Osmoregulation and excretion in invertebrates.

The Aquatic Environment

71% of the earths surface is covered in water. All water contains dissolved salts, gases & organic compounds. These, in addition, to temperature, are of the greatest importance physiologically. In these two lectures we will be exploring how animals regulate these dissolved salts and water.

Types of metabolic waste produced by living systems:

1. 2. 3.

4.

Digestive waste. Respiratory waste. Excess water and salts (through osmoregulation). Nitrogenous waste (through excretion).

Excretion and osmoregulationExcretion is the elimination of metabolic waste products from the body including CO2, H2O & excess nitrogenous products. (Elimination of respiratory CO2 will not be covered here). Osmoregulation is the regulation of water and ion balance within the body fluids.

Excretion and osmoregulation. osmoregulation.

Body fluids are dilute saline solns. assumed to reflect origin of life in sea. Some animals can tolerate greater changes in their body fluid concentrations than others. In many animals excretion & osmoregulation managed by same organs. Osmoregulation & excretion are closely linked phenomena.

Excretion and osmoregulation.Excretion, osmoreg. & ion regulation: Rid body of potential toxic wastes. Maintain conc. of components of body fluids at levels appropriate for metabolic activities. These processes tied structurally & functionally to overall level of body complexity, physiology & environment in which animal lives.

Basic terms.

Membranes of animals are permeable, to some extent, to solvent (water) & solutes (ions). Diffusion is mechanism by which solutes move from more concentrated to less concentrated solutions. Osmosis is process by which solvents move from less to more concentrated solutions, usually across semipermeable membrane. Osmotic pressure is the force that drives solvent from low to high concentration.

Osmotic states.

Animal is isoosmotic with medium in which it lives (isos = equal in Greek) when body osmotic concentration equals that of environment. Animals that have a higher osmotic concentration than environment are hyperosmotic. Animals that have body fluids less concentrated than surrounding medium are hypoosmotic.

Osmotic states.Isoosmotic Hyperosmotic

Animal

Environment Animal Environment

Hypoosmotic Animal Environment

Osmoregulatory strategies.

Osmoconformers are animals that conform their osmotic concentration with that of environment. Osmoregulators are animals that maintain their osmotic concentration despite changes in environment. Some osmoconformers do regulate certain ions at concentrations different from those in the environment Ionic regulation.

Ionic regulation in marine invertebrates.% concn. to that of sea water Na Mg K Ca Cl S04Aurelia (Cnidaria) 99 Pachygrapsus (Crustacea) 94 Sepia (Mollusca) 93 97 24 98 106 95 205 96 92 91 104 87 105 47 46 22

Aurelia = isoosmotic osmoconformer Pachy = hypoosmotic osmoregulator Sepia = hyperosmotic osmoregulator

Nitrogenous waste products.

Most nitrogen in animals system from amino acids digestion of proteins. These aas may be used to build new proteins or deaminated & residues used to form other compounds. Excess nitrogen released during deamination usually liberated from aas in form of NH3 (ammonia). NH3 highly soluble but quite toxic needs diluted, eliminated or converted quickly.

Nitrogenous waste products in invertebrates.

Not as well studied as vertebrates. Typically 1 nitrogenous waste form predominates in a given species & is generally related to availability of environmental water. Major excretory product in most marine & freshwater inverts is NH3. Such animals are ammonotelic.

Nitrogenous waste products in invertebrates.

Terrestrial inverts have water conservation problems. Cant afford to lose much body water in process of diluting wastes. Convert wastes into more complex but less toxic substances. Metabolically expensive to produce but conserve water & can be stored in body prior to excretion.

Detoxification of ammonia.2 major metabolic pathways for detox of NH3: Urea pathway ureotelic animals include amphibians, mammals & cartilaginous fish. Rare in invertebrates. Uric acid pathway uricotelic animals capitalise on relative insolubility (& low toxicity) of uric acid (generally precipitated as solid/semi-solid form). Most terrestrial arthropods & snails use uric acid pathway.

Osmoregulation & habitat in invertebrates.Osmoreg directly associated with habitat. 4 habitats to consider: Marine - Composition of seawater & body fluids of most inverts very similar. Body fluids of many marine inverts & their habitat close to isotonic. Freshwater - Body fluids of freshwater animals strongly hypertonic to environment. Terrestrial Animals exposed to air problem of water loss. Brine/brackish changes in salinity.

Osmoreg in marine /brackish invertebrates.

Body fluids of marine inverts almost isotonic to environment but not absolutely- still need absolutelysome degree of osmoreg. osmoreg. 1 strategy - control intracellular but not extracellular osmotic concentration. E.g. Mussel Mytilus edulis - osmoconformer. Regulates its intracellular osmotic concentration with changes in medium. Amino acids are used. In dilute medium, proteins broken down to liberate amino acids maintaining intracellular osmotic concentrations.

