Page 78 - The Toxicology of Fishes

58 The Toxicology of Fishes

TABLE 3.1
Generalized Structural, Functional, and Environmental Differences between Fish and Mammals
Characteristic Fish Mammals

Media
Surrounding media Water Air
Specific heat High Low
Solvent properties Universal solvent Nonsolvent
Dissolved gases Low levels High levels
Viscosity High Low
Temperature
Modality Poilkilotherms Homeotherms
Metabolic rate Slow Fast
Primary energy currency Amino acids and lipid Carbohydrates
Feed/weight conversion Very efficient Less efficient
Membrane composition Homeoviscous adaptation No adaptation necessary
Body temperature regulation Behavioral Internal setpoint
Respiratory surface
Primary organ Gill Lung
Blood/gas flows Counter-current Directional/tidal flow
Gas exchange Yes Yes
Acid–base balance Yes Yes
Nitrogen excretion Yes No
Osmoregulation Yes No
Primary driver of respiration Low oxygen High carbon dioxide
Acid–base balance
Primary organ Gill Lung and kidney
Primary mechanism Ion based Gas (lung) and ion (kidney) based
Nitrogenous waste elimination
Primary organ Gill Kidney
Primary form Ammonia Urea
Circulation
Heart Two chambers Four chambers
Arrangement Heart and gill in series Heart and lung in parallel
Vessels Arteries and veins less distinct Distinct arteries and veins

The gastrointestinal tract plays an important role in the absorption of xenobiotics in both fish and
mammals. The structure and function of the gastrointestinal tract in both taxa are similar in many respects.
In fish, however, the absence of a lymphatic system and lack of classical mammalian villi may substan-
tially influence the absorptive process. The feces represent an important route of elimination in fish,
both as a purveyor of bile and as a matrix with affinity for exsorbed compounds. Although bile formation
is slow in fishes relative to mammals, many xenobiotic compounds are readily excreted in bile.
In most cases, urinary elimination of xenobiotics is less important in fish than it is for mammals. This
is due in part to the contribution of the gills to chemical elimination and to the modified role of the
kidneys in comparison with mammals. In other cases, however, the urine may represent the primary
route of elimination. Urinary elimination is particularly important for ionic compounds that act as
substrates for active transport systems within the renal tubular epithelium.

The Environment

Large differences often exist among environmental toxicant concentrations, the concentration in the
systemic circulation of the fish, and the concentration involved in specific target organ toxicity. The basis
for these differences resides with both environmental factors and factors associated with the fish. The
amount of chemical released to the environment, its movement among compartments (e.g., air, water,
sediment), and susceptibility to transformation are primary determinants of toxicant exposure to the