In the Classroom
How Water Moves in Animals
Animals are about 75% water(2). Water is vital for cellular and physiological functions, such as the regulation of body temperature, nutrient uptake into cells (circulatory systems), and, in vertebrate animals, the clearance of metabolic waste products in urine, such as urea and uric acid.
• Animals, with thin body walls such as frogs, worms, etc. gain water by osmosis occurring across the skin, and thus live in moist environments.
• Most land-dwelling vertebrates are protected by thickened outer coverings and gain water largely by drinking; but also from food they eat.
• These animals are also exposed to evaporation of water from their skin. Water exits through the pores of the skin (in humans), and across the tongue (in panting animals) as a vapour.
• The change in state, from a solid to a gas, requires energy, and this energy escapes as heat, cooling the animal. It is called the heat of vaporisation.
• Body scales and hair are protection from evaporation, but are not strictly an adaptation, rather they are pre-adaptations.
HOW A LIZARD DRINKS
The Mountain devil (Moloch horridus) arrived in Australia possibly 10 million years before other dragon lizards, and is found throughout the hottest regions of Australia (see distribution map). These lizards have remarkable adaptions to aridity; they are protected by thorny scales that are impervious to water. A Moloch is able to drink with a closed mouth, by standing (even with one leg) on a wet surface.
How did scientists find that out?
• Scientists placed a dehydrated (and weighed) lizard on a watery surface and noticed that its jaws were moving, as if it were swallowing. Indeed it was, as evidenced by a gain in weight. In some way, water had travelled up the body, and passed into the mouth. How does this happen?
• Similar to the capillary action in plant cells, the impermeable skin of the lizard contains channels between the scales that direct columns of water upwards from the puddle below and into the mouth.
• These scientists went further. They taped the jaws shut and placed the animal into water containing a dye. Tracing the movement of the water showed it passing upwards and into the closed mouth (although less than before.)
• Looking at the cells of the skin of the mouth revealed tiny spaces between the outer layer of the skin around the mouth, and from the spaces, even tinier ducts into the cells.(3)
So, with a closed mouth, and standing on a wet surface, the Mountain devil can rehydrate.
HOW A FROG SURVIVES IN THE DESERT
https://www.youtube.com/watch?v=XZazXPrFzWw
Frogs are tied to wet places. Their eggs are laid and fertilised in water. The larval stage, the tadpole, is aquatic, and only becomes terrestrial when it develops limbs and its adult physiological systems. On land, the frog’s skin is highly permeable to water. For a frog to live in arid regions, species have developed morphological (structural), physiological, and behavioural adaptations.
• Behavioural. They burrow. Rainfall can be brief, or even non-existent for many years in arid regions and these frogs stay underground for years. How do they survive a long dry period?
• Physiological. In the early stages underground (i) they can absorb water from the soil by osmosis, with the skin acting as a semi-permeable membrane.
• Morphological. They shed successive layers of skin (about one every 2.5 to 5 days) that stick together to form a thickened, impermeable outside layer, or cocoon. This prevents the frog from drying out, (or desiccating), keeping it moist (hydrated). The mouth is also covered, so the frog does not ingest food while burrowed.
• Physiological. While cocooned, their rate of metabolism is very reduced. Metabolising food produces urine, which is stored in the bladder. Urine contains solutes and the waste product urea, which creates an osmotic gradient between the frog’s tissues and the outside water, a physiological adaptation to retain water in the body.
• Morphological. Burrowing frogs hasten the larval stage to take advantage of a short wet period. In wet habitats, a tadpole develops into a frog in 120 to 160 days. In a burrowing desert-adapted frog this process takes 16 to 40 days.
How do frogs know when to dig their way to the surface?
Scientists have not yet discovered the precise stimulus for the frog to dig itself out into the open, but suspect that a fall in barometric pressure, indicating onset of rain, may be detected by the burrowed frog(4).
In summary, adaptations shown by desert frogs may be classified as
i. Structural (or Morphological) – retaining skin layers as a cocoon, and accelerated larval development.
ii. Behavioural – by burrowing and by habitat selection (sand or clay).
iii. Physiological – by storing water in the bladder and absorbing water from the soil by osmosis.
https://www.uwa.edu.au/study/-/media/Faculties/Science/Docs/Worksheet-Waterholding-frog.pdf
HOW THE EGGS OF AN AQUATIC INSECT SURVIVE WITH NO WATER
Cryptobiosis in a non-biting midge (Tanytarsus sp.) https://bie.ala.org.au/species/
• Non-biting midge flies, or chironomids, commonly occur in both inland freshwater and coastal areas. They are commonly known as “blind mosquitoes” because they are mosquito-like in appearance, but do not bite. They are also called “fuzzy bills” because of the male’s bushy antennae.
• This midge species occurs across Australia and is found in swift moving streams, deep slow moving rivers, stagnant ditches, and in lakes and ponds that are rich in decomposing organic matter. It has been documented as an ‘indicator species’ for water quality (see Module Year 8).
• The midge’s life cycle includes egg, larval, and pupal phases that are all aquatic and form a significant part of the food chain in inland waters. When the winged adult emerges from the water, it lives for a short 3 to 5 days – just long enough to mate and develop about 1000 eggs! These are laid in shallow water, on the edge of inland rock pools or the edge of streams.
• Of the several adaptations in this life cycle, one that is remarkable is a form of cryptobiosis. Cryptobiosis is a state of extreme physiological inactivity in response to adverse environmental conditions. In the cryptobiotic state, all metabolic activity stops, preventing reproduction, development, and repair.
• For the midge, cryptobiosis involves a shut-down of the metabolic activity in the eggs – they lose all their water (dehydrate) and remain dried out (or desiccated) for long periods. They are brought back to life when their environment becomes moist. Water enters the eggs and revives the metabolic process of living. This is best described as a physiological adaptation.
Activity
Observe Rehydration in a Midge from a Local Water:
AIM
To demonstrate cryptobiosis, a physiological adaptation to aridity in the non-biting midge.
METHOD
1. Students, or the teacher, use a scraper to collect a sample of dried mud from the edge of a granite outcrop, or a stream. The sample should be large enough for each student to have approx. 2 cm2.
2. In the science room, each student (or pair) places a sample in a Petri dish. (Equipment in TOOLBOX).
3. Moisten the sample with about 10ml tap water, and mix gently, stirring with a glass rod, and cover. Non-biting midge Midge lifecycle Midge larva
4. Stand for 1 hour and observe under low power (X4) or (X10).
5. Look for the larval form of the midge. (5-10 mm). It is red in colour, due to the presence of haemoglobin. This is an iron-containing molecule, present in higher vertebrate blood that carries oxygen throughout the body.
6. Record presence or absence of midge larvae.
7. If absent, let stand a further hour, repeat Step 4, and record as before.
8. Depending on the presence of dehydrated invertebrates, rehydration may take up to 3 hours – so don’t give up!
9. Write up your experiment - introduce the animal, describe its life cycle, how it has adapted, the time taken for rehydration, and the environmental conditions to which it is adapted.