California.

Depth Range:  Low intertidal and mostly shallow subtidal

Habitat:  A. mitra can be found whereever red coralline algae is present.

Biology/Natural History:  Acmaea mitra is known by its tall shell, which can reach a height of 30 mm and a length of 35 mm.  The scientific name means "pointed cap".  This species eats coralline aglae using its radula.  The teeth of its radula are unique amoung marine invertebrates, as they are capped with goethite, and also with silica (opal).  Goethite is an iron compound that forms a hard crystalline cap around the teeth.  A. mitra is also fairly unique amoung limpets as it does not appear to have any defense response system to predators.  Predators include the seastar Orthasterias koehleri and birds such as black oystercatchers and white-winged scoters.

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Dichotomous Keys:

Flora and Fairbanks, 1966
Kozloff, 1987, 1996
Smith and Carlton, 1975

General References:
Brusca and Brusca, 1978
Harbo, 1997
Harbo, 1999
Kozloff, 1993
McConnaughey and McConnaughey, 1986
Morris et al., 1980.
Niesen, 1994
Niesen, 1997
O'Clair and O'Clair, 1998
Ricketts et al., 1985

Scientific Articles:

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

This individual is crawling on a Nucella lamellosa shell.  It is covered with coralline algae, which is usually pink.  The white color suggests that the coralline algae is dead on this individual.
Photo by Dave Cowles, July 2005


The head, tentacles, and foot of this species are white.  Photo by Dave Cowles, July 2010

An underwater photo of Acmaea mitra by Kirt Onthank, July 2007.  The limpet is encrusted with algae.
The two snails in the background are Calliostoma ligatum.

With coralline algae
Another individual, photographed in the intertidal.  The limpet grazes on coralline algae, but some coralline algae also often overgrow the limpet's shell.  I do not know whether the algal overgrowth comes due to the fact that the limpet often rests within the algal colony or whether the algae spores or gametes settle anew on the shell.  I rarely see anything else but coralline algae growing on the shell, so that may imply that the overgrowth comes from resting within the algal colony.  Note that many of the coralline algae encrusting the rock already seem to have been grazed in this photo.  Photo by Dave Cowles, July 2010.

Authors and Editors of Page:
Ryan Lunsford (2002)
Editors:  Dave Cowles 8-2002, 2005
             Hans Helmstetler 12-2002