Environmental tolerances of three species of freshwater crayfish
The only species of crayfish native to the British Isles, i.e. Austropotamobius pallipes, is presently widely distributed throughout much of England, Wales and Ireland. The main controlling factor behind this distribution is the predominantly "bicarbonate" freshwater system of the British Isles. In geological terms, native crayfish are generally found in areas of chalk, carboniferous or magnesium limestone, or drift deposits of boulder clay. More recently, native crayfish have been affected by a general decline in the quality of Britain's freshwater system largely due to sewage effluent and agricultural. By far the majority of native crayfish records are associated with water bodies of very good or good quality, with the result that many populations of A. pallipes are isolated in small, good quality headwaters, due to the degree of pollution in the main rivers acting as "chemical barriers". The last two decades have seen a number of alien species of crayfish imported into England and Wales for aquaculture and culinary purposes. Deliberate and accidental introductions have resulted in the establishment of populations of these species in the wild, the most widespread being the North American signal crayfish Pacifastacus leniusculus and the European slender-clawed crayfish Astacus leptodactylus. An important and devastating consequence of the introduction of alien species of crayfish has been the spread of the crayfish plague fungus Aphanomyces astaci, possibly initiated by infected P. leniusculus. Both A. pallipes and A. leptodactylus are susceptible to the disease, with the result that populations of A. pallipes have been eliminated from, whole lengths of river, such as the River Kennet and the Hampshire River Avon. The nature of the disease and the ease with which it is spread means that crayfish is an ongoing problem in England and Wales, with new populations of A. pallipes regularly becoming affected. The distribution of alien species in the British Isles is an artificial one, initially determined by man, although consideration of water quality may have determined the choice of aquaculture sites. However, established populations of alien species in the wild will be subject to the same constraints as A. pallipes, which will affect their subsequent spread and distribution in the freshwater system of England and Wales. A greater tolerance of environmental factors may allow alien species to inhabit waters not currently occupied by the native species, such as polluted or estuarial waters, and possibly threaten populations of A. pallipes protected by "chemical barriers". This study investigated and compared some of the environmental tolerances of A. pallipes, A. leptodactylus and P. leniusculus and consisted of two parts: 1.) Comparison of the tolerance of A. pallipes, A. leptodactylus and P. leniusculus to four common pollutants; chloride, copper, ammonia and lindane, using lethal and sublethal toxicity tests. No species was found to have a greater overall pollution tolerance on the basis of the toxicants used in this study. Median lethal concentrations (LC50) obtained from lethal studies with stage II juveniles indicated that A. leptodactylus juveniles were most tolerant of chloride, but were very sensitive to lindane. P. leniusculus stage II juveniles were least tolerant of chloride, but of equal or greater tolerance when tested in larger juvenile stages, and were most tolerant of copper. A. pallipes juveniles were very sensitive to copper, and all three species showed a similar sensitivity to ammonia. However, episodic experiments with ammonia and copper indicated that differential short-term tolerance to toxicants, i.e. less than 24 hours, may be important in determining the outcome of episodic pollution events on crayfish populations. Salinity studies indicated high resistance to chloride, with all three species able to tolerate salinities up to 50% seawater, implying freshwater crayfish may be able to tolerate estuarine conditions. Accumulation studies with P. leniusculus inferred resistance to high levels of copper in the environment, with a substantial storage capacity for the metal and a regulatory mechanism for excretion of excess amounts. However, establishment of populations under high saline or high copper conditions would be limited by the lower tolerance of early life stages. Similarly, sublethal studies implied that sublethal toxicant effects on early life stages may be important in determining presence or absence of crayfish from a waterbody, through effects on recruitment. 2.) Comparison of the thermal relations of A. pallipes, A. leptodactylus and P. leniusculus, using tolerance, growth and respiration experiments. Results from tolerance experiments showed that P. leniusculus had a greater overall thermal tolerance, so is more resistant to changes in environmental temperature, such as in waters affected by thermal discharges or in waters affected by drought and reduced flow. In growth experiments P. leniusculus grew faster at all temperatures tested and was predicted to grow at temperatures unsuitable for the other species. Therefore, P. leniusculus would not only be able to survive and grow in conditions unsuitable for A. pallipes and A. leptodactylus, but will also grow faster where favourable conditions exist for all three species. Field experiments showed that, although P. leniusculus juveniles were smaller on release from the female, they were released earlier and their faster growth rate allowed them to maintain a distinct size advantage over A. pallipes juveniles, which was very marked by the end of the growing season. Large size is a key element in the attributes leading to competitive success in other crayfish species. Field observations on mixed populations of plague-free P. leniusculus and A. pallipes indeed show that signal crayfish are superior competitors, eventually eliminating A. pallipes. Therefore temperature effects may be important in determining the outcome of competition in mixed crayfish populations.