COMPLEMENTARITY OF THERMOREGULATORY RHYTHMS IN MICROPTERUS SALMOIDES ANDM. DOLOMIEUI

William W . REYNOLDS & Martha E. CASTERLIN

Department of Biology, The Pennsylvania State University, Wilkes-Barre, Pennsylvania 18708 U .S.A .

Received December 7, 1977

Keywords : behavioral thermoregulation, temperature preference, blackbasses, Micropterus, niche segregation, circadian rhythms .

Abstract

Largemouth (Micropterus salmoides) and smallmouth (M.dolomieui) blackbasses tested in an electronic shuttlebox ex-hibited behavioral thermoregulatory rhythms which were tem-porally complementary . With a LD 12 : 12 photoperiod, M.dolomieui exhibited a preferred-temperature peak of 30 .1°Cduring the latter portion of the photophase, when M. salmoidesreached a minimum of 27 .1°C. M. dolomieui exhibited a mini-mum of 26 .6°C during the latter portion of scotophase, while M .salmoides remained at a significantly higher plateau of about29°C, with a peak of 29 .5°C at the midpoint of scotophase . Thephase relations of the thermoregulatory rhythms relative tophotoperiod suggest that they are endogenously timed circadianrhythms entrained by photoperiod . The thermotemporal com-plementarity of these rhythms suggests an aspect of niche segre-gation between these largely sympatric congeneric species .

Introduction

The largemouth (Micropterus salmoides) and small-mouth (M. dolomieui) blackbasses (Centrarchidae) aresympatric over much of their ranges, often occurring to-gether in the same bodies of water (Vogele & Rainwater,1975) . This raises the question of how these morphologi-cally similar congeneric species segregate their niches inspace and time in such a manner as to partition limitedresources between them with a minimum of interferencecompetition . Previous studies in field and laboratoryhave indicated subtle differences in light intensity prefe-rences and locomotor activity rhythms (Reynolds, 1977 ;Reynolds & Casterlin, 1976a), habitat and spawning-site

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preferences (Vogele & Rainwater, 1975), and frequencydistributions of preferred temperature (Reynolds & Cas-terlin, 1976b) . Body temperatures of behaviorally ther-moregulating bass correspond closely to occupied watertemperatures (Reynolds, McCauley, Casterlin & Craw-shaw, 1976), except when ambient temperatures arechanging rapidly (Reynolds, 1977) . Earlier work (Rey-nolds & Casterlin, 1976b) indicated differences in pre-ferred temperature between night and day, suggesting acircadian thermoregulatory rhythm . Such rhythms havebeen found to exist in the goldfish Carassius auratus (Rey-nolds, 1977; Reynolds, Casterlin, Matthey, Millington& Ostrowski, in press), and in the bowfin Amia calva(Reynolds, Casterlin & Millington, in press) . The purposeof the present investigation was to determine hourlymean preferred temperatures in relation to photoperiod,to learn the extent of circadian rhythmicity in the ther-moregulatory patterns of largemouth and smallmouthbasses, and to examine the thermotemporal patterns ofthe two species for evidence of niche complementarity .

Methods and materials

We tested individually twelve yearling (100-200 g) bass ofeach species in an electronic thermoregulatory shuttlebox(Ichthyotron) described previously (Reynolds, 1977 ; Rey-nolds, McCauley, Casterlin & Crawshaw, 1976), whichpermits a fish to control water temperatures, and therebyits own body temperature, by its swimming movementsbetween two chambers . Photocells and associated cir-cuitry monitor the movements and control heating andcooling equipment accordingly .

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All the fish were acclimatized to identical laboratoryconditions (22 ± 2'C, LD 12 : 12) for at least 2 weeks priorto testing . Individuals of each species were alternated inthe testing sequence . Each individual was permitted aninitial 24-hr adjustment period allowing gravitation tothe final preferendum (Richards, Reynolds & McCauley,1977), after which mean hourly occupied temperatureswere recorded for 48 hr . Data for all individuals of a spe-cies were pooled, and the pooled mean hourly preferredtemperatures were plotted relative to the LD 12 : 12photoperiod (Fig. I) .

Results

M. dolomieui exhibited a preferred-temperature peak of30.1°C during the latter portion of the photophase (light

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Fig. t . Mean hourly preferred temperatures of largemouth blackbass ( .) and smallmouth blackbass (o) in an electronicthermoregulatory shuttlebox . Dark horizontal bar indicates scotophase (2400 to 1200 hours EST) of the LD 12 : 12photoperiod (sudden on-off and off-on transitions) . Vertical bars indicate one standard error above and below the

mean at 1 too and at 2300.

