hydric & thermal constraints & opportunities introduction – why is water important? 70 –...

Hydric & Thermal Constraints & OpportunitiesIntroduction – Why is Water Important?

70 – 80% body mass is water

All biochemical reactions take place in solution & water is a nearly universal solvent.

All biochemical reactions depend on concentrations of cellular substrates & salts.

Spatial relationships of intracellular organelles greatly influenced by cellular water content.

Blood carries chemical substrates, products of chemical reactions, other biologically important compounds, and heat.

Water has great thermal capacity compared to other universal “fluids” (air for example).

Thus water and dissolved solutes must remain in proper balance for survival.

Herpetology (Bio 488), Spring 2011, Poe & Snell, HSL01 – Hydric & Thermal Restraints / Opportunities 1

Hydric & Thermal Constraints & OpportunitiesIntroduction B – Why is Temperature Important?

Maintenance of proper body temperature is equally important

Biochemical reactions greatly affected by temperature:Reaction rates increase as temperature increases & decrease as temperature decreases.

Temperature affects the cellular environment.Cellular temperature increases cause:

Decreased viscosity of cytoplasmIncreased permeability of lipid membranesIncreased speed & force of muscle contractions

Amphibians & Reptiles must regulate their water content and body temperature.

Herpetology (Bio 488), Spring 2011, Poe & Snell, HS_L01 – Hydric & Thermal Restraints / Opportunities 2

Hydric & Thermal Constraints & OpportunitiesWater Exchange

Balance of the gain & loss of water is required to maintain a relatively constant amount of body water

Gain: Liquid H2O + preformed H2O + metabolic H2O

=

Loss: evaporation + urine + feces + salt glands + reproductive products

Herpetology (Bio 488 Spring 2011, Poe & Snell, HS_L01 – Hydric & Thermal Restraints / Opportunities 3

Hydric & Thermal Constraints & OpportunitiesWater Exchange – Liquid water

Most water required by organisms enters the body through the mouth and/or across the skin as liquid or preformed water

Liquid water – comes into the body as “water” drank or absorbed across the skin (inhaled water vapor is a negligible source)Many reptiles drink -> bodies of water, droplets on plants, rocks, and/or channel water from surface of the body

Amphibians generally don’t drink -> absorb water through skin which is highly permeable, especially in terrestrial amphibians

Pelvic patch: area of thin skin underlain by capillary network in pelvic region of most terrestrial anurans



Desert anurans (Spea, Scaphiopus, Bufo, etc) well adapted for absorbing water from soil

Most of year spent in burrows.

As soil dries (which decreases water potential) the osmolarity of body fluids increases due to accumulation of urea.

That increased osmolarity means decreased water potential of the anurans’ tissues which increases the ability to absorb water from drier soil – how?



Semi-aquatic and aquatic amphibians in freshwater environments face different osmotic challenges than arid adapted species:

Osmolality of body fluids greater than that of surrounding medium & because they have highly permeable skin:

Water enters & ions escape across the skin

These amphibians produce large amounts of dilute urine & use active transport across the skin to replenish ions.



Salt water & Amphibians: approximately 12 species of salamanders and 60 species of anurans inhabit or tolerate brackish environments

Similar solution to that of desert anurans – increase body osmolarity

Accumulate/retain organic and inorganic solutes raises internal osmolalities to high levels (300 – 500 mmole/kg H2O)

Internal osmolarity then greater than surrounding medium



Water movement across skin in reptiles much less than amphibiansLipids in reptile skin reduce its permeability

Surface to volume ratio increases potential for loss in really dry environments (small versus large lizards; snakes versus lizards; Chelonians versus Lepidosaurs)

Minor gain of water and loss of ions in freshwater environments

Minor loss of water in marine reptiles


Hydric & Thermal Constraints & OpportunitiesWater Exchange – Preformed Water

Preformed water: water present in food, often a smaller component that liquid water, but not in all cases:

Important source of water for desert taxa:

70 – 80% of animal-prey body-mass is water

More variable in plants (diurnally & seasonally)

Some turtles & lizards herbivorous & some of those (Sauromalus, chuckwallas) cease feeding when plant water content gets low, so water > important than energy &nutrients

Besides water, ions also accumulate with food intake & getting rid of them can be costly in terms of water – thus use of salt glands in reptiles


Hydric & Thermal Constraints & OpportunitiesWater Exchange – Metabolic Water

Metabolic water – formed by cellular metabolism when H & O atoms combine

Amount of water produced depends on what is metabolized:

Protein yields 0.4 to 0.5 g H2O/g protein

Starches yield 0.6 g H2O/g starch

Fat yields 1.1 g of H2O/g fat – how does this work?


Hydric & Thermal Constraints & OpportunitiesWater Exchange – Routes of Water Loss

Routes of Loss:

Evaporation: Cutaneous evaporation major means of water loss in most terrestrial amphibians

Most terrestrial species have low cutaneous resistance

Exceptions:Some arboreal species of Hylidae & others can have high “cutaneous” resistance due to coatings including dried mucus with lipids and / or waxes & esters secreted by glands

Cocoon formation in some groups of hylids, myobatrachids, & the salamander Siren intermedia. Cocoon formed by retaining shed layers of the stratum corneum yielding resistance comparable to the arboreal forms.



Routes of Loss:







Routes of Loss (continued):

Urine & feces:

Urine = fluid containing nitrogenous waste plus ions

Nitrogenous end products (waste) can be ammonia, urea, and uric acid:

Ammonia (NH3) very soluble in water, very “cheap” energetically, but very toxic. Can’t be stored & must be immediately excreted across skin, gills thus used in aquatic species

Urea (CH4ON2): Very soluble in water & nontoxic, energetically expensive to convert from NH3. Most terrestrial amphibians

Can be stored in high concentrations, used to adjust osmolarity of body fluids




Urine & feces (continued):

Uric acid (C5H4O3N4): Insoluble in water & nontoxic. Energetically expensive (most expensive?). Most reptiles & a few amphibians.

Water reabsorbed from urine in bladder and/or cloaca

yielding uric acid precipitates excreted with feces as

white or grey material

Some reptiles facultatively switch between urea & uric acid

(some turtles & some crocs) depending on water availability

Feces = undigested food & other waste products excreted from cloaca, water reabsorbed in cloaca and large intestine.




Salt excretion & salt glands:

Most terrestrial vertebrates excrete salts via renal function.

Salt glands are additional organs found in many reptiles and provide extrarenal means of excreting salts.

Ions (potassium & sodium) in urine are reabsorbed in bladder and/or cloaca via active transport, then removed from body via

the salt glands (nasal in many lizards)

Salt glands require more energy, but save water since secretions are more concentrated. Often found where salt intake is

high Marine iguanas & many desert species




Salt excretion & salt glands:




Reproductive products:

Relative clutch mass in reptiles can reach 70% (possibly more), if 70% of egg mass is water then a 100g snake can lose 49 g of water (50% of its mass) in eggs!


hydric & thermal constraints & opportunities introduction – why is water important? 70 –...

Documents

preformed water liquid

water important

water potential

solution water

absorbing water

body water gain

cellular water content

gain loss of water