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The Blue mussel

– irreplaceable filter-feeder

and geneticist’s favourite

Kamila Sfugier

Summary:

Mussels from Mytilus spp. complex are important in aquatic ecosystems as well as their worldwide economic importance. Annual world production of marine mussels for consumption is around one million tons and in Eu-rope exceeds 600.000 tons. These bivalves contain nutri-tious proteins, carbohydrates, mineral salts and a small amount of fat, but apart from cooks they fascinate sci-entists. The sensitivity of mussels to environmental pol-lution allows their exploitation as bioindicators. Addi-tionally their inheritance of mitochondrial DNA is quite extraordinary. This article aims to present the blue mussel in the light of its ecology and genetics.

Key words: blue mussel, doubly uniparental inheritance,

hy-bridisation, masculinization

Biological and ecological characteristics

The Mytilus edulis spp. complex includes the three taxa: Mytilus edulis, Linnaeus, 1758; Mytilus gallopro-vincialis, Lamarck, 1819; Mytilus trossulus, Gould, 1850. All these species are widely distributed and hybridise within areas where their habitats overlap (McDonald et al., 1991).

Size, shape and colour of blue mussels depend on their habitat. Growth rate is influenced largely by water temperature, sa-linity, quality and avail-ability of food. The ideal temperature for growth varies between 10 and 200C, while temperatures above 270C are consid-ered lethal. Blue mus-sels have a  cosmopolitan distribution, inhabiting water bodies of salinity

between 0 ppm and 31 ppm. Their growth rate, how-ever, significantly decreases in salinity below 12.8 ppm. This bivalve attains an average length of 3 to 5 cm, but in deeper water forms larger shells of about 9 cm.

The outer part of the blue mussel shell is often dark blue, blackish or brown. The inner part is silvery and slightly pearly. Blue mussels grow a shell consisting of two valves that are opened by two dorsal muscles and closed by sphincters (Jura, 2004).

Blue mussels are gonochoric, but it is only possible to identify their gender during the breeding season. In the Atlantic Ocean breeding takes place from mid-May to the end of September. Duration is dependent on nu-merous factors, such as food, water temperature and physical processes in the water column. In stagnant wa-ter it is also possible to find hermaphroditic blue mus-sels (Saavedra, 1997).

The breeding strategy of blue mussels is a combina-tion of three features, i.e., relative fecundity (Bayne et al., 1983; Sprung, 1983), comparatively high mortality of larvae (Yap, 1977) and plankton dispersal (Crisp, 1975). Gametes are discharged once or several times directly into the water, where fertilisation occurs.

In-coherence with the Curriculum – see. p. 10

mgr Kamila Sfugier: Institute of Oceanology of the Polish

Academy of Sciences, Genetics and Marine Biotechnology Department, ksfugier@iopan.gda.pl

received: 16.01.2015; accepted: 12.02.2015; published: 27.03.2015

Scientific classification Kingdom: Animalia Phylum: Mollusca Class: Bivalvia Subclass: Pteriomorphia Order: Mytiloida Family: Mytilidae Subfamily: Mytilinae Genus: Mytilus Species:

Mytilus edulis (Linnaeus, 1758) Mytilus trossulus (Gould, 1850) Mytilus galloprovincialis (Lamarck, 1819)

Figure 1. Shape of the shell: Mytilus galloprovincialis, Lamarck, 1819; Mytilus trossulus, Gould, 1850; Mytilus edulis, Linnaeus, 1758

Source: http://naturalhistory.museumwales.ac.uk/britishbivalves The Author – Kamila Sfugier – is a participant of the project

„Stypendia dla doktorantów województwa podlaskiego”, co-financed within the Operational Programme Human Capital,

measure 8.2 Transfer of knowledge, sub-measure 8.2.2 Regional Innovation Strategies, from the European Social Fund, state budget and Podlaskie Voivodship budget.

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dividual development stages of Mytilus have been de-scribed by Field (1922) and Bayne (1976) and have been divided into three separate phases (Sprung, 1984). The first larval stage of Mytilus a trochophore is formed – characterized by the presence of cilia. A larva reaches a length of up to 120 µm and has a D-shaped shell. Dur-ing the growth phase the larva feeds and increases in size. It loses its D-shape with a velum in its prostomium, functioning as a swim organ. In the setting phase, the planktonic larva is distinguished by the germ of the foot, head, mantle and mantle cavity. In the pediveliger stage it has a shell, 360 µm in length. Throughout the various larval stages, the veliger struggles to settle on a firm foundation in order to transform into the ulti-mate juvenile form (Flyachinskaya and Kulakowski, 1991; Sprung, 1984).

