Distribution and Biology of the Blue Mussel (Mytilus edulis)
The mussel Mytilus edulis is found worldwide mostly in polar and temperate habitats ranging from slightly brackish shallow estuaries to highly saline deep offshore environments. It tends to occur in bays and estuaries that have elevated levels of nutrients from land runoff, causing an increase in phytoplankton. Very often the mussels are found in the rocky shores along the coastlines, bays, and river mouths, where they attach themselves to submerged surfaces.
In the coastal waters of Germany blue mussel are residents of the coastal waters of the North Sea and the western part of the Baltic Sea. In the North Sea blue mussel build large beds in the intertidal areas of the Wadden Sea and occur at the rocky shores around the island of Helgoland. In the Baltic Sea mussels are mainly distributed in the western parts, were waters are more saline. Due to the sweet water influence from river run-off they get rare in the east of the Baltic Sea.
The two shells of the mature mussels are smooth, identical, and pear-shaped. Projecting out from between the shells on one side is a bundle of tough, brown fibres called the byssal threads or byssus. The outside of the shell is usually glossy bluish or bluish black, but sometimes pale brown and rayed (see Fig. 1). The inside of the shell is generally violet in colour (see Fig. 2). The meat of the mussels is of optimum quality for human consumption around March, April, May, October and November when they are not in post-spawning condition (see Fig. 3).
Reproduction and Early Development
Sexes are usually separated (dioecious), but some individual with both sexes (hermaphrodites) may occur in the population. Mussels generally become sexually mature in late spring or early summer, the formation of sperm or eggs (gametogenesis) occurring in numerous ducts located principally in the mantle (see Fig. 4). Spawning generally occurs in the North Sea between May and August, usually in response to environmental triggers such as high food levels, temperature fluctuations, and physical disturbance. Main spawning is observed in May, followed by some smaller larval peaks during summer. While spawning, mussels release some or all of their eggs or sperms in the water and whitish or orange clouds can be seen. Fertilization occurs in the water column and in a very short time the embryos differentiate into free-swimming larvae.
It only takes five hours for the embryo beginning to swim; feeding on small phytoplankton cells and growing to the last larvae stage, the veliger larvae, often referred as spat. Larvae are free swimming for 3 to 4 weeks until their final metamorphosis when they seek to settle. At metamorphosis, the mussel extends the foot, withdraws the vellum, and secrets byssal threads to initially fasten themselves to solid substrate such as collectors, wharf-pilings, filamentous algae, or rocks. This occurrence is generally referred as mussel spatfall. The mussel is now called a juvenile mussel and they can easily detach themselves and change locations, either by using their foot to actively crawl or by floating passively in the water column. If stressed, the mussel can continuously reattach itself to another substrate until it settles permanently. However, movement becomes increasingly limited as it become heavier and when it finally anchors on a suitable hard substrate, it will grow into a mature mussel.
Growth and Feeding Habits
Once permanently set, the competition for space is vigorous. It is common to find mussels growing in layers of other mussels, both naturally in intertidal and sub-littoral levels, and when cultured on lines. The mussels have the ability to grow under a broad range of environmental conditions. They can tolerate a wide range of salinity - between 0ppt to 31ppt - the optimum being around 26ppt. It seems that lower intertidal individuals have more rapid growth than higher intertidal ones because mussels grow most quickly when continually immersed in water where they can almost constantly feed. In the intertidal areas of the North Sea, mussels have to withstand a wide range of temperatures - between sub-zero in winter to 35°C in summer- when mussel beds fall dry during low water. Although the growth rate of mussels is reduced at low temperatures, they can tolerate temperatures below zero and significant growth may still occur in cold waters (between November and April) where food availability is high.
Mussels are efficient suspension feeders. They feed by actively filtering particles from the water, which passes into and out of the mantle cavity through the frilled siphons (see Fig. 4). Breathing also occurs as this stream of water passes over the creature's gill. Phytoplankton cells both living and dead constitutes the main source of food, but other sources of carbon such as decomposed macrophytes or resuspended detritus may also supplement their diet. As stocking density increases, the demand for food eventually exceeds the supply, ultimately resulting in food limitation, which in turn could reduce growth.
In summary, time for development to the mature adult stage depends ultimately on various location specific factors. In general however, mussels in the North Sea take 18-24 months to grow to about the 50 mm-commercial size, pumping around 4 litres of water an hour.
In aquaculture the ability of mussel larvae to attach themself at all kind of hard substrates is used to catch the mussel spat from the water column. Therefore specially designed collectors are deployed where mussels grow for human consumption (see Fig. 5).
Different agents such as predators, parasites, diseases, or even environmental conditions may cause mussel mortality. Summer mussel mortality is more frequent because of high water temperature levels in conjunction with low food levels. As larvae, mussels are preyed on by zooplankton and small fishes. As juvenile and adult, they are preyed on mainly by sea birds, starfish, crabs, and of course, humans.
In recent times another threat for the mussel is identified. In Netherlands and Denmark the pacific oyster (Crassostrea gigas) is invading and overgrowing natural habitats of the blue mussel.
Certain species of natural occurring microscopic algae that bloom under favourable hydrographic conditions can produce biotoxins, such as domoic acid. Filter-feeding molluscs like mussels accumulate the toxins when utilizing toxic algae as a food source. The consumption of toxic mussels by human and even by predators can lead to illness and sometimes death. The toxins do not kill the mussels nor cause any discernible changes in appearance, smell, or taste that would alert mussel consumers of toxicity. As hydrographical conditions become less favourable, the bloom subsides and with time, the mussels rid themselves of the toxins and are once again safe to eat. The rates at which toxins are accumulated and eliminated vary between species. However, experiments on the pumping rates of bivalve molluscs indicate that mussels are the most efficient filter feeders thus exposing them to more risk of biotoxin accumulation.
There are several types of illnesses caused by marine biotoxins that are connected with the consumption of contaminated bivalve shellfish. They include Paralytic Shellfish Poisoning (PSP), Amnesic Shellfish Poisoning (ASP), and Diarrhetic Shellfish Poisoning (DSP). The toxins are named for the most notable symptom they cause, i.e., paralysis, amnesia and diarrhea, respectively. Therefore mussels cultivated in the North Sea for human consumption are carefully analysed by German authorities after the guidelines of the European Union.