Marine Biology

Mytilus edulis (The common or edible mussel)

Figure numbers refer to diagrams handed out during the lecture.

Phylum Mollusca

Molluscs are conspicuous invertebrates, including familiar forms such as clams, oysters, mussels squids, octopus, snails. In abundance the molluscs comprise the largest invertebrate phylum aside from the arthropods Ð 80,000 living species and 35,000 fossil species. Molluscs are a heterogeneous assemblage but all built on the same fundamental plan (Figure 1).

Molluscs are unsegmented coelomate animals with a head, ventral muscular foot, dorsal visceral hump, soft mantle covering visceral hump which secretes a calcareous shell, mantle cavity, anus and kidneys opening into mantle cavity, a pair of ctenidia (gills) (originally for breathing) within the mantle cavity (Figure1).

Class Lamellibranchiata (Bivalves)

Bivalves are molluscs with bilaterally symmetrical body, laterally compressed and enclosed by a shell that develops two valves hinged dorsally (Figure 2). The foot like the remainder of the body is also laterally compressed, the mantle cavity is capacious and the gills are large having assumed the role of food collecting as well as respiration. (Figure 2)

Most of the characteristics represent modifications that enabled Bivalves to leave the hard substratum, to which the ancestral molluscs were confined, and take up an existence in the much more numerous soft bottom habitats. The lateral compression and development of a strong muscular foot for burrowing led to a degree of specialisation that confined almost all Bivalves to the soft sediment environment. Only a few groups have subsequently migrated to other habitats.

The development of the gills (ctenidia) is the outstanding morphological and physiological character of the Bivalves (Figure 3). The arrangement of the valves allows the mantle cavity to extend the whole length of the body and makes possible a great extension of the gills. There are two limbs of the gill filaments on either side of the body mass, hanging down into the mantle cavity and joined together by ciliary junctions. The joining of the gill fillaments divides the mantle cavity horizontally into an upper and lower chamber. During activity a constant stream of water is maintained entering via an inhalent ventral siphon, passing through the gill lamellae into the dorsal chamber and out through a dorsal exhalent siphon. This system facilitates respiration and feeding. In some bivalves, including mussels, there has been morphological sealing of the mantle edges and the development of inhalent and exhalent siphons (Figure 4)

Filter feeding is achieved by the separation of minute plants and organic debris from the water current onto the surface of the gill lamellae. Inhalent water is largely a function of ciliary activity. As the water enters the mantle cavity larger particles fall to the floor of the cavity whilst smaller particles are filtered onto gill lamellae. These smaller particles are entrapped in mucous and transported in the direction of the mouth in ciliated grooves by ciliary action. Ciliated palps between the gills and mouth have the function of sorting food particles. Lighter particles are transported to the mouth whilst heavier particles are transported downwards to the bottom of the mantle cavity. Rejected particles which accumulate in the bottom of the mantle cavity are periodically ejected through the inhalent siphon. This achieved by closure of the two valves of the shell (Figure 5)

The lamellibranchs are divided into 4 Orders according to structure, degree of development of the gills and the extent to which gill filaments are joined together. Mytilus edulis belongs to the Order Filibranchiata with reflected gill filaments and adjacent filaments joined by ciliary junctions.

Mytilus edulis

Whilst the majority of lamellibranchs are semi-sedentary, the sea mussel has developed the sedentary tendency and marks a half way stage to the oyster which remains fixed throughout adult life. The mussel lives in associations in beds between the tide marks where conditions are favourable. The reduced but very extensive foot is tongue like in shape with a groove on the ventral surface, which is continuous with the byssus pit. In this pit a viscous secretion is poured out which enters the groove and hardens gradually when it comes into contact with sea water. The tip of the foot is pressed against the surface to which the mussel attaches itself, and in a cup like hollow which ends the groove the attachment plate is formed at the end of the byssal thread. When one byssal thread has formed the foot changes position and secretes another thread in another place. The byssus thus consists of a mass of threads arising from the byssus pit and by means of it the mussel is firmly attached to rock, stones or other mussels etc. (Figure 6). However, mussels, particularly when young, creep about both by using the cup at the tip of the foot as a sucker and by forming a path of threads along the substratum. The byssus is the most outstanding characteristic of the mussel but it is additionally worth noting that other features include a pair of simple eyes at the anterior end of the gills and invasion of the mantle by the gonads. Note also that most bivalves, including mussels, are dioecious. There is no copulation and fertilisation takes place in the surrounding water. The development of free swimming larvae is typical of bivalves.

The general biology of mussels

Mussels dominate in much of the low and mid intertidal region in temperate seas of the northern and southern hemispheres. There are many genera, all with a narrow anterior end and the anterior abductor reduced. The pointed umbo is at the anterior end of the shell in Mytilus, and slightly set back in other genera.