Larvae, capable of floating in the water column, may travel with ocean currents and long distance passive migration is also possible in ballast water (Carlton and Geller, 1993). Adult forms may travel attached to hard surfaces, like the hulls of ships.

The bivalve lives in shoals in coastal zones. The belt of blue mussels starts at a depth of several-metres and spreads to a depth of 30 m. Blue mussels are highly im-portant filter feeders. They transform the sea water sus-pension into high-quality proteins which can be used by animals and humans. Individual shoals filter hun-dreds of cubic metres of water daily. Within an area of 160 km2 near Asko (Sweden) in the northern part of the Baltic proper, all bivalves are capable of filtering the to-tal water mass in two and a half months. Shoals of blue mussels also provide a good food source for flounder, cod, ray and sturgeon. In the upper water zone, blue mussels are eaten mainly by common eiders which can dive to a depth of 10 m. The blue mussel’s main preda-tors in Kattegat, the Asterias rubens starfish and Carci-nus meanas littoral crab, were not able to adapt to the low salinity of the Baltic. This explains why the blue mussel has found such favourable conditions to develop in the Baltic Sea, despite its weakened shell structure in lower salinity waters (Reimer and Harms-Ringdahl, 2001).

Due to the low salinity of the Baltic Sea, blue mus-sels have developed a dwarf form (up to 5 cm long), but they constitute about 75% of the epifauna (Jura, 2004). The total population of blue mussels inhabiting the Bal-tic proper to a maximum depth of 25 m (with shell) is estimated at 8 million tons dry weight (Kautsky, 1991).

Aquaculture and use in food industry

In many countries blue mussels are harvested for consumption. In Poland, however, blue mussels are not commercial owing to their small size. The first men-tion of human blue mussel cultivamen-tion in Europe is de-scribed on wooden stakes in 1235 AD, in France. From that time, blue mussel breeding started in Europe over the full area of their distribution. Subsequently breed-ing techniques emerged in the late 19th century when aquaculture began to be regarded as a  cheap protein source. Blue mussels then became a very popular dish in Western Europe.

Aquaculture is always in phytoplankton rich zones. There are several methods used depending on type of

Fig. 2. Blue mussel anatomy

Source: http://www.design-site.net

Fig. 3. Mussel on a rocky shore

Source: http://en.academic.ru/pictures/enwiki/66/Blue_mus-sel_Mytilus_edulis.jpg

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coast. In Holland, the young are spread over boards in shallow gulfs or sheltered areas where they are attached to the sea floor. They are harvested by dredging with special nets. In France, cultivation is on rows of wooden stakes positioned within the tidal zone. In Spain, blue mussels are bred on lines. The Spanish Atlantic coast is favourable for blue mussel growth because of high tides which ensure regular exchange of water. Blue mussels reach commercial sizes after 7-8 months, while in other regions (e.g., England) such sizes might be reached after 4-5 years. In some places blue mussels are farmed just like oysters – in bags on platforms mount-ed within a  tidal flat or fixmount-ed directly to the bottom (Breber and Sirocco, 1998; Bishop, 2001; Bürgin et al., 2001).

Bivalves contain high-quality proteins which pro-vide essential building blocks for nutrition. They are also a perfect source of calcium, fluorine as well as sele-nium that can protect against cancer. They also contain B and D vitamins which are important for healthy skin, the central nervous system and structural and function-al maintenance of bone.

The blue mussel is usually boiled before eating and should be cooked quickly for about 1-2 minutes until the shell opens. If the shell does not open after this short time, it suggests that the bivalve was probably not fresh. Blue mussels are usually sold live or processed in tins or marinated. Cultivated bivalves are fleshier and have thinner shells. Depending on the feed they are given, the bivalves can also vary from those in the natural en-vironment in flavour and shell colour. The blue mussel’s filtration of water, however, poses a certain threat to hu-man health. In highly polluted regions, the bivalves can accumulate large amounts of toxic substances which can lead to poisoning (Bürgin et al., 2001; Sikorski, 2004; Rajski, 1997; Kwoczek, 2006).

