EXPERT TOPIC BLUE MUSSEL
INTRODUCTION
BLUE MUSSEL
EXPERT TOPIC
Mussels are the generic name for bivalve molluscs found in fresh water and salt-water habitats. They can tolerate living in a wide variety of microhabitats, everything from tidal areas to fully submerged zones, and can thrive in a wide range of water temperatures and salinities. Their hard oval shell is divided into two halves, or valves (hence the term “bivalve”) which can split down the middle to reveal a soft orange fleshy body inside. Mussels have two sets of gills, one in each shell half. The shells support the internal structures and form a harddefensive shield against predators. A muscular foot protrudes from between the shell halves which aids the mussel in moving, burrowing, or in anchoring the animal to the substrate. The shells are typically grey to black in colour but can sometimes have a bluish or even purple tint. Most mussels are a maximum of 4-5 cm in length, but they can grow to over 10cm in length. Mussels use their byssal threads (often referred to as their ‘beard’) to attach themselves to rocks or man-made structures such as groynes (a wall or jetty), often located between the high and low water marks. Mussels usually cluster together in large groups (mussel beds), which can consist of hundreds or even thousands of individual mussels. Mussels are filter feeders that draw in seawater and filter it to consume the plankton and other tiny sea creatures floating free in suspension. Breathing also occurs as this water is passed
by Vaughn Entwistle, Features Editor, International Aquafeed across the gills. The common blue mussel (Mytilus edulis) is highly fecund and has a life cycle divided into several distinct stages. Mussels reproduce in much the same way as they feed—using the currents of the water around them. Blue mussels, as with all mussels, reproduce sexually. There are separate sexes (male and female), which release gametes (eggs and sperms) directly into the water. In spring, each female mussel ejects about five to 12 million eggs into the water. At the same time, male mussels release sperm. After fertilisation, the fertilised egg begins to develop into a freefloating larval form of the mussel known as a trochophore. These larval mussels float around in the ocean and may be transported several hundred kilometres by sea currents, as their major body parts and systems develop. This free-floating larval form usually exists for a typical duration of three to four weeks, at which point it reaches what is known as the settlement stage Eventually, these floating larvae mature and attach themselves to a suitable growing surface, hopefully near other mussels. This could be a coastal rock, a pier, or as in the aquaculture industry a collector line. It takes a new generation of mussels between two to three years before they reach sexual maturity. The major spawning periods take place during the summer, although a major spawning may take place anytime from late spring to late summer. During their spawning season, the mussels lose about one half of their meat weight. Although it is not fully understood what triggers mussels to begin spawning, we do know that water temperature and salinity plays an important role in determining the best time to spawn.
38 | August 2018 - International Aquafeed