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period), when M. salmoides reached a minimum of27.1°C (Fig . i) . M. dolomieui reached a minimum of26.6°C during the latter part of scotophase (dark period),while M. salmoides reached a peak at 29.5°C during mid-scotophase (Fig . i) . Preferred temperatures increasedfollowing minima, and decreased following maxima,prior to changes in exogenous light cues (sudden onset oroffset of photophase) .

Discussion and conclusions

The fact that preferred-temperature changes anticipatedexogenous light cues (Fig . i) is evidence for endogenously-controlled circadian thermoregulatory rhythms, whichare likely entrained by photoperiod (Schwassmann,1971). The thermotemporal patterns of these two con-

generic species are distinctly complementary, suggestinga possible mechanism which, in conjunction with differ-ing light intensity (depth/cover) preferences, could aid insegregating their respective niches (Werner, Hall, Laugh-lin, Wagner, Wilsman & Funk, 1977) in space and time,an aspect of resource partitioning and species packing .

In addition to the possible ecological functions of thediffering thermoregulatory rhythms suggested above,temperature (and therefore thermoregulation) has ob-vious profound physiological and metabolic consequen-ces. For example, the preferred-temperature range oflargemouth bass, about 27-30°C (Fig . 1 ; 1, 5, 8, 9), coin-cides closely with the range of temperatures which pro-duces maximal growth rates in largemouth bass fry(Strawn, 1961) . The temporal patterning of temperaturealso has been shown to profoundly affect growth rates andreproductive readiness (Reynolds, 1977; Reynolds, Cas-terlin, Matthey, Millington & Ostrowski, in press ; Spieler,Noeske, De Vlaming & Meier, 1977), in a probablyspecies-specific manner .

References

Reynolds, W . W. 1977 . Fish orientation behavior : an electronicdevice for studying simultaneous responses to two variables . J .Fish . Res . Board Can . 34 : 300 - 304 .

Reynolds, W . W . 1977 . Thermal equilibration rates in relation toheartbeat and ventilatory frequencies in largemouth blackbass,Micropterus salmoides . Comp . Biochem. Physiol . 56A : 195-201 .

Reynolds, W . W . 1977 . Circadian rhythms in the goldfish Carras-sius auratus L . : preliminary observations and possible impli-cations . Rev . Can . Biol . 36 : 355- 356.

Reynolds, W . W . & Casterlin, M . E . 1976a. Activity rhythms andlight intensity preferences of Micropterus salmoides and M .dolomieui. Trans . Am . Fish . Soc . 105 : 400-403 .

Reynolds, W . W. & Casterlin, M . E . 1976b . Thermal preferendaand behavioral thermoregulation in three centrarchid fishes . InThermal Ecology II, ERDA Symp . Set . CONF-75o425 . Ar-lington, Virginia : U .S . Natl . Tech . Info . Serv ., pp . 185-190.

Reynolds, W . W., Casterlin, M . E ., Matthey, J . K ., Millington,S. T . & Ostrowski, A. C . in press . Circadian rhythms of pre-ferred temperature and locomotor activity in the goldfishCarassius auratus . Comp. Biochem . Physiol .

Reynolds, W . W., Casterlin, M . E . & Millington, S . T . in press.Circadian rhythm of preferred temperature in the bowfinAmia calva, a primitive holostean fish . Comp. Biochem.Physiol.

Reynolds, W . W., McCauley, R . W ., Casterlin, M . E . & Craws-haw, L . 1 . 1976. Body temperatures of behaviorally thermore-gulating largemouth blackbass (Micropterus salmoides) .Comp. Biochem . Physiol . 54A : 461-463 .

Richards, F . P., Reynolds, W . W . & McCauley, R . W . (eds .) . 1977 .Temperature preference studies in environmental impact

assessments : an overview with procedural recommendations .J . Fish . Res . Board Can . 34 : 728-761 .

Schwassmann, H . O . 1971 . Biological rhythms . In Fish Physio-logy, Vol . 6 . New York : Academic Press, pp . 371-428 .

Spieler, R . E ., Noeske, T. A ., De Vlaming, V . L . & Meier, A . H .1977 . Effects of thermocycles on body weight gain and gonadelgrowth in the goldfish, Carassius auratus . Trans. Am . Fish .Soc . 106 : 440-444 .

Strawn, K. 1961 . Growth of largemouth bass fry at various tem-peratures . Trans . Am . Fish . Soc . 90 : 334 -335 .

Vogele, L . E . & Rainwater, W . C . 1975 . Use of brush shelters ascover by spawning black basses (Micropterus) in Bull ShoalsReservoir . Trans . Am . Fish . Soc . 104: 264-269 .

Werner, E . E ., Hall, D . J ., Laughlin, D . R ., Wagner, D . J ., Wils-mann, L. A . & Funk, F. G . 1977 . Habitat partitioning in afreshwater fish community . J . Fish. Res . Board Can . 34 : 360-370 .

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