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Crayfish (Procambarus spiculifer) growth rate and final thermal preferendum
J. therm. Biol. Vol. 15, No. 1, pp. 79-81, 1990 Printed in Great Britain. All rights reserved
0306-4565/90 $3.00 + 0.00 Copyright © 1990Pergamon Press plc
CRAYFISH (PROCAMBARUS SPICULIFER) GROWTH RATE A N D FINAL THERMAL P R E F E R E N D U M ROBERT C. TAYLOR Department of Zoology, University of Georgia, Athens, GA 30602, U.S.A.
Abstraet--l. Crayfish growth rate was determined in a wild population and in the laboratory and compared to ambient temperatures. 2. The largest growth increment/molt and the largest mean individual growth rate were observed when habitat temperatures were most similar to the final preferendum of the species. 3. Growth slowed when temperatures exceeded the final preferendum. Key Word Index: Crayfish; Procambarus spiculifer; growth; thermal preferendum
INTRODUCTION
METHODS AND MATERIALS
The adaptive value of an exact thermal preferendum can be determined by comparing a rate optimum with the species' thermal optimum (Beitinger and Fitzpatrick, 1979); an adaptive value would be indicated if the two optima were close to each other. This approach was used to correlate the final preferendum of a crayfish (Procambarus spiculifer) with its growth rate. Lemke (1977) reported that groups of bluegills grew most rapidly at temperatures close to their final preferendum. Crayfish exhibit indeterminate growth, continuing to molt throughout their lives. Increase in size depends on both growth/molt (growth increment), that varies between 2-4 mm/molt (Weagle and Ozburn, 1978; Boyd and Page, 1978; Shimizu and Goldman, 1983) as well as molt frequency (Sadewasser and Prins, 1979; Thorp and Winteriter, 1981; Shimizu and Goldman, 1983). Higher ambient temperatures have been observed to produce larger molt increments (Shimizu and Goldman, 1983; Sv/irdson, 1949; Sadewasser and Prins, 1979) and higher molt frequencies (Thorp and Winteriter, 1981) but Hopkins (1967) found that neither molt increment nor frequency could be related to a single variable. Increment and frequency were also affected by population density, water quality, predator pressure and food availability (Momot, 1967; Momot and Gowing, 1977; Momot et al., 1978; Payne, 1978; Sadewasser and Prins, 1979; Thorp and Winteriter, 1981). I attempted to look at the effect of ambient temperature on the growth of individual crayfish (Procambarus spiculifer). I have several years of habitat temperature data (Taylor, 1983, 1988) from two different temperature regimes as well as information on the species' preferred temperature and final preferendum (23.4°C; Taylor, 1985). This study compared growth rates in two subpopulations of P. spiculifer over a 3 year period. Differences in growth rates between the two study sites indicated the need for a more controlled measure to serve as a reference. The field results were therefore compared to growth under constant laboratory conditions. The crayfish were held at a temperature near their final preferendum (23.4 _ 0.4°C; Taylor, 1984).
The study areas and trapping techniques have been described (Taylor, 1983, 1988). Individual crayfish were marked using pleural clips. Two study sites in the same drainage (Sandy Creek, Clarke County, Ga) were sampled for 1 week each month. Water temperatures were measured each day a sample was taken and on some occasions recording thermographs were set for a 1 month period (Taylor, 1984). One study site was in a 5th-order stream (5th-order) and the other was 13 km upstream in a 2nd-order stream (2nd-order). The results reported in this communication include only two normal rainfall years (1980 and 1982) and a drought year (1981). No significance between year growth differences was found within either site, indicating that the drought (1981) produced no changes in growth rate. In the 2nd-order in both 1983 and 1984 the number of sampled crayfish that had molted was, unfortunately, too small to be included and the population became extinct in 1985. Growth of individually marked crayfish, of both sexes, were determined by comparing their recapture size to their size at the previous capture. The majority of recaptures were made the month following initial marking, although some were recaptured up to 10 months later. The growth in millimetres was converted to mean mm growth/ month, using only those animals that exhibited some growth. This amount will be called the growth increment/molt. The average increase in size for the entire sample will be called the mean growth, calculated over a fixed period of time, usually one year. This measure included both crayfish that had molted and those that had not. Crayfish of both sexes, from a 3rd order stream, in the same drainage system, were held in the laboratory, at room temperatures (22-24°C), for variable periods of time up to 2 yr. Each crayfish was held individually in stainless steel containers (22 x 15 × 15 cm). They were covered with 2-3 cm of aged tap water and fed catfish pellets and composted yellow popular leaves. The size of each crayfish was measured with a calipers when brought into the laboratory and subsequently after each molt. A total 79
80
ROBERT C. TAYLOR
of 130 individuals molted from which an accurate growth increment was calculated.
