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
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
to at least 3 cm. When free from the animal the shell has a
groove around the margin and the ends may turn up slightly (see
photo). I do not know why it is called the two-spot
Fissurellidea bimaculata (Dall, 1851)
Common name(s): Two-spot keyhole limpet
|Fissurellidea bimaculata found at
Flattery, WA. The
shell is almost completely exposed. in this view
|(Photo by: Dave Cowles
How to Distinguish from
I know of no similar species in this area. Other keyhole
here do not have a shell which is so small in comparison to the body,
is the aperture equal to about a third of the shell length.
not to mistake it as a dorid
nudibranch (the apex looks much like the anus and gills of a
but is surrounded by a small, mostly hidden shell and is just behind
head instead of on the posterior end of the body as seen in dorids).
Range: Sitka Alaska
to Baja California, especially on the open coast.
Intertidal to shallow
On compound tunicates
or sponges under rocks, or on kelp holdfasts.
History: This limpet
is uncommon. Seems to feed on sponges and compound tunicates,
may also feed on phytoplankton (their stomach contains a crystalline
which is characteristic of plankton feeders such as bivalves). May be
in patches on breakwaters formed of boulders. Predators
ducks. The individual pictured above had a mass of yellowish
1993 (as Megatebennus bimaculatus)
and McConnaughey, 1985 (as Megatebennus bimaculatus)
et al., 1980 (as Megatebennus bimaculatus)
1966 (As Megatebennus bimaculatus)
and O'Clair, 1998
General Notes and
abundances, unusual behaviors:
I have rarely seen this keyhole limpet.
This closeup of the aperture in a living individual shows the
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
is to the left. There is not an obvious groove around the
of this shell but there are a series of
small concentric grooves, including on the margin.
and posterior ends of this shell do not turn up but slope down less
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
Authors and Editors of
Dave Cowles (2007): Created original page
Jonathan Cowles (2007): Updated page with CSS