Temperature is the environmental masterfactor which has a substantial effect on the growth, nutrition, reproduction,distribution and behaviour of ectothermic fish.
Increases and decreases ofambient water temperature will be reflected on the cardiac function in teleostfish which considers as a key physiological variable in environmentaladaptation and acclimation of aquatic vertebrates via delivery oxygen andnutrients to tissue cells and providing homeostatic balance between body parts.Also, temperature determines the metabolic rate, sets demands on bloodcirculation, cardiac function, and thus heart rate (fH) whichis the main regulatory for cardiac output and circulation in fishes withtemperature changes (Brett, 1971; Cech et al., 1976; Pörtner, 2001; Vornanen etal., 2002; Gamperl and Farrell, 2004; Gollock et al., 2006; Steinhausen et al.,2008; Mendonça and Gamperl, 2010; Badr et al., 2016).
To this end, temperature of waterenvironemnt acts as a leading factor which control the boundary of habitats ofaquatic organsms (Skelly, 2010). Climate warmingis considered to be the most serious environmental threat for freshwaterecosystems and their biodiversity. According to the current scenarios, theannual mean temperature in Finland is projected to rise by 2–5°C and 2–7°C bythe 2050s and 2080s, respectively. The duration of ice cover in lakes willbecome shorter, winters will be milder and extreme temperature peaks are likelyto be higher and occur more often. Changes of these magnitudes are likely toseriously affect fish populations in Finland.
For example, the Arctic charr (Salvelinusalpinus), a cold-water salmonid species, faces a real heat threatespecially in the shallow lakes of northern Finland (Lehtonen, 1998; Heino etal., 2009). Unlike the Arctic charr, which has a narrow thermal tolerancewindow, the roach tolerates a much wider range of temperatures. Putatively, theroach may be one of the fish species which may benefit from the warming waters;therefore its abundance in Finnish lakes and coastal waters may increase withsignificant impact on those ecosystems. Jeppesen et al. (2012) reviewed theimpacts of climate warming on fish data series from 24 European lakes, included2 Finnish lakes, with annual temperature increase of about 0.15-0.
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3°C per decade whereprofound changes have occurred in fish assemblagecomposition, biomass, abundance, body size and/or age structure and a shiftedtowards higher dominance of eurythermal species with decreasing in theabundance of cold-stenothermal species. Strongly response of fishes to thewarming makes them as indicator for detecting and documenting climate-inducedmodifications on freshwater ecosystems.As ectotherms,Fishes can acclimatize and adjusting their physiological and biochemicalprocesses during seasonal and daily thermal variations or vertical movementsthrough the water column (Matthews and Berg 1997, Wood and McDonald 1997). In north-temperatelatitudes, fish live and tolerate near-zero temperatures, while the limitationof upper thermal tolerance and plasticity different between species (Bennettand Beitinger, 1997; Beitinger and Bennett, 2000). According to the upperthermal tolerance, fish can be classified into 3 groups, stenothermal,mesothermal and eurythermal fish. Stenothermic fish have ability to toleratenarrow range of temperatures as fish in the Southern Ocean which living underthe constant cold and tolerate narrow range of temperatures usually less than10 degrees (Verde et al.
, 2006; Franklin et al., 2007). Contrary, someeurythemial fish have ability to tolerate wide range of temperatures between 0°C to 40°C (Bennett and Beitinger, 1997; Rantin et al.
, 1998; Beitinger andBennett, 2000) as crucian carp (0-36°C; Horoszewicz, 1973) androach (0-33°C; Cocking, 1959). Between both stenothermal and eurythermal fish,fish with intermediate thermal tolerance range called as mesothermal fish as salmonidfish which tolerate range of temperatures from about 0°C to22-28°C and classified as mesothermic fish (Elliott and Elliott, 2010). Physiologicalcharacteristics of animals (phenotypes) are not fixed within individuals, butinclude a plastic component. Hence, animals can remodel their physiology tocompensate for the effects of temperature variation via a process of thermalacclimation (also called physiological plasticity) and thereby confer resilienceto climate change. Knowledge on the physiological plasticity of ectotherms andits molecular and genetic basis is crucial when predicting the effects ofglobal warming on animal populations.
Importantly, the electrophysiologicalphenotype of the ectothermic heart is flexible, so that function of heart andnerves is reversibly changed between cold and warm seasons. The changephenotype involves a number of changes ion channel structure and compositionand in thermal properties of ion currents. Physiological thermal plasticity offish heart is crucial to maintain contractility and to avoid disturbances inelectrical excitability that should be sensitive to temperature changes toproduce temperature-dependent acceleration and deceleration of heart rate (fH)and coordinate changes in conduction rate of action potential (AP) over theheart (Hassinen et al., 2007; Haverinen and Vornanen, 2009; Vornanen et al.,2014; Badr et al., 2016).
Increases in beating frequency reduce duration and plateau height of the fishcardiac AP (Harwood et al., 2000), and similarly acute changes in temperaturecause profound changes in the shape of cardiac AP, including atrial andventricular myocytes as well as pacemaker cells (Harper et al., 1995; Haverinenand Vornanen, 2007; Vornanen et al.
