Aquatic animals have different modes of excretion depending on their habitat, body size, and evolutionary history. One of the major factors affecting excretion is the concentration of solutes in the environment. Aquatic animals can be categorized into three groups based on their mode of excretion: osmoconformers, ammonotelic, and uricotelic.

Osmoconformers:
Aquatic animals that live in environments with a stable salinity, such as seawater, are osmoconformers. These animals maintain the same osmotic pressure as their environment by allowing solutes to passively diffuse across their body surfaces. Osmoconformers excrete waste products, such as nitrogenous wastes, in the form of ammonia.

Ammonotelic:
Aquatic animals that live in freshwater environments, which have a lower concentration of solutes compared to their bodies, are called ammonotelic. These animals excrete nitrogenous wastes in the form of ammonia, which is highly toxic and requires a large amount of water to dilute it.

Uricotelic:
Terrestrial animals and some marine animals that have low water availability have evolved to excrete nitrogenous wastes in the form of uric acid. Uric acid is less toxic than ammonia and requires less water to excrete. Uricotelic animals include birds, reptiles, and insects.

In summary, aquatic animals can be osmoconformers, ammonotelic, or uricotelic depending on their habitat and evolutionary history. Osmoconformers excrete ammonia, while ammonotelic animals excrete ammonia and uricotelic animals excrete uric acid.

Ammonia is a toxic substance and since aquatic animals are in and around water the ammonia excreted by them can be easily diffused and diluted making it less toxic. Hence Ammonotelic.