Morphology & Physiology of
Mytilus trossulus
  The M. trossulus specimen shown here (left) is approximately 8 cm in length.  The species can reach lengths of 13 cm but is usually only 7- 10 cm in length.  The interior of the shell is a dull blue. The outer shell (view 1, view 2, view 3) is blue and black. The pointed anterior end is easily distinguishable from the broadly rounded posterior end. This unbalanced form, caused by the reduction of the anterior end, is thought to be an adaptation that allows the posterior end to be elevated and unobstructed by surrounding mussels.  The hinge ligament has three small teeth to aid in reduction of lateral valve movement. The mantle, shown in the feeding mussel on the index page, has three layers (folds). The inner most layer secretes the calcareous shell. The mussel absorbs the calcium it uses to lay down the shell directly from the seawater. 

Feeding and Digestion
M. trossulus feeds continuously when submerged in water. Even at near freezing temperatures, the mussel filters detritus and other particles from the water. Mucus and cilia on a filibranch gills capture particles as small as 4-5 µm. The particles are corralled into a food groove and transported to the labial palps. The particles are sorted and non-food particles may be rejected. As food enter the digestive tract, it is pulled into the stomach by a crystalline style which rotates on a gastric shield.  Carbohydrates are digested in the stomach (extracellularly) with help of carbohydrate splitting enzymes from the crystalline style. Digestion of proteins and fats are done intracellularly via the digestive gland.

Byssal Threads
M. trossulus uses byssal threads to anchor itself to its substrate. The threads are made of tough collagen-like proteins secreted by a gland in the base of the foot and in a groove that runs from the byssal gland to the tip of the foot. M. trossulus forms these threads by opening its shell and extending the foot to the substrate. The liquid secretion fills the groove from the foot to the substrate. In a few minutes, the thread hardens in the groove by a tanning process. The foot can then be withdrawn exposing a new byssal thread.  Mussels often produce more byssal threads at night as opposed to daytime. Thread formation is also a function of how crowded the mussel is and the temperature and salinity of the water.
 

Back to M. trossulus index page

Leah Behrends, 2002.