Modiolus (Modiolus) modiolus (Linnaeus, 1758)

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

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.
 


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