In: Mussels: Anatomy, Habitat and Environmental Impact ISBN Editor: Lauren E. McGevin, pp Nova Science Publishers, Inc. Chapter 2 ENVIRONMENTAL IMPACT OF ANTHROPOGENIC ACTIVITIES:
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In: Mussels: Anatomy, Habitat and Environmental Impact ISBN Editor: Lauren E. McGevin, pp Nova Science Publishers, Inc. Chapter 2 ENVIRONMENTAL IMPACT OF ANTHROPOGENIC ACTIVITIES: THE USE OF MUSSELS AS A RELIABLE TOOL FOR MONITORING MARINE POLLUTION Stefanos Dailianis Section of Animal Biology, Department of Biology, School of Natural Sciences, University of Patras, GR Patras, Greece ABSTRACT The current chapter is focused on a) the general anatomy and morphological characteristics of mussels (fresh water and saltwater species), b) the effect of both abiotic (temperature, salinity, congestion, pollution, air-exposure, food availability, etc.) and biotic (age, soft-body weight, reproductive cycle, predators, etc.) environmental factors on mussel behavior and physiology, c) the role of filter-feeding mussels as sensitive marker for assessing human-derived environmental impacts and d) the important ecological and environmental role of mussels, with emphasis to saltwater mussels, as reliable tool for monitoring the aquatic environment health status. Specifically, the role of mussels for monitoring aquatic environment is of great interest, since the presence of human-derived inorganic and organic pollutants into the water could affect environmental health status. The good knowledge of their physiology and behavior, as well as their study in cellular, genetic and biochemical level, are important parameters which reinforces the role of mussels as Bioindicators of the marine environment. Moreover, Biomarkers (general- and specific stress as well as genotoxicity), which represent biochemical, cellular,, genotoxical, physiological or behavioral variation that can be measured in mussels, providing evidence of exposure to and/or effects of, one or more chemical pollutants being present into the water, were briefly mentioned, in order to emphasize the use of mussels as bioindicators in a lot well-documented monitoring studies, as a result of the continuously anthropogenic-induced impacts on the environmental health status. Tel-Fax: , 2 Stefanos Dailianis 1. MUSSELS: GENERAL ANATOMY, HABITAT AND PHYSIOLOGY Mussels are members of several families of filter-feeding molluscs, whose habitat range from saltwater to freshwater. Freshwater mussels belong to the mollusk order Unionoida, a solely freshwater group and they inhabit lakes, ponds, rivers, creeks, canals, and similar habitats. Despite the fact that these freshwater species are not closely related to saltwater mussels and grouped in different subclass, similarities in appearance have been mentioned. Species belong to the family Unionidae, such as Unio pictorum, also known as the painter's mussel, are species of medium-sized freshwater mussel, being commonly present in rivers in Europe. Another species of freshwater mussels is the pearl mussel Margaritifera margaritifera, which belong to the family Margaritiferidae. This species is capable of making fine quality pearls (Hyman, 1967; Barnes, 1968; Morton, 1979). Another group of aquatic bivalve mollusc, belonging to the family Dreissenidae, is the zebra mussels Dreissena polymorpha. This species get its name from a striped pattern which is commonly seen on shells. They are usually about the size of a fingernail, but can grow to a maximum length of nearly two inches (5 cm). The shape of the shell is also somewhat variable. This species was originally native to the lakes of southeast Russia, but it has been accidentally introduced in many other areas, and has become an invasive species in many different countries. Despite the fact the zebra mussels are not related to previously mentioned groups, resembling many Mytilus species in shape, and live attached to rocks and other hard surfaces with the used of byssus, they are not at all closely related to the mytilids. Indeed, zebra mussels are much more closely related to the Veneridae, the genus clams they are classified with the Heterodonta, the taxonomic group which includes most of the bivalves commonly referred to as clams . The word mussel is most frequently used to mean the edible bivalves of the marine family Mytilidae, most of which live on exposed shores in the intertidal zone and are of great interest since they are intensively fished and cultured worldwide for human consumption. Specifically, mussels are farmed in many areas of the world with the most common species cultured being the blue mussel, Mytilus edulis and the Mediterranean mussel M. galloprovincialis. The main producers of mussels are countries such as China (over tones per year), Korea, Spain, The Netherlands, Denmark, France and New Zealand. Mussels of the genus Mytilus (Kingdom: Animalia, Phylum: Mollusca, Class: Bivalvia, Subclass: Pteriomorphia, Order: Lamellibranchia) are a group of filter-feeding bivalve molluscs, firstly