The blue mussel as an indicator species

Bio-indication uses life forms as indicators for envi-ronmental pollution. In general, it is used as a way to as-sess environmental degradation or observe changes in the biocoenosis or ecosystem. Animals and plants can act as indicator species if they exhibit a narrow band of tolerance to a  specific factor (stenobiont species). Bioindicators are chosen for their particular sensitiv-ity to substances of interest. Their reaction functions as the alarm to warn about contamination. Behavioural changes in bioindicators indicate stress from disad-vantageous or detrimental external factors. To classify species as bioindicators, they must meet several criteria. Primarily, a  bioindicator cannot have inherent prob-lems with identification and at the same time it must be accurately identified, morphologically, anatomically and physiologically. Bioindicators are characteristi-cally species which respond to specific changes in the environment in a manner, appropriate to the degree of contamination. Their reaction is permanent and repeat-able. A long life cycle is required to watch their reactions throughout the seasons.

Bivalves are sensitive to water pollution and their re-sponse to contamination with heavy metals (mercury, copper and cadmium), formaldehyde or pesticides (e.g., lindane) is immediate. Blue mussels immediately re-spond to increased amounts of chlorine, ammonia or iron. When a toxic substance first appears, the shell im-mediately closes and eventually the bivalve’s vital func-tions cease. At first, the bivalve restricts its activities by slowing down its metabolism and switching to anaero-bic respiration. However, this is a disadvantage in terms of energy intake, therefore the shell still opens from time to time. An increase in concentration of aqueous toxic substances shortens the interval at which the shell

Fig. 4. EU mussel aquaculture production (2009)

Source: Eurostat. Calories: 103 kcal Protein: 17 g Selenium: 50 µg Vitamin D: < 0,5 µg EPA: 340 mg DHA: 214 mg

Tab.1. Nutrition per 100 g (average values for cooked mussels, M. edulis)

EPA – Eicosapentaenoic acid, DHA – Docosahexaenoic acid Source: Eurostat.

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opens and increases the time when it is closed. This ac-tivity of the bivalves increases during exposure to pol-lutants. It is possible to observe increased frequency of shell movements and, in consequence, greater inten-sity of filtration and metabolic processes. The ultimate stress reaction to a toxic factor is death (Kees et al., 1989; Jenner et al., 1989).

Present distribution of the Mytilus edulis spp.

complex in the World’s Oceans

Species that belong to the Mytilus edulis spp. com-plex are scattered worldwide outside the tropics (fig. 5). There are numerous hypotheses explaining this. The most plausible explanation for their present distribution

is migration across the equator. Lindberg (1991) defined two periods in which expansion of the organisms from one hemisphere to the other took place. The first and long-term migration took place about 3.1 million years ago and coincided with the creation of the Isthmus of Panama. The expansion was asymmetric at that time, since as many as 90% of organisms made their way from the north to the south. The other migration occurred in the Pleistocene and was more balanced and less intense (Lindberg, 1991).

In the northern hemisphere Mytilus trossulus is found in the Pacific, north-west Atlantic and the Bal-tic (fig. 5). Mytilus edulis occurs in the AtlanBal-tic Ocean, while Mytilus galloprovincialis inhabits the Mediter-ranean, southern coasts of the Atlantic and North Af-rica. Introduction of Mytilus galloprovincialis to the Japanese Sea, southern California and Puget Bay has been recently discovered. In the southern hemisphere it is found along the South American Atlantic coast, the Kerguelen Islands, the Republic of South Africa, Aus-tralia, Tasmania and New Zealand (Hilbish et al,. 2000). Mytilus californianus occurs along the coasts of Pacific Ocean between San Diego and Humboldt Bay. This bi-valve is distinct genetically and in terms of morphol-ogy and is therefore not a member of the Mytilus edulis spp. complex (Sarver and Foltz, 1993). The blue mussel population along the coasts of Chile and the Falkland Islands is Mytilus chilensis (Hupe, 1854). Morphological analysis of M. edulis, M. trossulus and M. chilensis allow distinction between the three taxa. However, the mito-chondrial and other molecular markers (e.g. ITS; inter-nal transcribed spacer) do not reveal significant differ-ences between these forms, which leads many scientists to believe the Chilean blue mussel to be a subspecies, Mytilus edulis chilensis (Toro, 1998).

Numerous studies indicate that Mytilus trossulus evolved in the Pacific Ocean and colonized the north

Fig. 5. Mussel distribution in the World’s Oceans

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genes of both previously separated pools from two dif-ferent species. The newly created hybrid is characterized by increased variability. The heterosis effect is com-monly observed among hybrids. Such hybridisation is a  frequent phenomenon in marine organisms. A  few hybridisation zones have been described for the Mytilus species. All forms are able to reproduce fertile crosses and create hybridisation zones at diverse latitudes.

