California mussel (Mytilus californianus) ~ The California mussel is blue-black with strong radial ribs and irregular growth lines, often with surface eroded or worn. As they mature, they begin to secrete a string-like substance (called a byssal thread) by a gland at the base of the foot. Byssal threads hold the mussels firmly to rocks and other mussels. Once they are matured, the mussels are unable to regenerate their byssal thread.  They eat plankton suspended in the seawater. Aside from humans, their main predators are the ochre sea stars. California mussels are good to eat. In summer months mussels may become poisonous and they should be avoided because concentrations of toxins from red tide. In California, mussels are quarantined between May 1 and October 31.