|Mytilidae, the true mussels, have oblong shells that are usually narrower at the anterior end, with the beaks set slightly off the anterior end. They attach to hard substrates via their byssal threads and can be found in large colonies and among many other colonial organisms. Some, though, bury themselves in soft substrate. Certain species of mussels are a staple in many seafood restaurants. Although there are 20 species found in Texas, only the hooked mussel grows numerous enough to be collected in any number. However, they are generally too small to be valuable commercially. The invasive brown mussel (Perna perna) was once numerous on jetties and other hard substances in south Texas but slowly disappeared, possibly due to people collecting them as food.