Horse Mussel

Modiolus modiolus

This large mussel is usually around 10 centimetres in length, although some individuals have reached up to 22 centimetres. They're roughly oblong or triangular, with a solid, swollen shell. Typically dark blue or purple, but can appear glossy yellow or dark brown due to the periostracum (thin outer skin). Attach themselves to firm substrates using tough fibres known as byssal threads.

Authority

Linnaeus, 1758

Classification Details

Phylum: Mollusca (molluscs); Class: Bivalvia (bivalve molluscs).

Habitat

Found partly buried in soft sediment or attached to rocks and shells. Found on the lower shore down to 280 metres. Distributed in the Atlantic Ocean from the Arctic to the Bay of Biscay. Individuals often clump together and may form extensive beds or reefs.

Diet

Filter feeders. Mussels draw in water through their inhalant siphon. Cilia (hair-like structures) on the gills pump the water through them and particles are extracted. The water then exits through the exhalant siphon. The particles are moved to the mouth by the labial palps. Mussels feed on bacteria, phytoplankton, detritus, and dissolved organic matter.

Reproduction

Adults are male or female. Mussels release eggs and sperm in synchronized spawning events. Fertilization takes place in the seawater. The larvae are planktotrophic (feed on plankton). They stay in the water column for around one month before settling and developing into adults. Adults are often over 25 years old and can live for more than 50 years.

Fun Facts

Horse mussels can form large beds or reefs. Bed densities of up to 158 mussels per square metre have been recorded in the Bay of Fundy, and 400 mussels per square metre elsewhere. Horse mussel beds provide shelter, nurseries for young, and feeding grounds for many species, including commercially important species such as scallops. Bottom trawling can damage beds, and they can take many years to reform.

People have historically eaten horse mussels, but the older ones can be rather chewy. An old Scottish joke says the method to cook a horse mussel is to put a stone in the pot with it. When the stone is soft, it is ready.

References

Dinesen GE and Morton B (2014) Review of the functional morphology, biology and perturbation impacts on the boreal, habitat–forming horse mussel Modiolus modiolus (Bivalvia: Mytilidae: Modiolinae), Marine Biology Research 10:9, 845–870, DOI: 10.1080/17451000.2013.866250.

OSPAR (1999) Background document for Modiolus modiolus beds. OSPAR publication 425/2009.

Tyler-Walters H (2007) Modiolus modiolus Horse mussel. In Tyler–Walters H. and Hiscock K. (eds) Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on–line]. Plymouth: Marine Biological Association of the United Kingdom. Available from: https://www.marlin.ac.uk/species/detail/1532 Accessed online 22 January 2020.

Wildish D J, Fader GBJ and Parrott DR (2009) A model of horse mussel reef formation in the Bay of Fundy based on population growth and geological processes. Atlantic Geology 45, 157 – 170. https://doi.org/10.4138/atlgeol.2009.007