Final answer:
Marine teleost fish are stenohaline and cannot regulate at 300 mOsm in freshwater environments; they, along with osmoconformers like cartilaginous fish, have specialized osmoregulatory mechanisms to maintain solute balance in marine environments.
Step-by-step explanation:
The vertebrates that do not regulate at 300 mOsm are marine teleost fish, which are incapable of osmoregulation in freshwater environments because they are stenohaline. As osmoregulators, vertebrates possess specialized mechanisms for maintaining internal solute concentrations that differ from their external environments. In freshwater, these mechanisms involve producing a large amount of dilute urine and actively transporting salts through the gills to achieve electrolyte balance. In contrast, marine vertebrates that are osmoconformers, like cartilaginous fishes, maintain an isotonic state with the ocean through the accumulation of urea and TMAO, while bony marine fish and other organisms adapt through different regulatory mechanisms, including drinking seawater and secreting excess salts. Osmoregulatory organs, such as the kidneys and specialized glands, are critical for this process.