HomeostasisHomeostasis

Osmoregulation, pH regulation, Osmoregulation, pH regulation, and Nitrogen excretionand Nitrogen excretion

AnatomyAnatomy

Rectal GlandRectal Gland KidneyKidney GillsGills

Digitiform Rectal GlandDigitiform Rectal Gland Caudal region of Caudal region of

intestinal tractintestinal tract Atrophied and non-Atrophied and non-

functional in FW functional in FW elasmobranchselasmobranchs

Teleosts are lacking RGTeleosts are lacking RG Major site of NaCl Major site of NaCl

secretion (marine)secretion (marine) RG connected by rectal RG connected by rectal

duct to digestive tract, duct to digestive tract, secretions are expelledsecretions are expelled

KidneyKidney Paired organs on either side of vertebral columnPaired organs on either side of vertebral column Selachians: long and narrow; Batoids: short and Selachians: long and narrow; Batoids: short and

widewide Renal vascular anatomy…VERY intricate…what do Renal vascular anatomy…VERY intricate…what do

you need to know? Not much!you need to know? Not much! Elasmobranch nephron complexity rivals Elasmobranch nephron complexity rivals

mammalianmammalian

GillGill• 10 cartilaginous gill arches; 5 on each side of pharynx

•5 to 7 gills slits on either side of head

•Filaments: functional unit of gills; thin lamellaeshort diffusion distance for gas exchange between blood and environment thus increasing SA

•Countercurrent exchange between afferent and efferent arterioles increases gas exchange efficiency (of lamellae)

OsmoregulationOsmoregulation

MarineMarine Plasma HyperosmoticPlasma Hyperosmotic Large osmotic uptake Large osmotic uptake

of water via gillsof water via gills Rectal gland secretes Rectal gland secretes

NaClNaCl Kidneys concentrate Kidneys concentrate

urea and reabsorb urea and reabsorb most watermost water

FreshwaterFreshwater Plasma HyperosmoticPlasma Hyperosmotic Large osmotic uptake Large osmotic uptake

of water via gillsof water via gills Rectal gland Rectal gland

nonfunctionalnonfunctional Large urinary Large urinary

excretionexcretion NaCl uptake via gills, NaCl uptake via gills,

reabsorbed by kidneysreabsorbed by kidneys

Osmoregulation

NaCl

MarineFreshwater

NaCl

NaCl

NaCl

pH RegulationpH Regulation

Gills primarily responsibleGills primarily responsible

MarineMarine Rapid pH bufferingRapid pH buffering Subsequent net acid-base excretionsSubsequent net acid-base excretions

FreshwaterFreshwater Rapid pH buffering & acid-base excretionRapid pH buffering & acid-base excretion Primary Ion absorptionPrimary Ion absorption

Aborbs NaAborbs Na++ with H with H++ secretion secretion Absorbs ClAbsorbs Cl-- with HCO with HCO33

-- secretion secretion

Nitrogenous Metabolite Nitrogenous Metabolite ExcretionExcretion

Mostly excreted as urea, but must be Mostly excreted as urea, but must be retained for osmoregulationretained for osmoregulation

Ammonia is secreted with acid-base Ammonia is secreted with acid-base excretionsexcretions

Evolution of Freshwater Evolution of Freshwater ElasmosElasmos

Stenohaline marine elasmosStenohaline marine elasmos Moderately euryhalineModerately euryhaline Fully euryhaline - no life cycles in fwFully euryhaline - no life cycles in fw Fully euryhaline - complete life cycle Fully euryhaline - complete life cycle

in fwin fw Stenohaline freshwater - irreversibleStenohaline freshwater - irreversible

Stenohaline MarineStenohaline Marine

Large rectal gland Large rectal gland Cannot regulate Cannot regulate

excretory function in excretory function in lower salinitylower salinity

Squalus acanthiasSqualus acanthias

Moderately EuryhalineModerately Euryhaline

Moderately-large Moderately-large rectal gland that can rectal gland that can

Some rectal gland Some rectal gland regulationregulation

Dasyatis sayDasyatis say

Carcharhinus limbatusCarcharhinus limbatus

Fully EuryhalineFully Euryhaline

Rectal gland Rectal gland decreases in size with decreases in size with reduced salininty – reduced salininty – can be “turned off” in can be “turned off” in fwfw

• Carcharhinus leucas• Pristis perotteti & Dasyatis sabina

(reproduce in fw)

Stenohaline - FreshwaterStenohaline - Freshwater

Rectal gland Rectal gland atrophied and non-atrophied and non-functionalfunctional

(IRREVERSIBLE!!!)(IRREVERSIBLE!!!) PotamotrygonidsPotamotrygonids

Future ResearchFuture Research

Has only been studied in a few speciesHas only been studied in a few species updated and expanded to other speciesupdated and expanded to other species

New molecular techniques are being New molecular techniques are being appliedapplied genes and proteins involved in genes and proteins involved in processesprocesses

Current research indicates homology of Current research indicates homology of cellular and molecular processes to cellular and molecular processes to mammalsmammals future genomic and future genomic and proteomic approaches to determine proteomic approaches to determine evolution of transport processes evolution of transport processes

Questions?Questions?