Horse Mussel

Scientific Name:Modiolus modiolus

Phylum: Mollusca

Class: Bivalva

Description: Horse mussel is large boreal mussel with sub-terminal beaks. Periostracum (surface layer of shell) is shaggy, flaky, brownish or bluish-black. Shell is whitish or tinged with mauve. Approximately they can reach 150mm in size.

Distribution: Chiefly sub-tidal in deeper waters.

Locomotion: Sessile – attach themselves with byssal threads to any firm support available.

Food gathering/digestion: Filter feeders – will ingest plankton and non-living organic matter. Water is driven from inhalant to exhalent parts of mantle cavity by cilia borne on gills. Feeding material is sorted by size and rejected particles fall off gill edges into mantle cavity as pseudo-feces.

Gas Exchange: Mantle cavity allows ctenidia to develop a greatly enlarged surface area, which serves as both gas exchange and feeding.

Reproduction: Sexual – external fertilization from May to August in response to environment triggers (increased temperature and food). Larvae hatch and are planktonic for the first 3-4 weeks before settling for a benthic existence. Once attached, they metamorphose into juveniles.

Interesting Facts: Horse Mussel are seen mainly as beach shell and considered inedible.


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