Passage Within the Actinopterygii (Bony Fish) Lineage, Marine Teleosts (Bony Fish

Passage
Within the Actinopterygii (bony fish) lineage, marine teleosts (bony fish with mobile jaws) live in an aquatic environment where there is a higher external concentration of salt and a lower concentration of water relative to the internal concentrations.  Owing to these conditions, salt ions tend to diffuse into the teleost through its skin, whereas water molecules within the organism osmotically traverse the opposite path.Irrespective of lineage, many fish species actively assess their environment and regulate their internal concentration of fluids and electrolytes via a homeostatic process known as osmoregulation.  Teleosts with either glomerular kidneys (eg, eels, sculpin) or aglomerular kidneys that are purely tubular (eg, goosefish, toadfish) have served as experimental subjects for the study of marine teleost osmoregulation.  Analyzing the ionic (ie, Na⁺, Cl⁻, Mg²⁺, SO₄²⁻) concentration of intestinal fluids, urine, and plasma in each species led to the discovery that the renal tubule of the goosefish has both excretory and reabsorptive functions.Scientists have concluded that marine teleost osmoregulation involves the ingestion of seawater, retrieval of NaCl and water from the intestine, and primary excretion of divalent ions via urine and monovalent ions through the gills (thin barriers between the organism's blood and the aquatic environment) .In an experiment to further study osmoregulatory mechanisms, researchers emptied the gut of an eel and sealed the anus to prevent fluid loss.  The eel was transported into seawater that had been treated with the volume marker phenol red.  After 20 hours, the eel's gut contained 2.3 mL of fluid but showed a phenol red concentration equivalent to 12.3 mL of ingested seawater.  Researchers found that the eel lost 2.3 g of weight due to urine output.  They ultimately reported that the eel had lost all the seawater it absorbed through the gut via various mechanisms.
-Scientists also study freshwater teleosts, which live in an environment where there is more water and less salt outside their bodies than inside.  Osmoregulation in freshwater teleosts would most likely serve to combat their natural tendency to:

A) lose water and salt.
B) absorb water and salt.
C) absorb salt and lose water.
D) lose salt and absorb water.

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