| Summary: | During the last two decades, much public interest and concern has been expressed on the presence of metal ions in the environment and, particularly, in organisms of commercial significance. The concern has arisen because the amounts of such ions has, in local and sometimes general areas , increased to levels which are far higher than 'normal ' background values. The present thesis is concerned with studies on the effects of 3 metals on the brown shrimp, Crangon crangon - a species which is common in the inshore waters of the Yorkshire coast and which has a commercial significance to a number of local fisheries. Of the metals selected for study, cadmium is known to be neither biologically essential nor beneficial to living organisms (Eisler, 1971; Thorpe & Lake, 1974; Bryan, 1976; Pascoe & Mattey, 1977). Consequently, this metal is absent, or present only in trace amounts, in organisms from unpolluted waters. By contrast, copper is an essential element for several living processes, having first been associated with such in studies of blood proteins of Helix pomatia (Harless, 1847 - cited in Severy, 1923). Since then, its functional significance in the oxygen transport of crustacean and molluscan haemocyanins has became well documented (Redmond, 1955). Copper is known also to be an important constituent of certain enzymes such as tyrosinase and cytochrome oxidase (Scott & Major, 1972) and as an activator for certain others (e.g. malate dehydrogenase; Saliba & Krzyz, 1976). Furthermore, copper has been shown to be a necessary component for the successful accomplishment of specific behavioural and physiological phenomena (e.g. the settling and metamorphosis of the oyster, Crassostrea virginica; Prytherch, 1931). Possibly, the ubiquitous distribution of this metal accounts for its wide biological functions in organisms. Zinc also has been found to occur naturally in the tissues of many marine organisms (Bodansky, 1920) and, subsequently, has been shown to be an essential element in many metal-enzymes (Dixon & Webb, 1964). These enzymes include carbonic anhydrase (Keilin & Mann, 1940; Vallee, 1959; Coombs, 1972), alkaline phosphatase (Vallee, 1962; Wolfe, 1970; Coombs, 1972), carboxypeptidase (Vallee, 1962; Coombs, 1972), glutamate dehydrogenase, lactic dehydrogenase, alcohol dehydrogenase (Vallee, 1959) and -D- mannosidase, (Coombs, 1972). Parker (1962) has suggested that the capacity of organisms to concentrate zinc reflects the biological function of this metal. However, as with copper, zinc is usually found in tissues in quantities far in excess of those, required to satisfy the needs of enzymes. Coombs (1972), for example, used data of Vallee & Wacker (1970) to estimate that the oyster, Ostrea edulis used only 0·1% or less of its total zinc content for enzyme purposes. Heavy metals are ·normal constituents of estuarine and marine environments and are usually present in trace amounts. Normally, they reach the sea via rivers following the erosion of rocks. However, with the advent of industrialization, man has contributed to the base levels of metals found in coastal waters. Cairns, Dickson, Sparks & Waller (1970) summarised the industrial attitude as "the production of wastes by industry is not related to the capacity of the ecosystem to absorb and transform these wastes, but rather to market demand." Hence, large quantities of cadmium, copper and zinc, among other metals, have found their way in estuarine and coastal waters, mainly from copper and lead mines (McKee & Wolfe, 1963; Mount & Steven, 1967) and zinc smelting and electroplanting plants (Little & Martin, 1972; Jordan, 1975). By virtue of their physicogeochemical nature, estuaries have the capacity (albeit to a limited extent) to 'detoxify ' heavy metals by altering their biological availability. This is achieved by absorption to particulate material (Krauskopf, 1956) and by precipitation, chelation and sedimentation (Lewis , Whitfield & Ramnarine , 1972; Whitfield & Lewis , 1976; Batley & Gardner, 1978). However, those metal species which remain dissolved in seawater are likely to escape to the open sea (van Bennekom, Gieskes & Tijssen, 1975) and, if present in sufficiently high concentrations, are likely to be directly toxic to the fauna and flora. Many organisms , especially sessile bivalves, can accumulate cadmium, copper and zinc and tolerate high concentrations of these in their tissues without any apparent signs of harm (Brooks & Rumsby, 1965)'. This suggests that such organisms have very efficient methods for preventing these metals from poisoning essential enzyme systems . Other organisms (e .g. Paratya tasmaniensis; Thorpe & Lake, 1974) ,d.o not have the ability to tolerate these metals and are killed by very low concentrations of them. There appear to be few reliable data available on the susceptibility of C. crangon to heavy metals and these studies were undertaken to provide comprehensive data on the toxicity of cadmium, copper and zinc to this species . One widely used and accepted method for the assessment of the effects of pollutants to organisms, is that of toxicity testing (sometimes referred to as 'bioassays '). When death is used as the criterion of response in such studies, the method suffers serious disadvantages in that accuracy is limited because of the wide disparity of individual susceptibility. However, in toxicity studies, it is accepted universally that the most tolerant and the most susceptible individuals in a test group show greater variability of response than individuals near the median of this group. Consequently, the relevant stdies in this thesis are concerned predominantly with the median responses to the test parameters (i.e. the responses of the average individual ). The median lethal concentration (LC50) is the term used generally to describe the concentration at which 50% of the test population are killed Alderdice, 1967; Brown, Jordan & Tiller, 1967; Sprague, 1969; Eisler, 1971). In situations where time is the effect parameter, the median lethal time (LT50) is used to describe 50% mortality value. Concentration and time, however, are here inextricably linked and to maximise their usefulness in comparative studies, LC50 values need to be qualified by a prefixed time component. APHA (1965) has suggested that the time component may be expressed as hours, days or weeks, whichever is convenient in particular circumstances. Similarly, LC50 values should ~e qualified by the concentration of the toxicant used. Median lethal concentrations and LT50 values are determined by graphical means from plots of concentration or time respectively against the percentage mortality of the test population at specific times (LC50) or concentration (LT50). Brown (1973) suggested that 'quantal' bioassays (using concentrations and percentage mortalities) are superior to 'quantitive' bioassays involving exposure times and percentage mortalities. His reasoning is based on the fact that quantal bioassays yield mortality curves which are amenable to mathematical definition and thus enable confidence limits to be given in terms of units of concentration. On the other hand, quantitive bioassay mortality curves represent subjective estimates of effective concentrations. However, when experimental methodology imposes limitations to the design of experiments (e.g. in flow systems which permit but one concentration at a time to be tested) then quantitative methods offer an acceptable alternative. A very important concept in toxicity studies is that of incipient lethal levels (ILL). Sprague (1969) defined an ILL as "that level of the environmental entity beyond which 50% of the population cannot live for an indefinite time." The same concept has been named the 'lethal threshold concentration' (Lloyd & Jordan, 1963) or the 'asymptotic LC50' (Ball, 1967a).
|
|---|
