Baggy Mussel, also known as the Bearded Mussel or Hairy Mussel
Phylum: Mollusca, Class: Bivalvia, Order: Mytilida, Family: Mytilidae, Genus: Septifer, Species: Septifer bilocularis · Mytilidae (Mussel family) · Bivalve (clams/two-shelled); this specimen is a single valve (half-shell) with a sub-trigonal/wedge shape.
Species
Septifer bilocularis (Confidence: 90%)
Shell Type
Bivalve (clams/two-shelled); this specimen is a single valve (half-shell) with a sub-trigonal/wedge shape.
Family
Mytilidae (Mussel family)
Size
Approximately 20-30mm in length. This is typical for the species, which generally averages 25-40mm as adults.
Color & Pattern
Exterior shows variegated patterns of dark brown, olive-green, and creamy tan with fine radiating ribs. The interior usually possesses a dull greenish-white or bluish-grey nacre. The pattern is natural, showing some minor beach erosion.
Rarity
Common; frequently found on beaches near rocky outcrops or reef systems in tropical Australia.
Habitat
Intertidal and shallow subtidal zones. They prefer rocky substrates, crevices, or attached to coral rubble in reef environments.
Geographic Range
Widely distributed throughout the Indo-West Pacific, from South Africa to Japan and very common along the Queensland coast (including Brighton Beach/Moreton Bay).
Description
A sturdy, wedge-shaped bivalve characterized by dense, fine radiating ribs and concentric growth lines that create a somewhat reticulated or 'checkered' appearance. The shell tapers to a pointed beak (umbo) and has a distinct internal shelf or 'septum' near the hinge.
Key Features
The most diagnostic feature is the internal septum (a small horizontal shelf) located at the pointed end of the shell. It also features fine radial sculpture and a crenulated (toothed) inner margin.
Collector Value
Minimal financial value (typically $1-$5 USD for gem specimens), but high educational value for regional biological surveys. It is a classic representative of the Indo-Pacific intertidal fauna.
Condition Notes
The specimen is a single valve in 'Good' condition. It shows light beach wear (rounding of the ribs) and lacks the fibrous 'beard' (periostracum) usually found on living specimens. No visible predator bore holes.
Interesting Facts
The genus name 'Septifer' refers to the unique internal shelf ('septum') found inside the beak. These mussels use strong byssal threads to anchor themselves so firmly to rocks that they can withstand heavy surf and wave action.
Ecological Role
A suspension feeder that filters plankton from the water column. They provide micro-habitats for small invertebrates within the clusters they form on rocks. They are currently not listed as a conservation concern.
Similar Species
Modiolus species (Horse mussels) are similar but lack the internal septum. Brachidontes species are also similar but usually more elongated and darker.
Beach Finding Tips
Look in tide pools and among shell grit accumulated near rocky headlands or pier pilings at low tide. They are often found washed up after storms that dislodge them from reefs.
Notes
Brighton beach QLD