Aspects of the eco-physiology of the freshwater crayfish, Parastacoides tasmanicus (Clark 1936)
Creators: name: Fradd, PJ
Publisher: name: University Of Tasmania
Year: 2023

description: Parastacoides tasmanicus is a burrowing crayfish found on button grass plains in western Tasmania. While this project was in progress, the taxonomy of the genus Parastacoides was revised, and one species, P. tasmanicus, with three sub-species, P. tasmanicus inermis, P. tasmanicus tasmanicus and P. tasmanicus insignis, was recognized. P. tasmanicus inermis lives in drier areas than the other two subspecies and was not present at the site near Scott's Peak Dam where animals for this study were collected. P. tasmanicus insignis and P. tasmanicus tasmanicus, which have largely overlapping habitats, were both found to be present, but although they are considered to be subspecies, they differ only slightly morphologically, and physiologically, and so were not distinguished between. Except where it is otherwise stated, references to P. tasmanicus refer to both of these subspecies. Measurements of a number of environmental factors over the course of the study showed that P. tasmanicus at Scott's Peak Dam are exposed to low pH (3.7 - 5. 6), low oxygen levels (at least as low at 0.86 mL/L), and periods without free water in their burrows, although it is unlikely that the relative humidity at the bottom of the burrows would fall much below 100%, as even just inside the mouth of 'dry' burrows the relative humidity is 80% or more. Surface temperatures during the study ranged between -3¬¨‚àûC and 39¬¨‚àûC, but at depths of 40cm the yearly temperature range was probably not much more than 3¬¨‚àûC to 16¬¨‚àûC, the range of burrow temperatures on collecting trips. Almost 600 animals were collected at the Scott's Peak Dam site, 427 of which were over 1g weight. The weight-frequency distribution of these animals showed no detectable year classes, but there was a distinct difference in the weight-frequency distribution of males and females. Males appear to grow at a steady rate throughout their life, while the growth of females slows dramatically when a weight of approximately 3 g is reached. Females become sexually mature at this size, and 50% of the females over this weight are in berry or are carrying hatched young from the end of May until the end of January. No females are in berry in March. P. tasmanicus is very tolerant of a wide range of pH. Oxygen consumption was found to be unaffected by pH in the range of approximately pH 2.7 to 10.0 at 15¬¨‚àûC, and pH 2.7 - 7.6 at 5¬¨‚àûC, whilst haemolymph pH was only slightly affected by an external pH range of approximately pH 3 - 11 during a 110 hour exposure period. When P. tasmanicus was exposed to pH 2.5 there was a large loss of sodium ions to the external medium, although no loss of potassium was observed. At a pH of 4.8 the loss of both of these ions was negligible. It was concluded that whilst the tolerance to both high and low pH by this crayfish is quite remarkable, the causes of death at both very high and very low pH do not appear to be different from those of animals less tolerant to extremes of pH. When kept out of water at high humidities and at temperatures of 15¬¨‚àû and 20¬¨‚àûC, Parastacoides was found to have a low rate of water loss and a high lethal water loss compared to other crustaceans. Both adults and juveniles can survive indefinitely out of water at 100% relative humidity, if they have access to damp filter paper, but they are not able to moult successfully. Large animals without access to free water survived at 100% relative humidity for up to 7 \\(^1\\)/\\(_2\\) weeks, at 15¬¨‚àûC, while smaller animals survived for shorter periods. At lower relative humidities survival time is reduced, but survival times for P. tasmanicus are higher than those of most other semi-terrestrial and terrestrial decapods. The water lost by P. tasmanicus in humid conditions is almost entirely lost via the gill chambers, with negligible water loss via the integument. Parastacoides tasmanicus normally feeds on fresh or decomposing plant material, although animal food is taken when available. A study of the digestive enzymes of the crayfish, at test temperatures considerably above environmental temperatures, and at the optimal pH of the enzymes concerned, demonstrated a moderate lipase/strong esterase, strong protease, amylase and cellulase activities, and weak 'native cellulase' and chitinase activities. The lower activities observed at environmental temperatures would be counteracted by the slow