Fissurellidea bimaculata (Dall, 1851)

Common name(s): Two-spot keyhole limpet

Synonyms: Megatebennus bimaculatus
Phylum Mollusca
Class Gastropoda
Subclass Prosobranchia
Order Archaegastropoda
Suborder Pleurotomariina
Superfamily Fissurelloidea
Family Fissurellidae
Fissurellidea bimaculata found at Cape Flattery, WA.  The shell is almost completely exposed. in this view
(Photo by: Dave Cowles July 2007)
Description:  This keyhole limpet has a large brown, red, orange, or yellow mantle which covers so much of the shell that the animal looks very much like a nudibranch at first.  The body is much larger than the shell, which only covers the area around the dorsal opening (aperture) and may be mostly covered by mantle (photo).   The opening in the small shell is about 1/3 the shell length (photo).  Shell length to 2 cm (usually 1.6 cm or less), and has gray or brownish radiating ridges dorsally, smooth white ventrally.  Animal length to at least 3 cm.  When free from the animal the shell has a shallow groove around the margin and the ends may turn up slightly (see photo).  I do not know why it is called the two-spot keyhole limpet.

How to Distinguish from Similar Species:  I know of no similar species in this area.  Other keyhole limpets here do not have a shell which is so small in comparison to the body, nor is the aperture equal to about a third of the shell length.  Be careful not to mistake it as a dorid nudibranch (the apex looks much like the anus and gills of a dorid but is surrounded by a small, mostly hidden shell and is just behind the head instead of on the posterior end of the body as seen in dorids).

Geographical Range:  Sitka Alaska to Baja California, especially on the open coast.

Depth Range:  Intertidal to shallow subtidal

Habitat:  On compound tunicates or sponges under rocks, or on kelp holdfasts.

Biology/Natural History:  This limpet is uncommon.  Seems to feed on sponges and compound tunicates, and may also feed on phytoplankton (their stomach contains a crystalline style, which is characteristic of plankton feeders such as bivalves). May be common in patches on breakwaters formed of boulders.  Predators include harlequin ducks.  The individual pictured above had a mass of yellowish eggs inside.



Dichotomous Keys:
Smith and Carlton, 1975 (as Megatebennus bimaculatus)


General References:
  Harbo, 1997
  Kozloff, 1993 (as Megatebennus bimaculatus)
  McConnaughey and McConnaughey, 1985 (as Megatebennus bimaculatus)
  Morris et al., 1980  (as Megatebennus bimaculatus)
  Morris, 1966 (As Megatebennus bimaculatus)
  O'Clair and O'Clair, 1998

Scientific Articles:

Web sites:

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

I have rarely seen this keyhole limpet.

This closeup of the aperture in a living individual shows the anus and papillae which could look like retracted gills inside the opening.
The anterior end of the animal is to the left.
In this view the mantle has been almost completely retracted from covering the shell.

Here is the same shell, dissected out of the animal.  Anterior is to the left.  There is not an obvious groove around the margin of this shell but there are a series of
small concentric grooves, including on the margin.  The anterior and posterior ends of this shell do not turn up but slope down less steeply than elsewhere on the shell.

This view of the mouth (left) and foot of the animal show that the mouth and head looks more like that of a typical limpet than of a nudibranch.

Authors and Editors of Page:
Dave Cowles (2007):  Created original page
Jonathan Cowles (2007):  Updated page with CSS