1. Natural History of Sharks, Skates, and Rays
Homeostasis
MARE 394
Dr. Turner
Summer 2008

2. Homeostasis
It is well known that the osmotic composition of body fluids of elasmobranchs is significantly different than ambient seawater
Compared to teleosts:
- ↑ concentration of urea
- Stabilized by ↑ levels of TMOA (trimethylamine-oxide)
- ↑ Solutes compared to seawater causes water to passively invade the body

3. Homeostasis Compared to teleosts:
↑ Solutes compared to seawater causes water to passively invade the body
Salt balance also maintained by using:
Rectal gland, Kidneys, Gills

4. Osmoregulation
Osmosis = movement of pure H2O across membrane (permeable H2O but not to solute)
Water moves from area of lower solute concentration to higher
Water
Low High
Lower outside
Equal
Higher outside
More dissolved material in cell –
water moves into cell
Less dissolved material in cell
- water moves out of cell

5. Osmoregulation Body 1/3 solutes of SW….How to
Maintain osmotic homeostasis
Water loss by osmosis: gills and skin 35
Marine Teleosts – (bony fishes) 14
Drink SW
Salts pass through gut
Excrete concentrated urine; small volume
Selective excretion of salts (Mg2+ SO42-) at gills

6. Osmoregulation Freshwater Teleosts – (bony fishes)
Water gain by osmosis: gills and skin 14
Does not Drink
Salts pass through gut
Excrete dilute urine; Large volume
Selective absorption of salts (Mg2+ SO42-) at gills

7. Osmoregulation Some water & salt with food
Excess salts excreted by digitiform or rectal gland and in urine

8. Osmoregulation

9. Rectal Gland Major but not sole site of salt secretion
“So I says ‘Rectum…Damn near killed em!’” – Mike Donnelly
Major but not sole site of salt secretion
Few studies but secretory fluid contains high conc. of NaCl; higher than seawater
Movement of Na & Cl powers a K-transport protein

10. Rectal Gland
“So I says ‘Rectum…Damn near killed em!’” – Mike Donnelly

11. Kidney
Kidney were originally thought to be of little function in elasmobranchs, as they are in many marine teleosts
In teleosts – kidneys not able to regulate salts; even with Rectal gland, require gills*
Elasmo – have high glomerular filtration rate (GFR)
*Via Chloride Cells

12. GFR High in elasmobranchs (1-3 ml kg-1 h-1) Same order as FW teleosts
Sig. Higher than Marine Teleosts

13. Gills
Most evidence for salt transport by elasmo gill involved with salt uptake but not salt secretion
Unlike marine teleosts – no major role in NaCl extrusion