passage of food through the gut, since rate of passage decreases as temperature decreases. Measurements of assimilation efficiencies of P. tasnvnicus eating controlled diets, showed that animal food is assimilated with efficiencies of over 88%, while plant food is assimilated with an efficiency greater than 72%, at both summer and winter temperatures. When fed button grass mud, the crayfish is able to select the high-energy food components from the mud in preference to the lower-energy components and inorganic material. A study of the metabolic activity of P. tasmanicus (as measured by the rate of oxygen consumption), showed that the crayfish has a lower oxygen consumption, at normal environmental temperatures, than other decapod crustaceans. It shows very little in the way of compensation for seasonal temperature changes, and so its oxygen consumption exhibits a yearly cycle, with a maximum in February (for 1 g animals) or March (for animals of 5 g weight or over) and a minimum in August. The maximum rate is 2 to 4 times the minimum rate, with seasonal temperature changes affecting the respiration of smaller animals more than that of larger animals. Animals moult in summer and this is accompanied by an increase in oxygen consumption, with a maximum rate in early post-moult. Annual variation in the organic composition of the major tissues of male, juvenile and berried and non-berried females was measured. These measurements showed that females only breed once every two years, and exhibit a two year berried - non-berried cycle. In addition, the moult at the end of the 'berried' part of the cycle probably does not involve any increase in size. During the non-berried part of the cycle, mainly during the warmer months, energy stores in the midgut gland, in the form of lipids, increase in preparation for a 'growth' moult. At the same time, the gonads are increasing in size and stage of development, so that moulting can be rapidly followed by egg production. During the berried part of the cycle the energy stores in the midgut gland and other tissues remain low, and the gonads do not grow very large. Adult males moult once a year like the females, but every moult is a 'growth' moult. The males have a body composition similar to that of berried females. The eggs of P. tasmanicus are larger and have larger energy stores than the eggs of most other decapods. The relevance of this to the reproductive strategy of P. tasmanicus, and the survival strategy of juvenile animals,is discussed. It is concluded that there is only a small recruitment of juveniles into the population each year, and successful juveniles will be those that are a large size at birth, and grow rapidly so that they can find a burrow and defend it against other juveniles. It is estimated that crayfish live about 8 years. A study of the energy content of the tissues of P. tasmanicus supported the conclusions reached from body composition data. The energy content of P. tasmanicus is similar to that of benthic malacostracans; any differences can probably be attributed to incorrect techniques used by other researchers. Parastacoides tasmanicus exhibits a unique set of responses to low oxygen conditions. When exposed to oxygen concentrations of 0.8 mL/L at 17¬¨‚àûC, or to lower oxygen levels at lower temperatures, P. tasmanicus may leave the water to respire in air. The crayfish is capable of regulating its oxygen consumption down to approximately 4 mL O\\(_2\\) /L at both 5¬¨‚àû and 15¬¨‚àûC. Below this oxygen level, oxygen consumption decreases with decreasing oxygen tension. This is not a particularly low incipient limiting tension, but the important point is that the incipient lethal tension is low. Parastacoides tasmanicus can reduce its activity to reduce its oxygen demand, and respires anaerobically if oxygen levels are low enough. It does not have a very high tolerance towards lactic acid, but as long as the oxygen levels are not too low, it is able to excrete the lactic acid that it produces. It does not pay back an oxygen debt when it is returned to aerobic conditions after a period in anaerobic conditions. This would be wasteful in some situations, but is an important part of the strategy used by P. tasmanicus for coping with chronic low oxygen levels. The adaptations used by P. tasmanicus to cope with different aspects of its environment are interrelated, some mechanisms being used for multiple purposes, and other mechanisms affecting many aspects of the life cycle of the crayfish. These relationships are briefly considered in the final section of this thesis.