, 2014; Lin et al., 2014). The shape andduration of the AP is highly temperature-sensitive and appears to play animportant role in thermal acclimation of freshwater teleost cardiomyocytes(Bers, 2001; Haverinen and Vornanen, 2009, Galli et al.
, 2009). Acute changesof temperature extends also to include ion currents (K+, Na+and Ca2+), which control AP, and molecular composition of ionchannels. Generally, temperature virtually affects all levels of biologicalorganization from molecular diffusion to biochemical reactions, membranefunction and organ function which constitute the whole organism (Kalininet.al.
, 2009). 1.6.1 Effect of temperature on heart rate (fH) The rhythmicity of the heart is originatedby pacemaker cells in the region between sinus venosus and atrium. Thermalplacticity is variable between fish species and may limit the adaptability offishes in surviving with predicted climate warming (Somero, 2010). Both ofheart rate (fH) and the stroke volume (SV) determine the performanceof the heart and result in cardiac output. In exercising fish, changes incardiac output are achived by increases in both SV and fH,while fH is the main factor in regultation the cardiac outputduring temperature changes (Randall, 1982; Gollock et al.
, 2006; Steinhausen etal., 2008; Mendonça and Gamperl, 2010; Farrell and Smith, 2017). In most of fish,increase of temperature increases fH, e.g. from a few numberheartbeats per minute (bpm) to a maximum fH of 70-120 bpm (Lillywhiteet al., 1999; Gollock et al., 2006; Mendonça and Gamperl, 2010). However, themaximum fH can be increased to 300 bpm at high temperatures inzebrafish and frillfin goby (Bathygobius soporator) (Rantin etal.
, 1998; Lin et al., 2014; Sidhu et al., 2014; Vornanen and Hassinen, 2016). Decreasein temperature from 25° C to 10° C resulted in reducing the fHof yellowfin tuna (Thunnus albacares) from 106 bpm to 20 bpm (Blanket.al., 2002).
Similarly, Nile tilapia (Oreochromis niloticus) whichacclimated to 35° C follwed by acute temperature decline to 15° C(Maricondi-Massari et.al., 1998).
In contrast, fH in coldacclimated Pacu (Piaractus mesopotamicus) and goldfish (Carassiusauratus) was higher than that in warm acclimated fish (Morita and Tsukuda,1995; Aguair et.al., 2002). These results indicate that the cardiaccontractility (fH) alternations following temperature changesare not uniform among teleosts and the effects of temperature acclimation onthe heart rate are species-specific dependent. Changes intemperature cause compensatory changes in fH which also requiremodifications in the duration of pacemaker, atrial and ventricular APs tomaintain a proper balance between the durations of systole (cardiaccontraction) and diastle (cardiac relaxation) (Vornanen, 2016). In seasonalacclimated plaice (Pleuronectes platessa) and thermal acclimated rainbowtrout at 4°C, low temperatures induce increrases in fH by shortening in the pacemaker APD withoutchanges in the rate diastolic depolarization (Harper et al., 1995; Haverinenand Vornanen, 2007). In some other fishes, increases in fHinduced by cold temperatures is due to shortening of atrial and ventricular APs(Haverinen and Vornanen, 2009, Hassinen et al.
, 2014; Abramochkin and Vornanen,2015). Acutetemperature changes cause different types of arrhythmias in fish hearts appearewhen temperature approaching or exceeding the break point temperature (TBP)the upper thermal tolerance of fH. Cardiac arrhythmiasreported in fish hearts as missed beats, bradycardia, and bursts of rapidbeating, and finally complete cessation of heartbeat (asystole) (Casselman etal., 2012; Anttila et al.
, 2013; Verhille et al., 2013; Ferreira et al., 2014;Vornanen et al.
, 2014; Badret al., 2016). 1.6.2 Effect of temperature on action potential (AP) Acute temperature changes significantly changethe duration and shape of pacemaker, atrial and ventricular APs of fish heartsby altering the flow of inward and outward currents via the SL intemperature-dependent manner (Harper et al., 1995; Vornanen et al.
, 2002;Haverinen and Vornanen, 2007; Haverinen and Vornanen, 2009; Ballesta et al.,2012; Vornanen et al., 2014; Lin et al., 2014; Hassinen et al., 2014; Shiels etal.
, 2015). APD must inversly correlate with fH to allowsufficient time for systole and diastole durations, otherwise diastole willdisappear under high fH, i.e, increasing in temperatureincreases fH and decreases APD. In exercise and active fishes(as tunas) and tropical fishes (as zebrafish) have higher fHand shorter APD in comparison with dormant fishes (as crucian carp) or fisheslive in cold polar waters (as navaga). The shape and duration of fish cardiacAPs and the underlying ion currents are highly sensitive to temperature changesand crucial in thermal acclimation or acclimatization of both freshwater andmarine teleosts to seasonal temperature regimes (Haverinen and Vornanen, 2009;Galli et al.
, 2009; Hassinen et al., 2014; Abramochkin and Vornanen, 2015;Vornanen and Hassinen, 2016).
