Onchidella borealis Dall, 1871

Common name(s): Leather limpet, Northwest Onchidella

Synonyms:
Phylum Mollusca
 Class Gastropoda
  Subclass Gymnomorpha
   Order Onchidiacea
    Family Onchidellidae
(In Kozloff's key is a choice in the Opisthobranch key)
Onchidella borealis, 11 mm long, crawls along a blade of sea lettuce.  Found intertidally at Cape Flattery.
(Photo by: Dave Cowles July 2007)
Description:  This small, inconspicuous opisthobranch-like mollusk has a smooth dark dorsum which may be greenish, gray, brown, or black and sometimes has dots or streaks.  It has no cerata.  The foot is cream or tan.  It has no shell. The margins of the dorsum have short blunt tubercles or papillae, often colored lighter than the rest of the dorsum (photo).  The animal has eyes on short stalks (above photo) and has a pneumostome opening to an internal "lung" on the posterior end just dorsal and posterior to the anal opening (photos).  It is NOT closely related to the limpets and has no shell despite the common name.  Length to 1.5 cm.

How to Distinguish from Similar Species: (The systematics and distribution of another species in the genus, O. carpenteri (Binney, 1860), are unclear)

Geographical Range:  Alaska to San Luis Obispo county, California

Depth Range:  Intertidal.  This species spends considerable time above the water, especially in caves.  This may be related to the fact that it has a lung instead of gills.

Habitat:  Crevices, mussel beds, kelp holdfasts such as sea cabbage and Laminaria and red algae such as Odonthalia floccosa.

Biology/Natural History:  Often found in as a group of several.  Graze on diatoms from rocks and algae at low tide.  On mechanical stimulation, the marginal tubercles or papillae produce secretions that repel potential predators such as seastars, especially Leptasterias hexactis, which retreats from an encounter.  Hemigrapsus nudus will eat dead individuals but not live ones.  Lays a gelatinous mass of 6-40 encapsulated eggs.  Hatch as young juveniles.



 
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References:

Dichotomous Keys:
  Kozloff 1987, 1996
  Smith and Carlton, 1975

General References:
  Kozloff, 1993
  Morris et al., 1980
  O'Clair and O'Clair, 1998
  Sept, 1999

Scientific Articles:
Young, Craig M., Paul G. Greenwood, and Cynthia J. Powell, 1986.  The ecological role of defensive secretions in the intertidal pulmonate Onchidella borealis.  Biological Bulletin 171: 391-404.

Web sites:



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

Although Kozloff's key says this species is abundant throughout the region I have not noted it often except on the open coast.  Perhaps this is because it is inconspicuous and usually found in caves, crevices, and holdfasts.  In the summer of 2013, however, it was common along the north shore of Whidbey Island (see below).



The eyes are on stalks or tentacles.  The tubercles or papillae along the lateral margins of the dorsum, which secrete substances noxious to seastars, are also clearly visible.


Posterior view as the animal crawls under a blade of sea lettuce.  Note the wide-open pneumostome and the less obvious anus just ventral (left) of it. Dorsal view.  The animal is defecating from the anus and a bubble is visible in the pneumostome.
These views show the pneumostome opening to the lung, which is located on the posterior end under the dorsal flap of the mantle and just dorsal/posterior to the anus.

Most years I have found this species only occasionally.  However, in the summer of 2013 this species was very abundant on seaweed and rocks at North Beach and other beaches along the south margin of Deception Pass, Whidbey Island, WA just south of our marine station.  The photos below are several taken by my daughter, Joanna Cowles, at North Beach in the summer of 2013.
 
Dorsal view Posterolateral view Anterolateral view



Authors and Editors of Page:
Dave Cowles (2007):  Created original page