Ionic composition of the blood
The body fluids of simle marine animals resemble that of seawater,
allowing for the Donnan equilibrium. When holothurians, for example,
are placed in artificial seawater in which the ionic composition
has been changed, the body fluids alter in conformity with the
external medium and reach equilibrium within 5 days, the relative
rates for ionic movement across the body wall are K+>Na+>Ca2+>Mg
and Cl- > SO42-.
Ionic compositions of body fluids in animals possess a general pattern. Although in lower animals the fluids closely resemble seawater, higher forms tend to accumulate K+ relative to Na+ and to reduce levels of Mg2+ and SO42-.
| Component | Na+ | K+ | Ca++ | Mg++ | Cl- | SO4= | Urea | TMO |
| ANIMAL/FLUID | ||||||||
| Seawater | 455 | 9.4 | 9.8 | 52.4 | 524 | 27.3 | 0 | 0 |
| Aurelia aurita | 455 | 10.5 | 9.7 | 50.8 | 554 | 15.2 | ||
| Ascidia nigra | 403 | 17.6 | 10.4 | 38.5 | 14.8 | |||
| Myxine glutinosa /plasma | 529 | 10.4 | 6.4 | 25.6 | 534 | 18.3 | 5 | |
| Myxine glutinosa /urine | 481 | 12.8 | 8.9 | 29.7 | 536 | 22.3 | 9 | |
| Petromyzon marinus marinus in seawater | 156 | 31.8 | 3.3 | 7.0 | 159 | 4.4 | ||
| P. marinus marinus , freshrun in freshwater | 112 | 2.3 | 1.6 | 1.5 | 99.6 | |||
| Petromyzon marinus dorsatus in freshwater | 137 | 3.3 | 2.2 | 2.0 | 122 | 0.1 | ||
| Petromyzon marinus dorsatus urine | 11.6 | 2.0 | ||||||
| Lake Huron water | 0.02 | 0.05 | 0.9 | 0.25 | 0.05 | 2.3 | ||
| Lampetra planeri | 116.4 | 7.5 | 95.1 | |||||
| Squalus acanthias | 257 | 7.0 | 4.0 | 6.0 | 277 | 1.8 | 248 | 64 |
| Gadus morhua | 181 | 10.1 | 4.7 | 2.4 | 176 | |||
| Coregonus clupeiodes (freshwater) | 141 | 4 | 10.3 | 6.8 | 117 | 9.2 |
Cyclostomes
Myxinoidea - Hagfish - marine, developing from large yolky eggs
Petromyzontida - Lampreys- marine and FW, marine forms spawn in FW, lay small eggs which drevelop into filter feeding ammocyte larvae. Young lampreys return to sea after metamorphosis.
The sea lamprey, Petromzon marinus has a land-locked race (P. m. dorsatus) in the Great lakes.
Total body water content in cyclostomes is high (72-86%), extracellular body fluids make up approximately one third of this. Hagfish can live in aquaria for months without feeding and ions and water lost in the urine must therfore be replacedby absorption of equvivalent amounts. Most ions can diffuse in at the body surface of gills but Na has to be adsorbed against a gradient. Water in the gut has the same composition as seawater but drinking seawater has been found only in some specimens and may not be a normal feature. K, Ca, Mg, SO4 and urea are actively excreted.
The serum of hagfish is almost iso-osmotic with seawater (within 2%). Myxine glutinosa lives in water of salinity 31-35 (922-1047 mosmoles). Direct transfer to water of 25 leads to body swelling and death within a few hours. M. glutinosa can be acclimated to water outside its normal range provided this is done very slowly. Slow dilution of the sea water over a week allows the hagfish to live in water of 590 mosmoles. Similarly it is possible to adapt specimens to 1500 mosmoles. The hagfish remains iso-osmotic with the water when changes are made at this speed and is therefore an osmoconformer or poikilosmotic.
The Myxinoids are similar to cephalochordates, tunicates and almost all marine invertebrates in having blood and tissues isosmotic with seawater. As with many marine invertebrates the hagfish show marked ionic regulation, particularly a lowering of Mg2+ and SO42- relative to sea water. However, they also show a lowering of Ca2+, a feature found only in the tunicates and the mesogloea of jellyfishes. Myxine muscle fibres resemble those of decapod crustacea and cephalopods in having an organic fraction of trimethylamine oxide, betaine and free amino acids, especially proline. Since these features are present in marine invertebrates on distinct marine ancestry, it is likely that these features in myxinoids have been derived from their simpler marine chordate ancestors.
Lampreys, in contrast, are osmoregulators or homiosmotic. Those that live in freshwater have plasma concentrations of 205-245 mosmoles while Petromyzon marinus in its marine phase has a plasma concentration of 315 mosmoles, decreasing to as low as 135 mosmoles inthe rivers after spawning. These animals do not, under normal conditions, recover from spawning and will die in a few hours if returned to full-strength sea water. Degeberationn of internal tissues, e.g the intestine, in freshwater can be prevented by removing the gonad.
In Myxine - the osmolarity of the body fluids is due largely to inorganic ions with urea making up <0.5%.
Lampreys, with their anadramous habits, have the same physiological mechanisms in freshwater as freshwater teleostean fish, i.e uptake of ions by the gills and excretion of hypoosmotic urine. However their permeability to water is greater and, despite a lower osmotic concentration, they excrete a greater volume of urine (160-180 ml kg-1 day-1 for Petroyzon marinus in freshwater, compared with 25 160-180 ml kg-1 day-1 for Anguilla anguilla).
Elasmobranchs
Elasmobranchs are the only other vertebrates with as high an osmotic concentration in the blood and other body fluids as the hagfish. However, while in the hagfish inorganic ions make up 98-99% of the total osmotic constituents, inorganic ions are reponsible for only half the osmotic concentration in Elasmobranchs, the other half being made up of urea and trimethylamine oxide.In elasmobranchs the main nitrogen excretory product is urea. The branchial membranes are relatively, but not absolutely, impermeable to urea, but 75% is still lost via the gills. Both urea and TMO are conserved by the kidney.
In marine elasmobranchs, of the fluids taken into the alimentary canal, Mg2+ and SO42- are absorbed to only a slight extent compared with Ca2+, K+ and Cl-. The mechanism of ionic regulation is somewhat similar in marine teleosts, except that the blood is hypo-osmotic. Monovalent ions are adsorbed from the sea watrer drunk, while Ca2+, Mg2+ and SO42- are concentrated in the intestinal fluid. Excess Mg2+ and SO42- are excreted in the urine while Na+, K+ and Cl- are excreted extra-renally by the gills.
Teleosteans
In teleostean fish the relative excretion via the gills and the urine have been differentiated by using a rubber dam around the fish. 6-9 times as much nitrogen is excreted via the gills as via the urine, with the more diffusible compounds, ammonia, urea, amines and trimehylamine oxide, being lost via the gills and creatine, creatinine and uric acid via the kidneys.
|
|
|
|
| Myxine glutinosa | 5.4 | 4.6 |
| Petromyzon marinus dorsatus freshwater | 181 | 387 |
| Mustelus canis | 15 | |
| Anguilla anguilla marine | 6 | 10.5 |
| Anguilla anguilla freshwater | 26.4 | 36.1 |
| Carasius carrassius | 300 | |