GROWTH AND HABITAT TEMPERATURES (I &| MCNI~WI"ERVN.m A
RESULTS Growth using only a 1 month interval was used as a reference because over an interval longer than 1 month the chances increased that an animal might have molted more than once. Within sites, when the l month interval growth rates were compared to the average rates for all remaining monthly intervals there was a significant difference [5th-order: F(1,77) = I 1.05 P < 0.005; 2nd-order F(1,85) = 8.65 P <0.001]. However, the 2 month interval did not differ significantly from the 1 month interval, therefore both the 1 and 2 month intervals will be pooled. The percent distribution of the increments were similar in the fifth and second-order sites and the laboratory (Fig. 1). In the 5th-order, crayfish reached a maximum in both mean growth and growth increment in May with a second smaller increase in the fall [Fig. 2(a)]. Both measures, surprisingly, decreased during the warmest months for the year when the habitat temperature exceeded the final preferendum of the species (Taylor, 1985). The 2nd-ordcr never reached the final preferendum of the species (23.4°C) [Fig. 2(c)] and the monthly growth exhibited a rather flat curve during the summer for both mean growth and growth increment [Fig. 2(b)]. Differences between the mean growth and the growth increment resulted from the number of non-molting crayfish that in turn depended on the molt frequency. If the crayfish composing a population show a high molt frequency this will be reflected in a mean population growth that is close to the growth increment. The 5th-order significantly differed from the 2nd-order in both mean growth [ANOVA, F(5,182) = 2,47, P < 0.05] and the size of the growth increment [ANOVA F(5,108) = 3.50, P < 0.005]. The difference was caused by an increase in the 2 mm/molt increment and the decrease in the 3 mm and larger growth/molt in the 2nd-order (Fig. 1). Mean growth increment in the 5th-order was nearly identical to the increment in the laboratory whereas the mean increment in the 2nd-order showed little relationship to the laboratory measures. 5Or
"I
I
[~
P E 3O R C E N T 10
"flj 0
1
LABORATORY 5TH-ORDER 2ND-ORDER
4 5 6 2 3 GROWTH INCREMENT (MM)
i
t
_
7
Fig. 1. The distribution of growth increments/molt for the two study sites and laboratory populations. Note the large increase in the smaller sized increments in the 2nd-order.
M i M
_ ~
GFK)WTHINCREMENT
O R O W T H MAR
A R
MAY
J
JUL TIME
AUG
SEP
B GROWTHINCREMENT
M4" M
MEANGROWTH
G 3' R 02. W T H I 0 MA~
I
i
APR
MAY
I
I
JUN JUt. TIME
I
I
I
AUG
SEP
OCT
C
30 T E 2s M P2o E A T 15 U R to E
-+-- 5TH-ORDER AR
APR
MAY
JUN JUL TIME
AUG
SEP
OCT
Fig. 2. Growth increment and mean individual growth in the 5th-order (A) and the 2nd-order streams (B). Stream temperatures for both study sites are illustrated in (C). Mean growth exhibited a significant negative correlation with both adult and juvenile abundance (p = 0.862 and 0.796 respectively). DISCUSSION Optimal temperatures for complex biological processes, such as enzyme reactions, growth and development, metabolic rate are commonly described in the literature. However, the term "optimal" does not always describe the same circumstances. In some cases the decline in rate at higher temperatures occurs because of heat injury whereas in other cases it does not, and only in these latter instances does the use of the term "optimal" have meaning (Cossins and Bowler, 1987). The establishment of the final preferendum for a species is one such case where optimum temperature has a true biological meaning.
