Natural History of Sharks, Skates, and Rays Homeostasis MARE 394 Dr. Turner Summer 2009

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

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

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

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

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

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

Osmoregulation

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

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

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

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

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