Osmoregulation

Question 1

•Osmoconformer

Answer 1

•Osmoconformer - body fluid isosmotic with surrounding sea water.

Question 2

•Osmoregulator

Answer 2

•Osmoregulator - body fluid not isosmotic with sea water.

Question 3

Solute composition

Answer 3

•Solute composition - the dissolved substances (ions, proteins, etc.) in the body fluid.

Question 4

Haemolymph

Answer 4

•Haemolymph - body fluid contained in the primary body cavity (the haemocoele), this is the main body fluid in the Mollusca & Crustacea.

Question 5

Coelomic fluid

Answer 5

•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.

Question 6

Osmolarity

Answer 6

•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)

Question 7

Osmolality

Answer 7

•Osmolality – a variation of molality that takes into account only solutes that contribute to a solution’s osmotic pressure (Osmkg-1)

Question 8

Give the main solute composition in seawater

Answer 8

Question 9

What is the Donnan Equilibrium?

Answer 9

• 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.

Question 10

Mention how the ionic composition of seawater and some invertebrate body fluids vary between a few example organisms.

Answer 10

• 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.

Question 11

Give an example of the benefit of reducing heavy ions.

Answer 11

Reducing sulphate ion concentration aids buoyancy in squid and jellyfish (work by Eric Denton FRS at the MBA, Plymouth)

Question 12

Metion baltic sea starfish.

Answer 12

• 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.

Question 13

Mention osmoconformity in sipunculids

Answer 13

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

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Question 14

Why can some organisms control the amount it swells, but looses their ability to recover quickly afterwards?

Answer 14

• 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

Question 15

Describe the difference between an osmoconformer and an osmoregulator on a graph.

Answer 15

• Average seawater salinity ~34.5
• Chlorinity of 18 (18ppt)
• Osmolality of ~1000 mOsmol.kg-1

Question 16

What can be some examples of causes of osmoregulation failure ( example of the lug worm (Hediste diversicolor).

Answer 16

• 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

Question 17

What can allow for burrowing organsims to be buffered against changes in slalinity?

Answer 17

• 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

Question 18

What are the osmoregulatory abilities of Cephalopods?

Answer 18

• 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!

Question 19

What are the osmoregulatory abilities of Molluscs?

Answer 19

• 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.

Question 20

Acclimation to reduced salinity

Answer 20

• 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.

Question 21

‘The lugworm Arenicola marina: A model of physiological adaptation to life in intertidal sediments’

Zebe and Schiedek 1996

wider reading - osmoregulation in Lugworm.

Answer 21

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.

Question 22

Give examples of 4 different types of regulators.

Answer 22

• 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.

Question 23

The shore (green) crab Carcinus maenas

Answer 23

• 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

Question 24

The fiddler crab Uca pugilator

Answer 24

• 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).

Question 25

Chinese mitten crab – Eriocheir sinensis

Answer 25

* 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.

Question 26

What are some problems faced by hyper-regulators?

Answer 26

(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

Question 27

Mechanisms of permeability reduction

Answer 27

•Changes in the boundary tissue structure • –increase in phospholipid content of gills as seen for example in Eriocheir sinensis • •Changes in the haemolymph-medium interaction – –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

Question 28

Pathways of salt and water passage in hypo-regulators

Answer 28

(Homer-Smith, 1930 - for marine teleosts) a)Passive diffusion on water is offset by drinking the external medium ● b)Water absorbed by the gut carries with it a salt burden ● c)Urine is isosmotic or slightly hyper-osmotic to the external medium ● d)Net salt extrusion occurs at the gills