appeared before 1-2 millions years. Genetic analysis of mussels revealed 9 different species of the genus Mytilus, spreading throughout the world, such as: 1. M. edulis, (North hemisphere) 2. M. galloprovincialis, (the common species in the Mediterranean Sea) 3. M. trossulus, (coastal areas of North America, Pacific ocean) 4. M. coruscus,(coastal aereas of China and Japan) 5. M. californianus, (coastal areas of North America, Pacific ocean) 6. M. chilensis, (coastal areas of South America, Chile) 7. M. platensis, (coastal areas of Argentina, South Atlantic ocean) 8. M. planulatus, (coastal areas of Australia) 9. M. desolationis, (coastal areas of Kerguelen islands, Indic ocean). Environmental Impact of Anthropogenic Activities 3 Specifically, Mytilus galloprovincialis, M. edulis, M. trossulus and M.californianus represent the most common species being studied so far (Figure 1). Figure 1. Geographical distribution of the most common species Mytilus spp., according to morphologic and genetic analysis. In most marine mussels the shell is longer than it is wide, being wedge-shaped or asymmetrical. The external color of the shell is often dark blue, blackish, or brown, while the interior is silvery and somewhat nacreous. The mussel's external shell is composed of two hinged halves or valves . The valves are joined together on the outside by a ligament, and are closed when necessary by strong internal muscles. Mussel shells carry out a variety of functions, including support for soft tissues, protection from predators and protection against desiccation (Hyman, 1967; Barnes, 1968; Morton, 1979). The shell is made of three layers. In the pearly mussels there is an inner iridescent layer of nacre (mother-of-pearl) composed of calcium carbonate, which is continuously secreted by the mantle; the prismatic layer, a middle layer of chalky white crystals of calcium carbonate in a protein matrix; and the periostracum, an outer pigmented layer resembling a skin. The periostracum is composed of a protein called conchin, and its function is to protect the prismatic layer from abrasion and dissolution by acids (especially important in freshwater forms where the decay of leaf materials produces acids). The animal is enclosed by the large right and left mantle skirts (lobes) which line the inner surfaces of the two valves. The space between the two mantle skirts is the inhalant 4 Stefanos Dailianis chamber of the mantle cavity, which is filled with seawater. The two skirts are connected dorsally to each other and are attached to the valves along the pallial line. The mantle skirts are thick, containing the gonads. The male mantle is creamy beige whereas that of females is reddish. Like most bivalves, mussels have a large organ called a foot. In freshwater mussels, the large, muscular, and generally hatchet-shaped foot is commonly used to pull the animal through the substrate (typically sand, gravel, or silt) in which it lies partially buried. This procedure is performed by repeatedly advancing the foot through the substrate, expanding the end so it serves as an anchor, and then pulling the rest of the animal with its shell forward. It also serves as a fleshy anchor when the animal is stationary. In marine mussels, the foot is smaller, 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 exuded, entering the groove and hardening gradually upon contact with sea water, thus forming extremely tough, strong, elastic, byssus threads that secure the mussel to its substrate. Moreover, the byssus threads are used by mussels as a defensive measure, to tether predatory molluscs, such as dog whelks, that invade mussel beds, immobilizing them and thus starving them to death. Mussels and scallops have filibranch gills which are formed of the combined filaments attached to the central axis, holding together by ciliary interfilamentar junctions. The entire gill is a holobranch and includes the filaments on both sides of the axis. There is only one gill on the right and one on the left even though it may look to you as if there are two on each side. Each filament bears frontal cilia on its outer edge and lateral cilia on the flat surfaces facing adjacent filaments. The lateral cilia generate the feeding/respiratory current whereas the frontal cilia move food particles along the surface of the gill to the food grooves, thus representing the original condition of the lamellibranch gill. The eulamellibranch gills of most other bivalves, such as Mercenaria and Corbicula, are held together by solid, vascularized tissue junctions. The filaments of eulamellibranch gills have, in fact, grown together to form a continuous sheet perforated by small pores. The gills divide the mantle cavity into a ventral inhalant chamber and a dorsal exhalant chamber. Water enters the ventral edge of the shell, passes between gill filaments, enters suprabranchial chamber from the ventral inhalant siphon, proceeds posterioly and dorsally and finally flows back into the sea through the dorsal exhalant siphon. Although