Mytilus edulis is essentially an intertidal organism, which can form extensive beds dominating the rock surface. It can also form strips or patches. Competition, predation and physical factors are important in determining the distribution of Mytilus. Mytilus edulis can live high on the shore because it is tolerant of desiccation and frost. It is, however, susceptible to predation by starfish, dogwhelks, shore crabs and various birds. Mussels on exposed coasts are often free of predators while those in sheltered waters may suffer extreme predation.

The first larval stage of Mytilus edulis is called a trochophore, which lasts about one day. It is followed by various stages of veliger larvae lasting in all about one month. The veliger larvae has a pair of shell valves and carries a ciliated swimming organ or vellum. It settles first on filamentous seaweeds, with maximum settlement in June ÐJuly. In July-August these early plantigrades detach themselves and are carried once again by water currents. They settle again in their final position in mussel beds in the same months.

Growth rate of mussels varies greatly and is dependent largely upon the amount of time available for feeding i.e. in proportion to immersion. The position within mussel beds is also important: individuals within the centre grow slowly, while those at the edges grow faster but may suffer the penalty of higher predation. Growth can be measured by disturbance rings on the shell amongst other methods.

Adaptations of Mytilus edulis to life in the intertidal zone

1. An attached sedentary mode of life in dense communities enables mussels to withstand the physical exposure of waves. Their sedentary mode of life is facilitated by (1) nutrition by filter feeding which does not require locomotion; (2) reproduction without copulation and by pelagic larvae which achieves dispersal of the species without locomotion by the adult. Filter feeding is particularly effective in the intertidal zone where wave action causes the suspension and re-suspension particulate nutritive material which is readily available to the mussel.
2. Closure of the valves which enables mussels to withstand the physical, chemical and biotic factors associated with exposure to the atmosphere during low tide by trapping water in their valves and restricting water loss*. (Factors associated with exposure to atmosphere include dessication or water loss, light, heat, lack of dissolved oxygen, predation etc.).
3. Important in the context of 2 above are adaptations to facilitate (1)survival in the absence of a supply of dissolved oxygen; (2) survival of the build up carbon dioxide and toxic metabolites. Mussels can trap water within their valves and respire anaerobically, building up an oxygen debt which is paid off by aerobic respiration on the next incoming tide. Furthermore, the build up of carbon dioxide within the mantle cavity during low tide depresses ciliary activity and finally causes ciliary activity to stop so that the store of oxygen within the cells is conserved. In mussels ammonia constitutes a relatively small proportion of nitrogen excretion, much of it comprising less toxic amino acids*.
4. Closure of the valves affords mussels protection against many terrestrial predators during exposure to the atmosphere at low tide.

*Water loss can be fatal either because of a change in the internal environment as a result of an increase in body fluids (this affects the osmotic potential of the body, which may assist desiccation as the cells lose water by osmosis to their surrounding fluid), or because of lack of oxygen to the cells because of lack of water flowing over the surface of the gills(and so reducing gas exchange). Aquatic animals may suffocate even if they are surrounded by oxygen rich air because their gills will stick together. Water is also lost by excretion; many marine animals excrete ammonia which is highly toxic and must be diluted with large quantities of water. This is a serious problem on the sea shore where water loss is to be avoided at all costs.

Mytilus edulis as an indicator organism in marine pollution assessment

In many respects Mytilus edulis is the ideal marine indicator organism. It fulfils many of the basic prerequisites for the selection of an indicator organism (see Appendix ). Additional to the fulfilment of these requisites the following are particularly pertinent factors:

1. Mussels, like all filter feeding bivalve molluscs, process large volumes of water. This is necessary because the amount organic matter in sea water is low (average 1mg/l). Mussels filter, on average, 7.5 litres of sea water/hour. As a consequence of this they accumulate and concentrate many pollutants in sea water, particularly those which are particulate or are associated with particles. Like all bivalves, mussels are notorious for their ability to accumulate very high concentrations of metals. They also accumulate other pollutants such as faecal bacteria and radionuclides. This ability to accumulate materials facilitates the detection and measurement of pollutants that may be in the water column at very low concentrations.
2. The fact that mussels can accumulate faecal bacteria and viruses and the fact that mussels are harvested for human consumption has led to their use in routine determination of the quality of mussel flesh and water in terms of hygiene and public health.
3. Because mussels satisfy so many features of a good indicator organism they have been used widely in pollution assessment work, including survival (LC50), behavioural changes, reproductive changes, transplant experiments, and scope for growth experiments.
4. The scope for growth test was developed for use with bivalves, particularly mussels, to give a general measure of the overall physiological health of the animal based on its energy budget. The scope for growth is the difference between the energy assimilated from food, and the energy used in respiration, excretion and other maintenance activities. Any surplus energy is available for growth and reproduction. A reduced or even negative scope for growth results when energy intake from food is reduced, as it may be in winter, or if the energy expenditure on maintenance activities is increased by environmental stress.
5. Because of the filtration mode of feeding mussels also readily become tainted by hydrocarbons present in the water. This is of particular significance because of their commercial value for human consumption. Tainting of shellfish and finfish flesh is evaluated by taste panels.