Modiolus (Modiolus) modiolus (Linnaeus, 1758)

Common name(s): Horse mussel, bearded mussel, fan mussel, giant horse mussel, northern horsemussel

  Volsella modiolus, 
  Volsella modiola, 
  Volsella flabellatus, 
  (Modiolus rectus)
Phylum Mollusca
 Class Bivalvia
  Order Mytiloida
   Family Mytilidae
Modiolus modiolus from 100 m depth, San Juan Channel
(Photo by: Dave Cowles 7-2004)
Description:  This mussel is longer than high, umbones are not quite terminal, attaches by byssal threads to firm substrates, length is about 2-3 times the height, periostracum is usually brown (or black in large individuals) and grown out into long, soft bristles, especially in small individuals and near the margin of the shell.   Up to 15 cm long.

How to Distinguish from Similar Species: M. rectus is about 3x as long as wide and not likely found north of California. M. senshousia is smaller--up to 3 cm long and its periostracum is greenish.

Geographical Range: Atlantic and Pacific oceans, from the Arctic to subtropics

Depth Range: Usually subtidal.  Occasionally intertidal (mostly on the underside of rocks)

Habitat: Muddy bottoms with shell hash.

Biology/Natural History Predators include the sunflower star Pycnopodia helianthoides.  May have a symbiont, the pea crabs Fabia subquadrata or Opisthopus transversus. M. modiolus tends to form aggregates attached to one another and to empty shells.

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Dichotomous Keys:
Flora and Fairbanks, 1966
Kozloff 1987, 1996

General References:
Kozloff, 1993
O'Clair and O'Clair, 1998

Scientific Articles:
Carrington, Emily, Gretchen M. Moeser, Sean B. Thompson, Laura C. Coutts, and Carrie A. Craig, 2008.  Mussel attachment on rocky shores:  the effect of flow on byssus production.  Integrative and Comparative Biology 48:6 pp 801-807

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

The periostracum is shiny, and hairy near the posterior end.  This individual, found at Padilla Bay, also has byssal threads hanging out.

Side (ventral) view of the same individual as above.

Modiolus modiolus

This individual was riding on a float that washed up in Rosario Bay June 2012.  Shell length is 9 cm.  Photo by Dave Cowles, June 2012


This individual (the same one as above, found in Rosario Bay) had an unusual white "pearl" growth inside the hinge (see the growth to the left in the photo).  This is likely due to an unusual irritation inside the shell of this individual and is not normally found in this species.  Photo by Dave Cowles, June 2012

No pearl

This individual, found on the same float as the individual above, had no "pearl" near the hinge.  Shell length 8.8 cm, was a gravid female.  Photo by Dave Cowles, June 2012

Authors and Editors of Page:
Dave Cowles (2005):  Created original page