Biology 3601A/B Lecture 18: Lecture 18
12 Nov 2018
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Lecture 18 – Thermal Physiology
Hot Blooded Fish
Homeothermic Endotherm
• Maintaining a constant internal temperature by generating your own heat
o Via. metabolism
Why Fish Are Ectotherms
• Ectotherms don’t generate internal heat that contribute meaningfully to body temperature
• Fish can generate heat just like anything else, with metabolism and muscle contraction
o But they have problems maintaining this heat
▪ Mainly due to the gills
• Surrounded by thermally-conductive water
o And constantly processing this water through the gills
• Fish gills act like a heat sink
o Fish gills are processing huge volumes of water
o The blood from the body is constantly meeting this water
o This leads to the constant loss of body heat to the water
• Overall, fish are ectotherms because the high blood flow across their gill surface meaning that they lose heat to the
enviro really quickly
Tuna Temperature Gradients in 19°C Water – Regional Endothermy
• Tuna are highly metabolically active fish
• Gradient of temperature in the tuna
o Ranges from 19°C (same as external enviro) → 31°C
• Dark red muscle in tuna sits ~31°C
o Core area is maintained relatively hot
o This tells us that this tissue is not cooled like other tissues in the body
Rete Mirabile (“miraculous net”)
• Edge of this red muscle area
• Complex net of multiple exchange systems
• Countercurrent exchange in the Rete Mirabile
o Cool blood in, warm blood out in opposite direction
o Warm blood heats up the cool blood
o Meaning that heat is not lost from this tissue
• A recurring theme – especially in (but not exclusive to) fish
o Swim bladder
o Heater organs
o Oxygen delivery to retina
o Cooling during feeding in kissing bugs
• Allows for VERY effective countercurrent exchange of heat
(and other things)
Red Muscle in Tuna
• Red muscle with Rete that use counter current exchange to
maintain heat in this muscle area
Red Muscle Temperature is Elevated Above Water Temperature
• Water temp vs. Red muscle temp
• Isothermal line – dotted in figure
• Skipjack tune
o Body temp is increased slightly above the isothermal line
o Body temp above external water temp at any given time
o Note: not at homeothermic levels
• Bluefin tuna
o Much bigger organism – larger body size, with smaller SA
o Maintaining body temp well above the isothermal line
o This may be due to thermal inertia
▪ Larger organisms can retain heat better
Where does the heat come from?
• Heat comes from the normal heat produced by contractile activity of the red muscles
o The only difference is that the heat is retained (by the Rete)
• Other different factors of red muscles
o More myoglobin
• Red muscle is used for prolonged high-speed swimming
• Advantage – allows them to swim into cold water and not slow down very much
Heater Organ in Billfish
• Billfish – marlin, swordfish, etc.
• These organisms heat up the brain and eye
o The retina is an oxygen demanding and metabolically active
tissue – increased temp to increase performance
• Heater organ used to heat up the brain and the eye to maintain function
o This is maintained separate from the rest of the body
o Via. carotid Rete
o Only by about 5-6°C warmer than the enviro
• Regional endothermy – very specific location
• This organ is derived from an extra-ocular eye muscle
o No contractile ability left of this muscle
o Muscle has turned into solely a heater organ
o This organ has lots of mitochondria 63% mitochondria (by volume)
o Another 30% is sarcoplasmic reticulum
▪ Generates heat through similar process used in brown adipose tissue and protons
▪ Heat is produced by futile cycling of Ca2+ pumping across the SR
Large Sharks Are also Regional Endotherms
• Around heart and gut to maintain performance of digestive system
Regional Endothermy Evolved in Bony Fishes on At Least Three Separate Occasions
• → Red muscle heaters
o Tunas
• → Brain heaters
o Butterfly mackerel
o Billfishes
o → Both of these organs are derived form an extra ocular eye muscle,
but the actual muscle used to make the heater differs
Document Summary
Homeothermic endotherm: maintaining a constant internal temperature by generating your own heat, via. metabolism. Why fish are ectotherms: e(cid:272)tother(cid:373)s do(cid:374)"t ge(cid:374)erate i(cid:374)ter(cid:374)al heat that (cid:272)o(cid:374)tri(cid:271)ute (cid:373)ea(cid:374)i(cid:374)gfully to (cid:271)ody te(cid:373)perature. Fish can generate heat just like anything else, with metabolism and muscle contraction: but they have problems maintaining this heat, mainly due to the gills. Surrounded by thermally-conductive water: and constantly processing this water through the gills. Tuna temperature gradients in 19 c water regional endothermy. Red muscle in tuna: red muscle with rete that use counter current exchange to maintain heat in this muscle area. Red muscle temperature is elevated above water temperature: water temp vs. red muscle temp. Heater organ in billfish: billfish marlin, swordfish, etc. Large sharks are also regional endotherms: around heart and gut to maintain performance of digestive system. Whole-body endothermy in a mesopelagic fish the opah, lampris guttatus.