Osmoreg in freshwater invertebrates.

Body fluids of freshwater animals strongly hyperosmotic (hypertonic) to environment must remove excess water taken in. Useful ions extracted by organs of excretory system & excess water eliminated. Many freshwater inverts produce lot of urine! E.g. Crayfish Astacus ~ 40% of body weight/day. Crustacean Daphnia >200% body weight/day Clam Anodonta >400% body weight/day.

Crustacea: Crustacea: permeability of epidermis as a function of environment.E Permeability D C A B

Freshwater

Marine

Hypoosmotic invertebrates.Usually both marine & freshwater invertebrates hyperosmotic or isoosmotic to the medium. Some exceptions: Palaemonetes (salt marsh shrimp) hypoosmotic in sea water but hyperosmotic in freshwater. Artemia (brine shrimp) live hypoosmotically as hyporegulators under salt conditions many times > than sea water. Active uptake of water & NaCl & elimination of excess NaCl

Concept of active transport.

Active transport - ability to move solutes (salts, ions) from a less to higher concentrated environment against osmotic gradient. E.g. Artemia drinks water but transports excess salts back outside, probably through gills. Requires energy. However, in general, the cost is not high: e 1% of metabolic rate.

Terrestrial invertebrates.Danger: dehydration. Solutions: Epicuticular wax layer Insecta. May be restricted to moist environments. Obtain water in food. Drink through anus via uropods (woodlice). Absorb water directly from air (mites). Metabolic water, e.g., from sugar (Insecta). C6H12O6 + 6O2 6CO2 + 6H2O Structural, behavioural & physiological adaptations.

Arthropods that can absorb water from air.Animal Tick Desert Roach Flour mite Flea Limiting relative humidity (%) 92-94 83 70 50

STENOHALINE VS. EURYHALINE VS.

Stenohaline narrow salt; Most animals salt; cannot tolerate substantial changes in external osmolarity. osmolarity. Euryhaline broad salt; Animals that can salt; survive large fluctuations in external osmolarity. osmolarity. Examples: Salmon, Tilapia

Euryhaline

Stenohaline

MARINE ANIMALS

Most marine invertebrates are osmoconformers. osmoconformers. Marine vertebrates and some invertebrates are osmoregulators. osmoregulators. Ocean is strongly dehydrating because it is much saltier than than internal fluids and water is lost by osmosis. osmosis. Balance water loss by drinking large amounts of seawater. seawater. Salt is actively pumped out of gills and passed through urine. urine.

FRESHWATER ANIMALS

Problems are opposite those of marine animals. animals. Freshwater animals are constantly gaining water by osmosis and losing salt by diffusion. diffusion. Maintain water balance by excreting large amounts of very dilute urine and taking in salt by the gills. gills.

Osmoregulatory organs in invertebrates.

Four to consider: 1. Contractile vacuole of protozoa & sponges. 2. Nephridial organs of platyhelminths & annelids. 3. Antennal gland (Green gland) of crustaceans. 4. Malpighian tubules of insects.

1. Contractile vacuole (water expulsion vesicles).

Main osmoregulatory sponges.

organ

of

protozoa

&

Primarily for osmotic & volume control - actively excretes water from organism. CVs accumulate cytoplasmic water & expel it from cell. Contractile vacuole surrounded by layer of small vesicles inside ring of mitochondria. Mitochondria presumably provide maintain hypoosmotic vacuole. energy to

Contractile vacuole (water expulsion vesicles).Evidence supporting osmoregulation function of Contractile Vacuoles:

Rates of filling & emptying change dramatically when cell exposed to different salinities. E. g. marine flagellate, Chlamydomonas pulatilla lives in supralittoral tidal pools & is exposed to low salinity during rain, at which times it regulates cell volume & internal osmotic pressure via action of CVs (which increase in activity as salinity of rock pool drops).

Excretion and osmoregulation in Paramecium

2. Nephridia.

Ectodermally derived nephridia used for osmoregulation and/or excretion. excretion. Types to consider: consider:

Protonephridia & metanephridia. metanephridia.

2. Nephridia - protonephridia.

Probably earliest type nephridium to evolve was protonephridium. protonephridium. Usually have tubular arrangement opening to outside by 1 or more nephridiopores & terminating internally in closed cells. unicellular units called cap or terminal cells. Each cell leads to excretory duct nephridoduct & eventually to nephridiopore. nephridiopore. Two types of protonephridia - flame bulbs, bearing tuft of cilia bulbs, solenocytes, within cavity & solenocytes, usually only 1 or 2 flagella.

Nephridia - protonephridia.

