Inducible heat tolerance in Antarctic notothenioid fishes

Transcription

1 Polar Biol (26) 3:39 43 DOI 1.17/s y ORIGINAL PAPER Inducible heat tolerance in Antarctic notothenioid fishes Jason E. Podrabsky Æ George N. Somero Received: 27 February 26 / Revised: 3 April 26 / Accepted: 8 May 26 / Published online: 22 June 26 Ó Springer-Verlag 26 Abstract Significant increases in heat tolerance (time of survival at 14 C) were observed for some, but not all, species of notothenioid fishes collected from McMurdo Sound, Antarctica (77 51 S) following acclimation to 4 C. The increase in thermal tolerance was rapid in Trematomus bernacchii, developing within 1 2 days of acclimation to 4 C. Long-term (6 8 weeks) acclimation to 4 C led to greater heat tolerance in Trematomus pennellii than in T. bernacchii. Unlike its demersal congeners, the cryopelagic notothenioid Pagothenia borchgrevinki did not increase heat tolerance during warm acclimation. A deep-living zoarcid fish, Lycodichthys dearborni, also failed to increase heat tolerance, but survived significantly (> threefold) longer at 14 C than the notothenioids. Introduction Ectothermic animals endemic to Antarctic waters are markedly stenothermal; many invertebrates and fishes acclimatized to typical ambient water temperatures ( 1.8 to 2 C) have upper incipient lethal temperatures near or below 5 6 C (Somero and DeVries 1967; Clarke and Johnston 1996; Peck 22, 25; Peck et al. J. E. Podrabsky (&) Department of Biology, Portland State University, Portland, OR 9727, USA jpod@pdx.edu G. N. Somero Hopkins Marine Station, Stanford University, Pacific Grove, CA 9395, USA 24). Failure of important physiological activities such as locomotion and burrowing may occur at even lower temperatures (Peck et al. 24). Little is known, however, about the capacities of Antarctic ectotherms to acclimate to warmer temperatures, a question that is significant in the context of increasing seawater temperatures due to climate change. In some areas of the Southern ocean bordering Antarctica, sea surface temperatures already have risen by approximately 1 C since the 195s (Meredith and King 25). Here, we report the effects of short- (1 4 day) and long-term (6 8 weeks) acclimation to 4 C on thermal tolerance (survival at 14 C) of selected notothenioid and zoarcid fishes from McMurdo Sound, Antarctica (77 51 S, E). Previous work showed that specimens of notothenioids acclimated to 1.86 C had upper lethal temperatures near 5 6 C (Somero and DeVries 1967). We wished to determine whether notothenioids and zoarcids could alter their sensitivities to elevated temperatures during warm acclimation and, if so, how rapidly this capacity was gained. Materials and methods Specimens of four species of the Family Nototheniidae (Suborder Notothenioidei), Trematomus bernacchii, Trematomus pennellii, Trematomus hansoni and Pagothenia borchgrevinki, and the zoarcid Lycodichthys dearborni were caught in McMurdo Sound by hookand-line fishing or in baited wire traps during the months of December 25 and January 26. Specimens were returned to the aquarium facility in the Crary Science and Engineering Center at McMurdo Station and held in flow-through seawater aquaria at

2 4 Polar Biol (26) 3:39 43 ambient temperature ( 1.8 C) until used for experimentation. Fish were fed on chopped fish muscle every few days during the acclimation period. The protocol used to measure thermal tolerance followed that of Somero and DeVries (1967). Fish were transferred acutely from the 1.8 C holding tank to a 3 l clear plastic aquarium containing aerated seawater at 14 ±.1 C. The choice of 14 C as the thermal stress temperature is based on the finding that 1.8 C-acclimated notothenioids survive for approximately 1 min at this temperature (Fig. 1a). Thus, 14 C exposure allowed rapid screening of many specimens. Acclimation at 4 C involved two protocols: direct transfer into 4 C water or a slow increase in water temperature from 1.8 to 4 C over 24 h. Long-term acclimation involved holding specimens of T. bernacchii and T. pennellii for 6 8 weeks following direct transfer to 4 C. Shorter term acclimation was done with T. bernacchii, P. borchgrevinki, and L. dearborni to examine the kinetics of the acclimation response (change in survival time at 14 C) over the first few days of 4 C acclimation. Due to limitations in the number of available animals we were only able to follow the T. bernacchii exposed to a slow temperature increase for 3 days compared to 4 days for the fish directly transferred to 4 C. Analysis of variance (ANOVA) and analysis of covariance (ANCOVA) were used to examine the effects of body mass, species, and acclimation history of the