Q.Most bony freshwater fish excrete their nitrogenous waste chiefly as which of the following, a substance that is highly toxic but extremely soluble in water and so can be washed away continuously through the gills?

The adult frog excretes nitrogenous waste mainly as urea (ureotelic excretion).

Animals are classified by the chief nitrogenous waste they excrete: ammonotelic (ammonia — most bony/aquatic animals such as tadpoles, many aquatic invertebrates), ureotelic (urea — most terrestrial/semi-terrestrial amphibians and mammals), or uricotelic (uric acid — reptiles, birds, insects, adapted to conserve water). The adult frog, being amphibious but spending significant time on land where water conservation matters more than in its aquatic larval stage, converts toxic ammonia to the less toxic, less water-demanding urea and is therefore classified as ureotelic. (Its tadpole, being fully aquatic, is ammonotelic.)

Uric acid (a) is characteristic of uricotelic animals like reptiles and birds, not frogs; option (b) mixing uric acid and ammonia does not correctly describe the adult frog's excretion; ammonia alone (d) describes the aquatic larval stage, not the adult frog referred to generally in this question.

Adult frogs, like humans, are ureotelic animals that excrete nitrogenous waste chiefly as urea.

Animals excrete nitrogenous waste in one of three main forms depending on their habitat and need to conserve water: ammonia (ammonotelism, seen in aquatic animals like bony fish and tadpoles, where ammonia can be diluted and washed away quickly since it is highly toxic and needs plenty of water), urea (ureotelism, seen in terrestrial and semi-terrestrial animals like adult amphibians, mammals, and humans, where the more toxic ammonia is converted in the liver to less toxic urea, which is more water-soluble and requires far less water to be safely excreted), and uric acid (uricotelism, seen in birds, reptiles and insects, an almost non-toxic, poorly soluble semi-solid form that conserves the most water). Tadpoles, being aquatic, are ammonotelic, but as frogs metamorphose into adults and move onto land, their excretory pattern shifts to ureotelism, matching the pattern seen in humans.

Uricotelic animals excrete uric acid, so the answer is (C).

Animals are grouped by their main nitrogenous waste:

• Ammonotelic — excrete ammonia (e.g. many aquatic animals, bony fishes); needs a lot of water.
• Ureotelic — excrete urea (e.g. mammals, adult amphibians).
• Uricotelic — excrete uric acid (e.g. birds, reptiles, insects); uric acid is nearly insoluble and needs very little water, conserving water.

The term 'uricotelic' itself means uric-acid excreting. So uricotelic animals excrete uric acid.

The adult frog is ureotelic — it excretes nitrogenous waste mainly as urea.

Animals are classified by the form in which they excrete nitrogenous waste:

• Ammonotelic: excrete ammonia directly (needs large amounts of water to dilute the toxic ammonia) — e.g., many bony fishes, aquatic amphibian larvae (tadpoles).
• Ureotelic: convert toxic ammonia to less toxic urea in the liver (ornithine cycle) before excretion — e.g., adult amphibians (including the adult frog), mammals.
• Uricotelic: convert ammonia to insoluble uric acid — e.g., reptiles, birds, insects (water-conserving).

A frog begins life as an aquatic tadpole, which is ammonotelic like a fish. After metamorphosis, the adult frog becomes semi-terrestrial and switches to the ureotelic mode of excretion, converting ammonia to urea, which needs comparatively less water to be excreted safely.

The cockroach is uricotelic — it excretes nitrogenous waste chiefly as uric acid.

Animals are classified by the form in which they excrete nitrogenous waste: ammonotelic (ammonia — most bony fish, aquatic amphibians, aquatic insects, where earthworm and frog fall depending on habitat), ureotelic (urea — mammals, many terrestrial amphibians like frog on land, marine fish), and uricotelic (uric acid — insects, reptiles, birds, land snails). Uric acid is largely insoluble in water and can be excreted as a semi-solid paste with very little water loss, which is a major water-conservation adaptation for terrestrial animals such as insects. Among the given options, the earthworm is ammonotelic, the frog is ureotelic (on land) / ammonotelic (in water), and humans are ureotelic. Only the cockroach, an insect, is uricotelic.

Aquatic animals live surrounded by abundant water, so highly toxic ammonia can be rapidly diluted and diffused away through their body surface/gills as fast as it is formed, without needing to spend extra energy conve...

Unlimited surrounding water lets aquatic animals safely excrete toxic ammonia directly, without converting it to a less toxic form.

Ammonia is the most toxic of the nitrogenous excretory products, but it is also extremely soluble in water. Aquatic animals (most bony fishes, aquatic amphibians, many invertebrates) are constantly surrounded by a large volume of water, which allows the ammonia they excrete (mainly by diffusion across gills or the general body surface) to be immediately and continuously diluted to non-toxic levels and carried away. Because this dilution happens so efficiently, aquatic animals do not need to expend additional metabolic energy converting ammonia into the less toxic but energetically 'costlier' urea (as in ureotelic terrestrial animals) or uric acid (as in uricotelic animals adapted to water conservation). Hence, aquatic animals can afford to remain ammonotelic, excreting ammonia directly.