Crayfish growth and temperature The results reported here indicate that the 2ndorder stream temperature never reached the final preferendum of Procambarus spiculifer and the growth rate, likewise, never reached reference levels. The temperature in the 5th-order stream, however, not only reached the species final preferendum but during the summer months exceeded it by about 6 ° . The crayfish growth responded with a maximum during the spring and a second smaller one in the fall and a minimum in mid-summer when habitat temperature exceeded final preferendum. The differences in growth rate between the two streams must contribute to the differences in the mean body size in the two subpopulations. The 5th-order stream has significantly larger annual mean body size than the second (Taylor 1983, 1988). Although the two subpopulations live in different temperatures they are within 15 km of each other thus there are no latitudinal differences to confound the results. However, the two study sites differ in their rate of warming and cooling. The difference is due to the effect of leaf growth in the spring and loss in the fall. The 2ndorder, for example, exhibits a plateau in temperature change in the fall, due to increasing solar insolation due to leaf fall that occurs at the same time as the gradual reduction in ambient temperature. A similar plateau is observed in the spring. Shimizu and Goldman (1983) compared cool Lake Tahoe to the warmer Sacramento River and found the same number of molts in each crayfish age class but greater "increase in size at each molt". They suggested that the difference in temperature was the single most influential environmental factor. Similar changes in increment were also found by Sv~irdson (1949). The spring and fall growth in the 5th-order was greater than growth in the laboratory even though temperatures in the two were very similar. In addition to temperature there are numerous environmental factors in the field that could contribute to an increase in growth rate over the laboratory controls. These could include food, both its abundance and quality, trace elements, flowing water, etc. Stress, including density, can suppress the normal growth response resulting in decreased growth rate that will be reflected in a reduction in mean size of individuals at a given age ( M o m o t and Gowing, 1977). Density, likewise, had a negative effect on growth in this study. It would appear that temperatures near the final preferendum of Procambarus spiculifer will produce the maximum growth increment/molt and the temperatures either above or below that temperature are unfavourable for optimum growth. Temperature
TB ~5/I--F
81
does not appear to have a pronounced effect on the molt frequency. REFERENCES
Beitinger T. L. and Fitzpatrick L. C. (1979) Physiological and ecological correlates of prefered temperature in fish. Am. Zool. 19, 319-329. Boyd J. A. and Page L. M. (1978) The life history of the crayfish Orconectes kentuckiensis in big creek Illinois. Am. Mid. natn 99, 398-414. Cossins A. R. and Bowler K. (1987) Temperature Biology of Animals, p. 339. Chapman, London. Hopkins C. L. (1967) Growth rate in a population of the freshwater crayfish Paranephrops planifrons White. N.Z. Mar. Fw Res. 1, 464-474. Lemke A. B. (1977) Optimum temperature for growth of juvenile bluegills. Prog. Fish Cult. 37, 55-57. Momot W. (1967) Population dynamics and productivity of the crayfish Orconectes virilis in a marl like. Am. Mid. natn 70, 55-81. Momot W. and Gowing H. (1977) Results of an experimental fishery on the crayfish Orconectes z~irilis. J. Fish Bd Can. 34, 2056-2066. Momot W. T., Gowing H. and Jones P. D. (1978) The dynamics of crayfish and their role in ecosytems. Am. Mid. natn 99, 10-35. Payne J. (1978) Aspects of the life histories of selected species of North American crayfishes. Fisheries 3, 5-7. Sadewasser S. G. and Prins R. (1979) The effects of temperature and photoperiod on molting in seasonal population of the crayfish Orconectes rusticus rusticus. Trans. Ky Acad. Sci. 40, 129-140. Shimizu S. and Goldman C. R. (1983) Pacifastacus leniusculus (Dana) production in the Sacramento River. Freshwater Crayfish (Edited by Goldman C. R.), pp. 210--220. Westport, Conn. Svardson G. (1949) Stunted crayfish population in Sweden. Rep. Inst. F Res. Drottingholm. 29, 135-145. Taylor R. C. (1983) Drought induced changes in crayfish populations along a stream continuum. Am. Mid. natn 110, 286-298. Taylor R. C. (1984) Thermal preference and temporal distribution in three crayfish species. Comp. Biochem. Physiol. 77A, 513--517. Taylor R. C. (1985) Absence of Form I to Form II alternation in male Procambarus spiculifer (Cambaridae) Am. Mid. natn 114, 145-151. Taylor R. C. (1988) Populations dynamics of the crayfish Procambarus spiculifer observed in different-sized streams in response to two droughts. J. crust. Biol. 8, 401-409. Thorp J. and Winetriter S. (1981) Stress and growth response of juvenile crayfish to rhythmic and arrhythmic temperature fluctuations. Archs Envir. Contain. Toxicol. 10, 69 77. Weagle K. V. and Ozburn G. W. (1972) Observations on aspects of the life history of the crayfish Orconetes virilis (Hagen), in Northwestern Ontario. Can. J. Zool. 50, 366 370.