Four principal zones of hybridisation are known:

Northern Europe: where the Baltic Sea and the North Sea meet (Mytilus trossulus × Mytilus edulis);

Western Europe: (Mytilus edulis × Mytilus gallo-provincialis);

The Pacific Ocean: along the northern coast of Ca-lifornia and Oregon (Mytilus galloprovincialis × Mytilus trossulus);

Eastern Canada: between the boreal and Arctic zo-nes (Mytilus trossulus × Mytilus edulis), (Gardner, 1994);

The Japanese Sea: (Mytilus galloprovincialis × My-tilus trossulus), (Inoue et al., 1996)

There are two ideas used to explain the existence of the hybrid zones. The first assumes that zones are inde-pendent of the environment and constitute a by-prod-uct of relations between the two populations. Accord-ing to this model, hybrids are worse adapted, compared with their parental populations and the hybridised zone is maintained by a balance between proliferation of pa-rental genotypes and selection against hybrids, which leads to emergence of buffer zones (Barton and Hewitt, 1985). The hybridisation zone between Mytilus edulis × Mytilus galloprovincialis, where, despite the high possi-bility of contact, hybridisation occurs only in a separate buffer zone and in addition, the life-span of the hybrids at the larval stage is greatly reduced in comparison with parental forms (Bierne et al., 2002).

The hybridisation zones can also be present within areas in which hybrids demonstrate better adaptation than non-hybrids. If hybrids are favoured in terms of specific features, the parental forms and hybrids will be arranged according to the environmental conditions which they favour (Moore, 1977).

Doubly uniparental inheritance (DUI)

Uniparental inheritance and lack of recombination allow mtDNA to be used as a molecular marker in much animal research into populations. Until a mutation oc-curs, the whole progeny line of a female share the same haplotype. Such inheritance prevents the distribution of defective mitochondrial genomes beyond the progeny of an individual female.

Atlantic through the Bering Strait about 3.5 million years ago (Riginos and Cunningham, 2002). M. edu-lis emerged in the Atlantic through allopatric specia-tion (Vermeij, 1991). M. galloprovincialis evolved in the Mediterranean when it had limited connection with the Atlantic. The second, recent migration from the Pa-cific into the Atlantic took place in the Pleistocene or the Holocene and as a result, M. trossulus colonized the Baltic and Canadian coastal waters (Riginos and Cun-ningham, 2002).

Hybridisation

Hybridisation can induce exchange of genes be-tween species, with consequent mixing of the gene pool for both taxa. The process creates a form that possesses

Fig. 6. Polar maps with the migration pathways of Mytilus

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Baltic Sea are smaller and characterized by thin shells. The low salinity explains their smaller size. Based on morphological examination and analysis of allozymes it was established that populations in the North Sea and Skagerrak belong to Mytilus edulis, while popula-tions in the Baltic Sea are Mytilus trossulus (McDonald et al., 1991; Väinölä and Hvilsom, 1991). In the Katte-gat population, the number of alleles is somewhere be-tween the count for the Baltic and the North Sea, which

may be the consequence of frequent changes in salinity. While examinations of allozymes show diversification along the Scandinavian hybridisation zone, blue mus-sel DNA sequence analysis reveals extensive movement of genes. Asymmetrical introgression of female Mytilus edulis mtDNA into Baltic Sea populations has caused the F-mtDNA (female mtDNA) of M. trossulus to disap-pear (Rawson and Hilbish, 1998; Quesada et al., 1999). Moreover, the standard male mtDNA was not observed In the animal world, mitochondria and their own

mtDNA are inherited almost exclusively by the ma-ternal (female) line, because all, or almost all originate from the oocyte. Few are transferred with the sperma-tozoon. What is more, those arriving with the male gamete (along with their mtDNA) are destroyed during the early phases of zygote development. Hence, the mi-tochondrial genes are passed from the mother to male and female progeny.

Bivalves from the Mytilus family are characterized by unusual inheritance of mtDNA – doubly uniparental inheritance (DUI). Apart from genomes inherited along the maternal line, males of these bivalves have another mitochondrial haplotype which is inherited exclusively along the paternal line, being a typical example of het-eroplasmy. In these molluscs recombination between mtDNA genomes has also been documented (Zbawicka et al., 2007). This peculiarity of bivalves from the Myti-lus family stimulates particular interest.