specimens. Specific comparisons between groups were made using t tests or the Student Newman Keuls (SNK) test where appropriate. Results Fish placed into 14 C water typically showed rapid bursts of swimming immediately after transfer. Subsequently, fish generally became quiescent until shortly before death, when another, often brief bout of swimming occurred. Death was commonly accompanied by loss of equilibrium (fishes lying on their backs or sides), flaring of the opercula, and rigidity of the trunk muscle. No effect of body mass on survival time was found for any species (ANCOVA p =.378). Specimens of notothenioids acclimated (or field acclimatized) to 1.8 C survived at 14 C for 1 2 min, in agreement with earlier measurements made on some of these species (Somero and DeVries 1967; Fig. 1a). Field-acclimatized and 1.8 C-acclimated specimens of T. bernacchii did not differ in survival time at 14 C, suggesting that no effects on Log 1 (min to death) b a Temperature, C A -1.8 C 4. C Field B B L.d. P.b. T.h. T.b. T.p. Species Fig. 1 Thermal tolerance of Antarctic notothenioid and zoarcid fishes following long-term acclimation to 1.8 or 4 C. a Time to mortality (min) of four notothenioid species acclimated to 1.8 C [L. dearborni (open square, L.d., n = 7), P. borchgrevinki (open triangle, P.b., n = 4), T. hansoni (inverted triangle, T.h., n = 3), T. bernacchii (open circle, T.b., n = 8), T. pennellii (open diamond, T.p., n = 5)]. Filled symbols are data from Somero and DeVries (1967); open symbols are data from the present study. b Time to mortality (min) of notothenioid and zoarcid fish following long-term (6 8 weeks) acclimation to 4 or 1.8 C. Bars represent means ± SEM. Bars labeled with the same letters are not significantly different. Field-acclimatized T. bernacchii (n = 5) did not differ in tolerance from fish laboratory-acclimated (n = 8) at 1.8 C (lower case letters, t test p =.134). For 1.8 C-acclimated specimens, T. pennellii survived significantly longer at 14 C than other notothenioids, and L. dearborni survived significantly longer than all notothenioids (upper case letters, ANOVA, SNK p <.21). Long-term acclimation to 4 C led to significant increases (ANOVA p =.4) in tolerance of 14 C int. bernacchii (n = 6) and T. pennellii (n = 5); the latter species was significantly more heat tolerant than T. bernacchii following 4 C acclimation thermal tolerance resulted from several weeks of laboratory holding at 1.8 C (Fig. 1b). T. pennellii was significantly more tolerant of 14 C than the other notothenioid species. All four notothenioids were less tolerant of 14 C than the zoarcid species B,a a D C E

3 Polar Biol (26) 3: L. dearborni (Fig. 1b). Long-term acclimation of T. bernacchii and T. pennellii to 4 C led to significant increases in heat tolerance, seven- and sixfold, respectively (Fig. 1b). The heat tolerance of 4 C-acclimated T. pennellii was significantly greater than that of T. bernacchii. The rate of acquisition of heat tolerance by T. bernacchii was rapid: within a day, fish subjected to either acute transfer to 4 C or a slow increase to this temperature over 24 h showed significant increases in survival times at 14 C (Fig. 2). The time-dependence of acquisition of heat tolerance by T. bernacchii may reflect a biphasic process. For the acutely transferred specimens, the rapid increase in tolerance over the first 2 days of acclimation was followed by a decline in tolerance by day 4. Presumably, a second phase of warm acclimation was responsible for the higher level of heat tolerance observed in fish acclimated to 4 C for 6 8 weeks (Fig. 1b). Neither P. borchgrevinki nor L. dearborni (Fig. 3) exhibited a significant increase in survival time at 14 C during short-term acclimation at 4 C. However, linear regression analysis indicates a significant positive slope in the data for P. borchgrevinki, which may indicate a slow acclimation of thermal tolerance is possible over a longer time period than examined in our study Days at 4 C Fig. 2 Time course of acquisition of resistance to high temperature in T. bernacchii. Acute transfer of T. bernacchii to 4 C water (open circles) and increasing water temperature from 1.8 to 4 C over 24 h (filled circles; 1 day point = 24 h at 4 C) led to similar kinetics of warm acclimation (ANOVA p =.6). Both acclimation regimes induced a significant increase in thermal tolerance within 24 h (ANOVA p =.8). Asterisk denotes tolerances to 14 C that are significantly greater than the tolerance at t = (ANOVA, SNK p <.15). Symbols are means ± SEM. Time zero points are data presented in Fig. 1. For all other data points n = Days at 4 C Fig. 3 Thermal tolerance of the zoarcid L. dearborni (open circles) and the notothenioid P. borchgrevinki (filled circles), during acclimation to 4 C (acute transfer). No significant increase in tolerance of 14 C occurred during the 3 4 day acclimation period for either species (ANOVA p >.3). However, a significant slope was identified for the P. borchgrevinki data (p =.15), but not the L. dearborni data (p =.79). Symbols are means ± SEM. Time zero points are data presented in Fig. 1. For all other data points n = 3 5 Discussion The teleost suborder Notothenioidei primarily comprises species endemic to Antarctic waters (Gon and Heemstra 199; Eastman 1993). Notothenioids have radiated to fill many niches that opened during the extreme cooling of the high latitude Southern Ocean, which began approximately 5 million years ago and was greatly accelerated when deep-water flow began in the Drake Passage approximately 25 million years ago (Eastman 1993). Approximately 1 15 million years ago, a substantial sea ice cover began appearing near the Antarctic continent, and water temperatures began falling to their current low levels near the freezing point of seawater, 1.86 C. The Antarctic notothenioids thus have evolved in cold, thermally stable waters for millions of years, and many aspects of their physiology and biochemistry indicate a high level of cold adaptation (Clarke and Johnston 1996; Chen et al. 1997; Fields and Somero 1998; Kawall et al. 22). Evolution in thermally stable waters also has led to a pronounced stenothermy in these fishes, as indicated by their upper incipient lethal temperature of 5 6 C in the case of specimens acclimated or acclimatized to normal ambient temperatures. The present study suggests, however, that at least some notothenioids may be capable of significantly increasing their heat tolerance. For example, in T. pennellii acclimation to 4 C increased the time of tolerance of 14 C from 25 min to nearly 15 min

4 42 Polar Biol (26) 3:39 43 (Fig. 1b). In T. bernacchii, tolerance time rose from near 1 min to almost 68 min. These times of survival to acute lethal heat shock do not, of course, reveal the maximal temperatures at which indefinite survival can occur. Further studies of long-term acclimation to increasing temperatures may reveal this ultimate limit. Future work may also reveal which physiological processes acclimate to temperature. No change in tissue oxygen consumption during acclimation of T. bernacchii to 4 C was observed (Somero et al. 1968), but Seebacher et al. (25) reported alterations in locomotory capacity and enzymatic activities in P. borchgrevinki following acclimation to 4 C. The discoveries that T. pennellii is more heat tolerant than the other notothenioids studied and that inter-specific variation exists in the ability to acquire increased heat tolerance during acclimation to 4 C were unanticipated in view of the common thermal exposure the species are likely to encounter in their habitats and the evolutionary histories of these coldadapted species (Eastman 1993). In McMurdo Sound, T. bernacchii, T. pennellii and P. borchgrevinki normally live at temperatures close to the freezing point of seawater throughout their depths of occurrence. Annual variation in seawater temperature in McMurdo Sound is no greater than.5 C (Hunt et al. 23). All three species are circum-antarctic in distribution (Gon and Heemstra 199; Eastman 1993), and all likely encounter similar temperatures throughout their biogeographic ranges. The physiological and genetic differences among these species that determine their different thermal tolerances and degrees of acclimatory plasticity merit investigation. The zoarcid fish L. dearborni has only been collected in the Ross Sea, at depths of m (Anderson 199; Gon and Heemstra 199). Little is known about the thermal physiology of zoarcids, a primarily deep-sea family. A study by Mark et al. (22) indicated that another Antarctic zoarcid, Pachycara brachycephalum, tolerated temperatures up to 12 C, but optimal physiological function occurred only below about 6 C. Thus, while Antarctic zoarcids appear to be less stenothermal than notothenioids, they too tolerate only a relatively narrow range of temperatures compared to temperate eurythermal fishes. The kinetics of the change in heat tolerance during warm-acclimation of T. bernacchii may be biphasic. While this statement is presently based on limited data, we feel that the implications of these data warrant discussion. A rise in tolerance of 14 C occurred rapidly and the kinetics of acclimation following acute- and slow ramp-up exposure to 4 C were not statistically different. The apparent reduction in tolerance of 14 C between days 2 and 4 of acclimation in the