0306-4565/90 $3.00 + 0.00 Copyright © 1990Pergamon Press plc
CRAYFISH (PROCAMBARUS SPICULIFER) GROWTH RATE A N D FINAL THERMAL P R E F E R E N D U M ROBERT C. TAYLOR Department of Zoology, University of Georgia, Athens, GA 30602, U.S.A.
Abstraet--l. Crayfish growth rate was determined in a wild population and in the laboratory and compared to ambient temperatures. 2. The largest growth increment/molt and the largest mean individual growth rate were observed when habitat temperatures were most similar to the final preferendum of the species. 3. Growth slowed when temperatures exceeded the final preferendum. Key Word Index: Crayfish; Procambarus spiculifer; growth; thermal preferendum
INTRODUCTION
METHODS AND MATERIALS
The adaptive value of an exact thermal preferendum can be determined by comparing a rate optimum with the species' thermal optimum (Beitinger and Fitzpatrick, 1979); an adaptive value would be indicated if the two optima were close to each other. This approach was used to correlate the final preferendum of a crayfish (Procambarus spiculifer) with its growth rate. Lemke (1977) reported that groups of bluegills grew most rapidly at temperatures close to their final preferendum. Crayfish exhibit indeterminate growth, continuing to molt throughout their lives. Increase in size depends on both growth/molt (growth increment), that varies between 2-4 mm/molt (Weagle and Ozburn, 1978; Boyd and Page, 1978; Shimizu and Goldman, 1983) as well as molt frequency (Sadewasser and Prins, 1979; Thorp and Winteriter, 1981; Shimizu and Goldman, 1983). Higher ambient temperatures have been observed to produce larger molt increments (Shimizu and Goldman, 1983; Sv/irdson, 1949; Sadewasser and Prins, 1979) and higher molt frequencies (Thorp and Winteriter, 1981) but Hopkins (1967) found that neither molt increment nor frequency could be related to a single variable. Increment and frequency were also affected by population density, water quality, predator pressure and food availability (Momot, 1967; Momot and Gowing, 1977; Momot et al., 1978; Payne, 1978; Sadewasser and Prins, 1979; Thorp and Winteriter, 1981). I attempted to look at the effect of ambient temperature on the growth of individual crayfish (Procambarus spiculifer). I have several years of habitat temperature data (Taylor, 1983, 1988) from two different temperature regimes as well as information on the species' preferred temperature and final preferendum (23.4°C; Taylor, 1985). This study compared growth rates in two subpopulations of P. spiculifer over a 3 year period. Differences in growth rates between the two study sites indicated the need for a more controlled measure to serve as a reference. The field results were therefore compared to growth under constant laboratory conditions. The crayfish were held at a temperature near their final preferendum (23.4 _ 0.4°C; Taylor, 1984).
The study areas and trapping techniques have been described (Taylor, 1983, 1988). Individual crayfish were marked using pleural clips. Two study sites in the same drainage (Sandy Creek, Clarke County, Ga) were sampled for 1 week each month. Water temperatures were measured each day a sample was taken and on some occasions recording thermographs were set for a 1 month period (Taylor, 1984). One study site was in a 5th-order stream (5th-order) and the other was 13 km upstream in a 2nd-order stream (2nd-order). The results reported in this communication include only two normal rainfall years (1980 and 1982) and a drought year (1981). No significance between year growth differences was found within either site, indicating that the drought (1981) produced no changes in growth rate. In the 2nd-order in both 1983 and 1984 the number of sampled crayfish that had molted was, unfortunately, too small to be included and the population became extinct in 1985. Growth of individually marked crayfish, of both sexes, were determined by comparing their recapture size to their size at the previous capture. The majority of recaptures were made the month following initial marking, although some were recaptured up to 10 months later. The growth in millimetres was converted to mean mm growth/ month, using only those animals that exhibited some growth. This amount will be called the growth increment/molt. The average increase in size for the entire sample will be called the mean growth, calculated over a fixed period of time, usually one year. This measure included both crayfish that had molted and those that had not. Crayfish of both sexes, from a 3rd order stream, in the same drainage system, were held in the laboratory, at room temperatures (22-24°C), for variable periods of time up to 2 yr. Each crayfish was held individually in stainless steel containers (22 x 15 × 15 cm). They were covered with 2-3 cm of aged tap water and fed catfish pellets and composted yellow popular leaves. The size of each crayfish was measured with a calipers when brought into the laboratory and subsequently after each molt. A total 79
80
ROBERT C. TAYLOR
of 130 individuals molted from which an accurate growth increment was calculated.
GROWTH AND HABITAT TEMPERATURES (I &| MCNI~WI"ERVN.m A
RESULTS Growth using only a 1 month interval was used as a reference because over an interval longer than 1 month the chances increased that an animal might have molted more than once. Within sites, when the l month interval growth rates were compared to the average rates for all remaining monthly intervals there was a significant difference [5th-order: F(1,77) = I 1.05 P < 0.005; 2nd-order F(1,85) = 8.65 P <0.001]. However, the 2 month interval did not differ significantly from the 1 month interval, therefore both the 1 and 2 month intervals will be pooled. The percent distribution of the increments were similar in the fifth and second-order sites and the laboratory (Fig. 1). In the 5th-order, crayfish reached a maximum in both mean growth and growth increment in May with a second smaller increase in the fall [Fig. 2(a)]. Both measures, surprisingly, decreased during the warmest months for the year when the habitat temperature exceeded the final preferendum of the species (Taylor, 1985). The 2nd-ordcr never reached the final preferendum of the species (23.4°C) [Fig. 2(c)] and the monthly growth exhibited a rather flat curve during the summer for both mean growth and growth increment [Fig. 2(b)]. Differences between the mean growth and the growth increment resulted from the number of non-molting crayfish that in turn depended on the molt frequency. If the crayfish composing a population show a high molt frequency this will be reflected in a mean population growth that is close to the growth increment. The 5th-order significantly differed from the 2nd-order in both mean growth [ANOVA, F(5,182) = 2,47, P < 0.05] and the size of the growth increment [ANOVA F(5,108) = 3.50, P < 0.005]. The difference was caused by an increase in the 2 mm/molt increment and the decrease in the 3 mm and larger growth/molt in the 2nd-order (Fig. 1). Mean growth increment in the 5th-order was nearly identical to the increment in the laboratory whereas the mean increment in the 2nd-order showed little relationship to the laboratory measures. 5Or
"I
I
[~
P E 3O R C E N T 10
"flj 0
1
LABORATORY 5TH-ORDER 2ND-ORDER
4 5 6 2 3 GROWTH INCREMENT (MM)
i
t
_
7
Fig. 1. The distribution of growth increments/molt for the two study sites and laboratory populations. Note the large increase in the smaller sized increments in the 2nd-order.
M i M
_ ~
GFK)WTHINCREMENT
O R O W T H MAR
A R
MAY
J
JUL TIME
AUG
SEP
B GROWTHINCREMENT
M4" M
MEANGROWTH
G 3' R 02. W T H I 0 MA~
I
i
APR
MAY
I
I
JUN JUt. TIME
I
I
I
AUG
SEP
OCT
C
30 T E 2s M P2o E A T 15 U R to E
-+-- 5TH-ORDER AR
APR
MAY
JUN JUL TIME
AUG
SEP
OCT
Fig. 2. Growth increment and mean individual growth in the 5th-order (A) and the 2nd-order streams (B). Stream temperatures for both study sites are illustrated in (C). Mean growth exhibited a significant negative correlation with both adult and juvenile abundance (p = 0.862 and 0.796 respectively). DISCUSSION Optimal temperatures for complex biological processes, such as enzyme reactions, growth and development, metabolic rate are commonly described in the literature. However, the term "optimal" does not always describe the same circumstances. In some cases the decline in rate at higher temperatures occurs because of heat injury whereas in other cases it does not, and only in these latter instances does the use of the term "optimal" have meaning (Cossins and Bowler, 1987). The establishment of the final preferendum for a species is one such case where optimum temperature has a true biological meaning.
Crayfish growth and temperature The results reported here indicate that the 2ndorder stream temperature never reached the final preferendum of Procambarus spiculifer and the growth rate, likewise, never reached reference levels. The temperature in the 5th-order stream, however, not only reached the species final preferendum but during the summer months exceeded it by about 6 ° . The crayfish growth responded with a maximum during the spring and a second smaller one in the fall and a minimum in mid-summer when habitat temperature exceeded final preferendum. The differences in growth rate between the two streams must contribute to the differences in the mean body size in the two subpopulations. The 5th-order stream has significantly larger annual mean body size than the second (Taylor 1983, 1988). Although the two subpopulations live in different temperatures they are within 15 km of each other thus there are no latitudinal differences to confound the results. However, the two study sites differ in their rate of warming and cooling. The difference is due to the effect of leaf growth in the spring and loss in the fall. The 2ndorder, for example, exhibits a plateau in temperature change in the fall, due to increasing solar insolation due to leaf fall that occurs at the same time as the gradual reduction in ambient temperature. A similar plateau is observed in the spring. Shimizu and Goldman (1983) compared cool Lake Tahoe to the warmer Sacramento River and found the same number of molts in each crayfish age class but greater "increase in size at each molt". They suggested that the difference in temperature was the single most influential environmental factor. Similar changes in increment were also found by Sv~irdson (1949). The spring and fall growth in the 5th-order was greater than growth in the laboratory even though temperatures in the two were very similar. In addition to temperature there are numerous environmental factors in the field that could contribute to an increase in growth rate over the laboratory controls. These could include food, both its abundance and quality, trace elements, flowing water, etc. Stress, including density, can suppress the normal growth response resulting in decreased growth rate that will be reflected in a reduction in mean size of individuals at a given age ( M o m o t and Gowing, 1977). Density, likewise, had a negative effect on growth in this study. It would appear that temperatures near the final preferendum of Procambarus spiculifer will produce the maximum growth increment/molt and the temperatures either above or below that temperature are unfavourable for optimum growth. Temperature
TB ~5/I--F
81
does not appear to have a pronounced effect on the molt frequency. REFERENCES
Beitinger T. L. and Fitzpatrick L. C. (1979) Physiological and ecological correlates of prefered temperature in fish. Am. Zool. 19, 319-329. Boyd J. A. and Page L. M. (1978) The life history of the crayfish Orconectes kentuckiensis in big creek Illinois. Am. Mid. natn 99, 398-414. Cossins A. R. and Bowler K. (1987) Temperature Biology of Animals, p. 339. Chapman, London. Hopkins C. L. (1967) Growth rate in a population of the freshwater crayfish Paranephrops planifrons White. N.Z. Mar. Fw Res. 1, 464-474. Lemke A. B. (1977) Optimum temperature for growth of juvenile bluegills. Prog. Fish Cult. 37, 55-57. Momot W. (1967) Population dynamics and productivity of the crayfish Orconectes virilis in a marl like. Am. Mid. natn 70, 55-81. Momot W. and Gowing H. (1977) Results of an experimental fishery on the crayfish Orconectes z~irilis. J. Fish Bd Can. 34, 2056-2066. Momot W. T., Gowing H. and Jones P. D. (1978) The dynamics of crayfish and their role in ecosytems. Am. Mid. natn 99, 10-35. Payne J. (1978) Aspects of the life histories of selected species of North American crayfishes. Fisheries 3, 5-7. Sadewasser S. G. and Prins R. (1979) The effects of temperature and photoperiod on molting in seasonal population of the crayfish Orconectes rusticus rusticus. Trans. Ky Acad. Sci. 40, 129-140. Shimizu S. and Goldman C. R. (1983) Pacifastacus leniusculus (Dana) production in the Sacramento River. Freshwater Crayfish (Edited by Goldman C. R.), pp. 210--220. Westport, Conn. Svardson G. (1949) Stunted crayfish population in Sweden. Rep. Inst. F Res. Drottingholm. 29, 135-145. Taylor R. C. (1983) Drought induced changes in crayfish populations along a stream continuum. Am. Mid. natn 110, 286-298. Taylor R. C. (1984) Thermal preference and temporal distribution in three crayfish species. Comp. Biochem. Physiol. 77A, 513--517. Taylor R. C. (1985) Absence of Form I to Form II alternation in male Procambarus spiculifer (Cambaridae) Am. Mid. natn 114, 145-151. Taylor R. C. (1988) Populations dynamics of the crayfish Procambarus spiculifer observed in different-sized streams in response to two droughts. J. crust. Biol. 8, 401-409. Thorp J. and Winetriter S. (1981) Stress and growth response of juvenile crayfish to rhythmic and arrhythmic temperature fluctuations. Archs Envir. Contain. Toxicol. 10, 69 77. Weagle K. V. and Ozburn G. W. (1972) Observations on aspects of the life history of the crayfish Orconetes virilis (Hagen), in Northwestern Ontario. Can. J. Zool. 50, 366 370.