Scandinavian zone of hybridisation

The Scandinavian zone of hybridisation includes the Baltic Sea – a  region that bivalves have recently colonized. During the last North-Poland glaciation, the Baltic Sea was entirely ice-covered. As the result of accumulation of snowmelt waters, the sea trans-formed into a  lake. Contact with the North Sea took place about 7500 years ago (Donner, 1995) and enabled bivalves and other marine organisms to colonize. Cur-rent salinity of the Baltic is estimated at 5–10‰. In the Bothnian Sea the salinity is about 4‰ and the Bay of Bothnia can be considered a body of freshwater. Signifi-cant fluctuations in salinity are present in the Kattegat, between 10–20‰ and 20–30‰ in the Skagerrak Strait. The bivalves of the Baltic differ from the North Sea and Skaggerak population. Mature forms of bivalves in the

Fig. 7. Distribution of Mytilus edulis spp. complex mussels in Europe

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either. In heteroplasmic individuals, mtDNA originates from M.edulis females which suggests that F-mtDNA in M.edulis has acquired M-mtDNA (male mtDNA) func-tion (Wenne and Skibiński, 1995; Quesada et al., 1999). This phenomenon is often referred to as virilization and is most common within the hybridisation zones (Raw-son et al., 1996).

There are two hypotheses to explain the above phe-nomenon. According to Vainola and Hvilsom (1991), this situation is a result of introgression of genes through hybridisation between Mytilus edulis of Skagerrak and the Mytilus trossulus of the Baltic Sea. However, Bul-nheim and Gosling (1988) claim that Mytilus trossulus evolved in the Baltic Sea from Mytilus edulis. This hy-pothesis, however, does not explain the resemblance of the Baltic population to the Pacific and west-Atlantic populations. Taxonomy and distribution of the Myti-lus edulis spp. complex was first based on morphology and allozymes (McDonald et al., 1991), but later studies identified many markers based on DNA. Selected nu-clear DNA markers now permit more detailed compari-son of species that belonging to the Mytilus edulis spp. complex.

Conclusions

Blue mussels occupy many diverse habitats, from tidal areas to entirely submerged zones with wide tem-perature and salinity ranges. Bivalves of the Mytilus spp. complex play a  major role in aquatic ecosystems and belong to the most economically significant group of invertebrates. They are effective cleaners of large bodies of water, removing of excess organic matter by filtration. The mussels are a  quality food source for other organisms, including humans. Edible species are cultivated and eaten worldwide and their sensitivity to environmental pollution also recommends them as

use-ful bioindicators. Owing to their important roles, blue mussels are a popular subject for ecological and biologi-cal research. Modern methods in genetics have allowed discovery and investigation of processes such as hybrid-isation or doubly uniparental inheritance. Hybridisa-tion zones offer miraculous systems for analysis using molecular markers. The main stimulus for the intense study of blue mussels is that they provide an unusually rich population for progress in the understanding of evolution and speciation.

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Compatability with

the Polish core curriculum:

Biology – 4th educational stage – basic scope: Educational goals:

I. Searching, utilization and creation of information.

A student receives, analyses and judges information coming from various sources, taking into account particularly the press, the media, the Internet.

Contents:

1. Biotechnology and genetic engineering. A student:

6) provides examples of utilization of research DNA (judicial system, medicine, science);

2. Biological diversity and threats for it. A student:

1) describes biological diversity on a genetic, species and ecosystem level; indicates causes of a de-crease in genetic diversity, extin-ction of species, deterioration of habitats and ecosystems;

Biology – 4th educational stage – extended scope: Educational goals:

IV. Searching, utilization and creation of information.

A student reads, selects, compares and processes information obtained from various sources, including this obtained by means of information-communication technologies.

VI. Attitude towards nature and environment.

A student understands the importance of nature and environ-ment protection, and knows and understands principles of sustained development; presents an attitude of respect towards themselves and all living creatures, the environment; describes an attitude and behaviour of a human who utilizing goods of nature and environment in a responsible way, knows rights of animals and analyzes their relation towards living creatures and the envi-ronment.

Contents of education:

11. Invertebrate animals. A student:

13) presents the importance of molluscs in nature and for humans; 6. Genetic variability. A student:

1) defines sources of genetic variability (mutations, recombination); 8. Molecular biotechnology, genetic engineering and molecular

medicine. A student:

7) presents various applications of genetic methods, i.a. (...) evolu-tionary research;

Cytaty

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