acute transfer experiment and the subsequent acquisition of a higher level of tolerance later in the 6 8 week period of 4 C acclimation may reflect the two-stage stress response described recently by Kültz (25). The initial effects of stress lead to a cellular stress response (CSR), which develops rapidly (within hours to days) and leads to repair of cellular damage, especially to membranes and proteins. Thereby, the CSR facilitates the short-term survival of the cell following stress. Then, a cellular homeostatic response (CHR) is triggered, which involves a host of other molecular level changes that differ from those associated with the CSR and effect a restoration of a cellular homeostasis, e.g., in ion balance, redox balance, and cell division. On-going studies of temperature-induced changes in gene expression in Antarctic notothenioids, using cdna microarrays, may reveal how the molecular underpinnings of thermal tolerance change over poststress recovery in these species (B. Buckley and G. N. Somero, in preparation). Notably, in T. bernacchii the CSR does not involve the production of increased amounts of heat-shock proteins (Hofmann et al. 2) or messenger RNA (Place and Hofmann 24; B. Buckley and G. N. Somero, in preparation). Although rapid induction of heat-shock proteins is commonly thought to be a key event in the CSR (see Kültz 25), T. bernacchii and at least some Antarctic notothenioids (Place and Hofmann 24) lack this capacity. Thus, the induced thermal tolerance found in T. bernacchii and other Antarctic notothenioids may occur through different mechanisms than those common to other species. Acknowledgement We gratefully acknowledge the assistance of Dr. G. Hofmann and colleagues in collection and maintenance of specimens (through support of NSF grant OPP3-1927). We also express our gratitude to the staff of the Crary Science and Engineering Center for their valuable assistance. This work was supported by NSF grant OPP5-472 to Dr. Donal Manahan. References Anderson ME (199) Zoarcidae. In: Gon O, Heemstra PC (eds) Fishes of the Southern Ocean. JLB Smith Institute of Ichthyology, Grahamstown Chen LA, DeVries AL, Cheng CHC (1997) Evolution of antifreeze glycoprotein gene from a trypsinogen gene in Antarctic notothenioid fish. Proc Natl Acad Sci USA 94: Clarke A, Johnston IA (1996) Evolution and adaptive radiation of Antarctic fishes. Trends Ecol Evol 11: Eastman J (1993) Antarctic fish biology: evolution in a unique environment. Academic, San Diego

5 Polar Biol (26) 3: Fields PA, Somero GN (1998) Hot spots in cold adaptation: localized increases in conformational flexibility in lactate dehydrogenase A 4 orthologs of Antarctic notothenioid fishes. Proc Natl Acad Sci USA 95: Gon O, Heemstra PC (199) Fishes of the Southern Ocean. JLB Smith Institute of Ichthyology, Grahamstown Hofmann GE, Buckley BA, Airaksinen S, Keen JE, Somero GN (2) Heat-shock protein expression is absent in the Antarctic fish Trematomus bernacchii (family Nototheniidae). J Exp Biol 23: Hunt BM, Hoefling K, Cheng CHC (23) Annual warming in seawater temperatures in McMurdo Sound in relationship to endogenous ice in notothenioid fishes. Antarct Sci 15: Kawall HG, Torres JJ, Sidell BD, Somero GN (22) Metabolic cold adaptation in Antarctic fishes: evidence from enzymatic activities of brain. Mar Biol 14: Kültz D (25) Molecular and evolutionary basis of the cellular stress response. Annu Rev Physiol 67: Mark FC, Bock C, Pörtner HO (22) Oxygen limited thermal tolerance in Antarctic fish investigated by magnetic resonance imaging (MRI) and spectroscopy ( 31 P-MRS). Am J Physiol 283:R1254 R1262 Meredith MP, King JP (25) Rapid climate change in the ocean west of the Antarctic peninsula during the second half of the 2th century. Geophys Res Lett 32:L1964 Peck LS (22) Ecophysiology of Antarctic marine ectotherms: limits to life. Polar Biol 25:31 4 Peck LS (25) Prospects for survival in the Southern ocean: extreme temperature sensitivity of benthic species. Antarct Sci 17: Peck LS, Webb KE, Bailey DM (24) Extreme sensitivity of biological function to temperature in Antarctic marine species. Funct Ecol 18: Place SP, Hofmann GE (24) Constitutive expression of a stress-inducible heat shock protein gene, hsp7, in phylogenetically distant Antarctic fish. Polar Biol 28: Seebacher F, Davison W, Lowe CJ, Franklin CE (25) A falsification of the thermal specialization paradigm: compensation for elevated temperatures in Antarctic fishes. Biol Lett 2: Somero GN, DeVries AL (1967) Temperature tolerance of some Antarctic fishes. Science 156: Somero GN, Giese AC, Wohlschlag DE (1968) Cold adaptation of the Antarctic fish Trematomus bernacchii. Comp Biochem Physiol 26: