Osmoregulation Flashcards
•Osmoconformer
•Osmoconformer - body fluid isosmotic with surrounding sea water.
•Osmoregulator
•Osmoregulator - body fluid not isosmotic with sea water.
Solute composition
•Solute composition - the dissolved substances (ions, proteins, etc.) in the body fluid.
Haemolymph
•Haemolymph - body fluid contained in the primary body cavity (the haemocoele), this is the main body fluid in the Mollusca & Crustacea.
Coelomic fluid
•Coelomic fluid - body fluid contained in the secondary body cavity (the coelom), this is the main body fluid in the annelids, echinoderms and cephalopod molluscs.
Osmolarity
•Osmolarity – now known as osmotic concentration, it is the measure of solute concentration, defined as the number of osmoles (Osm) of solute per litre (L) of solution (OsmL-1)
Osmolality
•Osmolality – a variation of molality that takes into account only solutes that contribute to a solution’s osmotic pressure (Osmkg-1)
Give the main solute composition in seawater
What is the Donnan Equilibrium?
- The presence of a non-diffusible ion (e.g. a protein) on one side of a semi-permeable membrane can lead to an unequal distribution of diffusible ions.
- This Donnan Equilibrium, relying on passive diffusion, can be demonstrated by dialysis.
- The equilibrium that occurs over a membrane,
- If something can’t be moved, all the diffusible ions will continue to move until the osmotic pressure is equal.
- Unequal distribution of diffusible ions.
Mention how the ionic composition of seawater and some invertebrate body fluids vary between a few example organisms.
- Jellyfish map very closely to water – slightly less sulfate
- Crab – actively regulating against the external environment
- Nephrops – low magnesium. Magnesium effects nerve transmission, and fast impulses needed for escape mechanisms.
- Squid – low sulfate, which is a heavy ion. Removing it increase its buoyancy.
Give an example of the benefit of reducing heavy ions.
Reducing sulphate ion concentration aids buoyancy in squid and jellyfish (work by Eric Denton FRS at the MBA, Plymouth)
Metion baltic sea starfish.
- Echinoderms are stenohaline marine
- North Sea starfish have a salinity tolerance of ~20 they have reasonable heat tolerance and breed successfully once a year
- Baltic Sea starfish have a salinity tolerance of ~8.
- They have a soft integument;
- Increased water content; poor heat tolerance, reduced metabolism and do not breed.
- Individuals are recruited from N Sea populations.
- Do not breed, entirely depndant on supply from north sea.
- Give up tolerance to temperature and to breed.
Mention osmoconformity in sipunculids
The peanut worm Themiste
This acts as a simple osmometer in dilute seawater the animal swells
- Water enters passively in dilute media
- After about 10h they reach a new stable, larger volume
- The worm shrinks again back to its original volume on return to normal seawater
Why can some organisms control the amount it swells, but looses their ability to recover quickly afterwards?
- In Golfingia, some amino acid loss reduces the degree of hydration in dilute seawater
- There is no recovery of volume
- In Dendrostomum salt loss via the gut and nephridia allows limited volume recovery in dilute seawater
Describe the difference between an osmoconformer and an osmoregulator on a graph.
- Average seawater salinity ~34.5
- Chlorinity of 18 (18ppt)
- Osmolality of ~1000 mOsmol.kg-1
What can be some examples of causes of osmoregulation failure ( example of the lug worm (Hediste diversicolor).
- Reduces permeability to salts & H2O in dilute media
- Exhibits increased 02 uptake in dilute media –(as muscles resist stretching when the animal swells)
- In low [02] osmoregulation fails
- In the presence of CN- osmoregulation fails
- In absence of Ca2+ osmoregulation fails
- In seawater from 100-75% Na+ uptake is passive
- Below 75% Na+ uptake is active
- In seawater from 100-50% Cl- uptake is passive
- Below 50% Cl- uptake is active
- Low temperature also exerts an effect and sets the Northern geographical limit for the species
What can allow for burrowing organsims to be buffered against changes in slalinity?
- Burrowing organisms in estuaries are ‘buffered’ from salinity variation
- Where you live – important can act as buffer
- Below 8 cm no variability occurs in salinity
What are the osmoregulatory abilities of Cephalopods?
- Cephalopods are very stenohaline – strictly marine.
- They do not have osmoregulatory ability but do show ionic regulation.
- Shell-less gastropods like sea slugs are osmoconformers that are also stenohaline.
- Aplysia shows limited regulation in 95% seawater but 80% seawater kills it!
What are the osmoregulatory abilities of Molluscs?
- Mussels are euryhaline and can tolerate 50% seawater.
- Similarly, clams and oysters can tolerate reduced salinity by closing the shell valves for several days.
- Reduces predation from starfish & dogwhelks.
- Mud snails, Hydrobia ulvae, are less active in low salinity.
- Haemolymph (blood) concentration alters slowly in inactive snails and rapidly in active snails.
Acclimation to reduced salinity
- In reduced salinity, Mytilus loses amino acids and the nitrogenous base taurine
- This reduces salt loss
- Acclimation is measured by ciliary activity which is a measure of mussel filtration (feeding)
- Mussels can acclimate to reduced salinity but this process takes about 30 days
- Acclimation – 1 variable
- Animals will acclimate over time to a new salinity regime.
- The North Sea, and exposed to what you would expect in the Baltic
- Over time after exposed (30 days) the mussels perform
- Capacity in the genotype/phenotype to adapt to changes in salinity.
‘The lugworm Arenicola marina: A model of physiological adaptation to life in intertidal sediments’
Zebe and Schiedek 1996
wider reading - osmoregulation in Lugworm.
Osmoregulation
- The lugworm is an osmoconformer so when the salinity of the ambient water changes, this induces corresponding changes of the cells.
- Osmoregulation in the lugworm is accomplished by two distinct and independent mechanisms: one extracellular and one intracellular.
- Extracellular volume regulation is probably based on a variation of the rate of urine production.
- Intracellular volume regulation is affected by changing amino acid concentrations, glycine and alanine in particular, in the cytoplasm.
- After heavy rains, during low tide, lugworms can start swelling and subsequently reduce their volume. In these cases, the amino acid concentration in the body-wall did not change, as this is likely to happen over long -term (i.e. seasonal) salinity changes.
Give examples of 4 different types of regulators.
- Carcinus (Shore Crab) - a hyper-regulator - euryhaline
- Uca - (Fiddler Crab) a hyper-hypo-regulator - warmer tropical environments
- Mudflats – an osmotic imbalance
- Can switch hyper/hypo. As the tide gets fresher it will maintain its internal body fluids – like carcinus.
- Eriocheir - (Chinese mitten crab a strong hyper-regulator
- Native to SE Asia, very successful invasive species – they are able to regulate
- Tends to conform, but in really fresh systems is can regulate efficiently
- Artemia - (Brine Shrimp) - A strong hypo-regulator
- Range of different responses within one taxonomic group.
The shore (green) crab Carcinus maenas
- The shore crab hyper-regulates when in salinities below ~25
- (c.75% seawater)
- Living in estuaries, the shore crab osmoconforms at higher salinities and therefore probably needs only to osmoregulate during the peak low tide period
- This reduces the overall energy expenditure associated with regulation
The fiddler crab Uca pugilator
- Uca forages over the surface of exposed estuarine sediment at low tide.
- It lives in burrows which give it some protection from temperature and salinity variation.
- Uca hyper-regulates at lower salinities (<~25) and hypo-regulates at higher salinities (>~25).
Chinese mitten crab – Eriocheir sinensis
- The Chinese mitten crab has a complex life cycle (catadromous), growing to the adult in freshwater and returning to the estuaries to breed
- In freshwater and salinities below ~12 (i.e. ~30 % seawater) it hyper-regulates.
- Early life stages are not able to hyper-regulate and survive in freshwater.
- This ability is only acquired in the juvenile stage.
What are some problems faced by hyper-regulators?
(when haemolymph is hyper-osmotic to the external medium)
- A tendency for water to enter to achieve osmotic equilibrium
- Solutes tend to be lost
- –because the internal concentration is higher than the external concentration
- –water must be excreted and this carries with it some salts
- These problems can be overcome by:
- Permeability reduction - – change the composition of the exoskeleton – how porous is it?
- Active uptake of ions - over gill area
- Excretion of a dilute urine
Mechanisms of permeability reduction
•Changes in the boundary tissue structure
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–increase in phospholipid content of gills as seen for example in Eriocheir sinensis
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•Changes in the haemolymph-medium interaction
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–Changes in blood flow rate through gills
–Shunt pathways in the gills
–Changes in ventilation rate or heart rate
–Changes in the water flow over the gills
Pathways of salt and water passage in hypo-regulators
(Homer-Smith, 1930 - for marine teleosts)
a)Passive diffusion on water is offset by drinking the external medium
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b)Water absorbed by the gut carries with it a salt burden
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c)Urine is isosmotic or slightly hyper-osmotic to the external medium
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d)Net salt extrusion occurs at the gills