the gills are the most important, respiratory organs of mussels, the inner surfaces of the mantle skirts are also responsible for gas exchange but the chief respiratory surfaces are the plicate organs. Each of the two plicate organs is a longitudinal row of transverse folds of epithelium between the gill and the visceral mass. Gills are the principal organs of food capture and selection of materials. Food passes into the short esophagus and then into the stomach, covered by the liver (digestive gland system). Digestive cells are the main cell type in the digestive gland, which in turn are the main metabolic organ in molluscs. Lysosomes in digestive cells of molluscs are involved in the uptake and intracellular digestion of food material. Most of the digestive tract is embedded in the dorsal region of the foot and visceral mass. Extracellular digestion of food is followed by intracellular accumulation and digestion in the well-developed endolysosomal system of digestive cells. Environmental Impact of Anthropogenic Activities 5 All species of the genus Mytilus are dioecious, but hermaphroditism (either protandrous or synchronous) is not excluded. Nuclear genetic material consists of 28 chromosomes, while there is evidence for the presence of sex chromosomes. It seems likely to suggest that mt- DNA could play an important role in sex differentiation in mussels (Fisher and Skibinski, 1990). In fact, differences between Mytilus species, regarding the presence of different types of enzymes and expression of different mt-dnas, conducted electrophoretically, reveals that local/geographical parameters could mediate gene expression. Moreover, environmental pollution could influence genetic incorporation of mussels population. Heterozygoty in mussels seems to favour survival under environmental stress, occurred by heavy metals. Gametes are developed in germ follicles of mantle lobes, while germ tissue is actually emerged within visceral tissue of mussel s body. The development of germ follicles within the mantle lobes of mussels is revealed during early autumn and a new gametogenesis is performed. During spring, gametogenesis is reduced, while a significant enhancement is performed during summer. Sexual maturation in marine mussels is performed after the first year of their life, while significant alterations are revealed due to growth rate-mediated environmental factors. For example, violent detachment and handling, temperature and food availability could affect both gametogenesis and spawning processes. In marine mussels, fertilization occurs outside the body. Fertilization in seawater mussels, such as Mytilus edulis was carried out at temperatures between 5-22 ο C and salinity ranged from 15 to 40. In normal condition, embryogenesis is carried out at temperature ranged between 15 and 20 ο C and salinity ranged from (at 15 ο C) to (at 20 ο C), while maximum developmental ability was recorded at In laboratory conditions, there is no embryonic development in larva of Mytilus galloprovincialis at 25 ο C. Fertilization of yellow-orange oocytes from male gametes (spermatozoa) is considered as a short-time process (Strathmann, 1987), depending on the environmental conditions and habitats and leads to the development of a free-swimming larva, named trochophore. Trochophore larva develops a velumlike fold and becomes the characteristic molluscan veliger larva. The duration of trochophores developmental period ranges between 1-4 weeks and depends on water temperature, salinity and food availability. Trochophores drift for three weeks to six months, before settling on a hard surface as a young mussel. Development of trochophore larva and metamorphosis is obtained during spring and early summer and characterized by larva ability to move slowly by means of attaching and detaching byssal threads to attain a better life position, in order adult mussels to be occurred. In fresh-water mussels, especially the members of genus Unio, a parasitic larva, called glochidium, was discharged into the water and encyst upon the skin or gills of fishes came in contact with. During a period of several weeks of parasitism, glochidia undergo metamorphosis into the adult. Planktonic organisms (diameter 3-5 μm) and other microscopic sea creatures which are free-floating in the seawater, are the main food during development of larva, while its daily food requirements are equivalent to 30-60% of its total weight. Larva mortality could reach at 99%, due to food deficiency, the presence of predators, such as fishes and other invertebrates (sea stars, such as Asterias rubens, gastropods in the family Muricidae, such as the dog whelk, Nucella lapillus) and environmental stress factors. 