Cilia/flagella drive fluids down nephridoduct lowered pressure in duct lumen. Lowered pressure draws body fluids (incl. wastes) across cell membranes into duct. Selectivity based on molecular size. Protonephridia common in flatworms (acoelomates), rotifers (blastocoelomates) & acoelomates), (blastocoelomates) some annelids. Protonephrida more important in osmoreg than excretion. Nitrogenous wastes primarily expelled by diffusion across body surface.

Protonephridium of flatworm

Protonephridia: Flame-bulb system Protonephridia: Flame Flatworms Functions in osmoregulation (wastes diffuse out through body surface).

Protonephridia

Protonephridia

ProtonephridiaNucleus of cap cell Flame bulb Cilia

Tubule Tubules of protonephridia

Interstitial fluid flow Opening in body wall Tubule cell

Nephridia - metanephridia.

Probably more advanced excretory structure in invertebrates is metanephridium. metanephridium. Important structural difference to protonephridia. protonephridia. Metanephridia open to outside but also open internally to body fluids. fluids. Multicellular. Multicellular. Inner end bears ciliated funnel nephrostome. nephrostome. Duct long & convoluted & may have bladderbladderlike storage region. region.

Metanephridia in annelids (the earthworm)

Nephridia - metanephridia.

Take in large amounts body fluid through nephrostome & selectively absorb most nonnon-waste components back through walls of bladder or excretory duct. Metanephridia found in molluscs, sipunculans, eichurans & some annelids.

Metanephridia in annelids.

Type of Organism

Structure

Product of excretion

Other features

Coelom

Capillary network

Flatworms

Flame cells

Components of a metanephridium: Collecting tubule Internal opening Bladder

Unsegmented roundworms

Protonephridia, closed network of dead-end tubes lacking openings

External opening

Annelids

Metanephridia, open-ended network of tubes with internal openings that collect body fluids

Excretion & osmoregulation in arthropods:

With evolution of haemocoelic circulatory systemin arthropods nephridia with open nephrostomes became functionally untenable. Could not simply drain blood directly from open haemocoel to outside! Arthropods therefore evolved highly efficient excretory structures that share common adaptive feature in that they are internally closed. Reduction in overall number of excretory units.

Excretion & osmoregulation in arthropods:

Marine crustaceans excrete 70-90% nitrogenous waste as NH3. Rest is urea, uric acid, aas & other compounds. Terrestrial arachnids, miriapods & insects excrete predominantly uric acid. Conservation of water success of life on land.

3. Antennal (Green) glands of Crustacea. Crustacea.

A pair of glands in the head: green glands Components: An end sac (+ podocytes) interfacing with blood vessels. A long coiled excretory tubule. A bladder. Excretory pore opens at base of antennae.

3. Antennal glands of Crustacea.

Podocytes: specialized interlocking cells allow small compounds e.g. aas, sugars & water to pass through but retain larger molecules e. g. proteins. They ultrafilter blood.

Antennal gland function.Examples:

Lobster (marine): Ultrafiltration but no water reabsorption in the tubule. Active secretion of substances. Carcinus crab (penetrates brackish water): Ultrafiltration with reabsorption: ions are resorbed followed by water through active transport.

Other functions of the antennal glands.

Retention of K+ and Ca++ Elimination of excess Mg++ and SO4. Mg++ seems to be actively transported into urine when urine is in bladder. Concentration of Mg++ in urine increases with increasing water salinity.

4. Malpighian tubules of insects.

Arise as blind tubules extending into haemocoel from gut wall. Malpighian tubule uptake from haemocoel relatively non-selective. nonResulting primary urine emptied directly into gut. Little reabsorption of non-waste material nonin tubule itself. Hindgut responsible for concentrating urine by reabsorbing non-waste nonfractions.

Malpighian tubules in the ant (an insect).

The Insect Malpighian tubule serves as both an osmoregulatory organ and an excretory device.

Type of Organism

Structure

Product of excretion

Other features

Digestive tract

Molluscs

Nephridia or metaphridia

Rectum Hindgut Intestine Midgut Malpighian (stomach) tubules Salt, water, and Feces nitrogenous and urine wastes

To anus

Crustaceans

Antennal/green gland

Malpighian tubule Rectum Reabsorption

HEMOLYMPH

Insects

Malpighian tubules and digestive tract

Uric acid

Type of Organism

Structure

Product of excretionInsoluble crystals

Other features

Stomata, lenticels

Plants

Crystals are kept inside plant cells

No excretory organ

-

Cnidarians and echinoderms

Osmoconformers, isoosmotic with environment

Contractile vacuole

Freshwater protists and sponges

Summary.In this part we have:

Described terminology associated with osmoregulation. osmoregulation. Discussed various strategies used by invertebrate animals to regulate water/salt balances. Described osmoregulatory requirements of invertebrate animals living in freshwater, marine & terrestrial environments. Explored some of the organs of osmoregulation found in invertebrates.