There's an old saying that you should only eat mussels in months containing an “r” because, like other shellfish, they spawn as waters warm-up during the summer months and so quality declines as they expend energy on reproduction rather than their own well being. Producers tend to harvest from different geographic areas so that they have a constant supply throughout the year but, if you want to enjoy local mussels, then they will be at their best outside the summer months.
Sometimes described as "poor man's shellfish", mussels are abundant and extremely good value for money. In the wild, they grow for up to 50 years on coastline rocks. However, the majority sold in the UK are rope-farmed and harvested at about 18 months. Due to their availability, farmed mussels are some of the most sustainable seafood you can buy.
As with most shellfish, they should be checked to ensure they are still alive before cooking; once dead, enzymes quickly break down the meat and rapidly make it inedible. Live mussels should close when tapped on a hard surface and unresponsive mussels should be discarded – it is better to be over-cautious if in doubt. A thorough rinse and removal of the "beard" (the tough fibre used by the mussels to attach themselves to rocks) is normally the only other preparation required.
In terms of their eating, mussel flesh is sex-specific. Bright orange mussels are female and, some argue, tastier than their lighter coloured male counterparts. Either way, they take just a few minutes to cook and, in many ways, are the ultimate fast-food.
For a delicious introduction to mussels, try my recipe for moules marinière.