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Aspects of the eco-physiology of the freshwater crayfish, Parastacoides tasmanicus (Clark 1936)

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Fradd, Philip John (1979) Aspects of the eco-physiology of the freshwater crayfish, Parastacoides tasmanicus (Clark 1936). PhD thesis, University of Tasmania.

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Abstract

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 0C and 39 0C,
but at depths of 40cm the yearly temperature range was probably
not much more than 3 0 to 16 0C, 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 °C, and pH 2.7 - 7.6 at 5 ° C, 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
150 and 20 0 C, 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.weeks, at 15 °C, 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 0C, 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 02 /L at both 50 and 150C. 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.

Item Type: Thesis (PhD)
Keywords: Parastacidae, Crayfish, Fishes, Fishes
Copyright Holders: The Author
Additional Information:

Thesis (Ph.D.)--University of Tasmania, 1981. Bibliography: l. 206-240

Date Deposited: 08 Dec 2014 23:54
Last Modified: 11 Mar 2016 05:56
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