6 Stefanos Dailianis 1.1. Environmental Factors and Their Role in Mussels Physiology and Behavior Physiology and behavior of mussels can be affected by various environmental factors that could lead to environmental stress, thus affecting growth rate, food uptake, reproductive cycle and homeostasis of mussels in general. Abiotic factors, such as salinity, temperature and airexposure of mussels could affect normal growth rates of bivalve molluscs and their response to various environmental stressors (Philips, 1976a,b; Cossa et al., 1979; Davies and Pirie, 1980). Bivalves posses great resistance to low temperatures due to the presence of anti-freeze proteins in their haemolymph, thus reinforcing tissue and cellular resistance against freeze during cold periods, such as winter (Kanwisher 1959; 1966; Williams, 1970; Aarset, 1982). On the other hand, temperatures higher than 29 ο C could result in increased mortality of mussels. Exposure of mussels in air during tide, could affect their metabolic rate, increasing anaerobic metabolism, which in turn could lead to significant alterations of enzyme activity and concomitant changes of their physiological behavior. Moreover, mussels lived in water with time-dependent alterations in salinity, maintain different homeostatic mechanisms, in order to face with environmental changes occurred during their annual life. As a result, reduced rate of shell s opening is obtained, in order acute alterations of osmolarity to be prevented, thus maintaining cellular homeostasis. During this period, energy requirements of the organism are supplemented by anaerobic processes, while similar responses are observed during changes in ph levels, both in cellular level and physiological behavior of mussels (Philips, 1976; Davies and Pirie, 1980). High congestion of mussels could lead to increased levels of mortality, representing an important stress-mediated factor from mussels populations. High competition between mussels increases their food demands and could lead to increased levels of water uptake through their gills, thus resulting in increase accumulation of non-self substances in their tissues, including toxins and pollutants. Furthermore, high congestion and competition are related with decrease of both shell length and body weight, probably due to the low rates of food uptake from mussels. Moreover, various biotic factors (such as age, soft-body weight and gametogenesis) could affect normal growth rate of mussels, as well as their response to environmental stress, occurred by pollutants (Philips, 1976; Cossa et al., 1979; Davies and Pirie, 1980). Moreover, sea stars, crabs and fishes represent natural predators of mussels, moderating their spreading geographical zone. Mussels are both aerobic and anaerobic organisms, but oxygen is the primarily source of energy, in order to cover their increased demands for energy (De Zwaan et al., 1992). The percentage of oxygen is depleted in their mitochondria, conjugating with catabolic processes. Despite the fact that glucose, triglycerides, free lipids and amino acids are the main types of energy source under aerobic metabolism, carbohydrates and lipids are considered as the main energy molecules, under anaerobic conditions. Environmental Impact of Anthropogenic Activities 7 2. ENVIRONMENTAL IMPACT OF ANTHROPOGENIC ACTIVITIES: THE USE OF MUSSELS AS A RELIABLE TOOL FOR MONITORING MARINE POLLUTION The strong interaction among living organisms and the various spheres of the abiotic environment, such as atmosphere, hydrosphere and geosphere, are best described by cycles of matter that involve biological, chemical, and geological processes and phenomena. Such cycles are called biogeochemical cycles. Organisms participate in biogeochemical cycles, which describe the circulation of matter, particularly plant and animal nutrients, through ecosystems. An ecosystem consists of a variety of communities of organisms and their surrounding environment existing in a generally steady state. Ecosystems can be divided generally into terrestrial ecosystems, which are consisted of those that exist primarily on land, and aquatic ecosystems, which are composed of those that exist in water. There are many interconnections between terrestrial and aquatic ecosystems, and many ecosystems have both terrestrial and aquatic components. An important aspect of ecosystems is the flow of energy and materials between living organisms and the various spheres of the abiotic environment. Harmonic circulation of energy and materials among them is performed via natural processes, which have been developed during the pass of time. However, during their brief time on earth, humans have used their ingenuity and technology to cause enormous perturbations in these naturally occurring processes. Indeed, this has occurred to such a degree that it is now necessary to recognize a fifth sphere of the environment that is constructed and operated by humans, the anthrosphere. Industrial revolution performed in the last decades lead to the importance of monitoring ecosystems, in order to predict anthrosphere impacts both on ecological and organism level. Since, intricate relationships existed among organisms may be perturbed by the effects of toxicants being present in the ecosystem, it became it bec
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