Trying to use new words:
Walruses are the biggest pinnipeds in the fauna of Russia. Tusks of the upper jaw are the most prominent feature of walruses. They don't have ear pinnae. Searching for food, the animal dives to the ocean bottom and digs the prey out of the ground with tusks and snout. Walruses spend significant part of their life resting and sleeping. During the mating season, severe fights between males occur. The female gives birth to one calf, twins are rare.
Habitat: ocean and ocean shore (seashore?)
Walruses are the biggest pinnipeds in the fauna of Russia.
This sentence is fine as it is, but it sounds better (to me) as "Walruses are the biggest pinnipeds in the Russian fauna."
Tusks of the upper jaw are the most prominent feature of walruses.
The tusks of the upper jaw are the most prominent feature of walruses.
They don't have ear pinnae.
This is fine as it is. If you write "Walruses don't have ear pinnae", then you have the previous sentence ending in "walruses" and this sentence starting with the same word, which looks wrong.
Searching for food, the animal dives to the ocean bottom and digs the prey out of the ground with tusks and snout.
When searching for food [or "When feeding," - shorter sounds better], the animal dives to the ocean bottom and digs the prey out of the ground with
the [or
its] tusks and snout. [Technically the sea floor isn't "ground" (it needs to be on land to be ground), so you can replace "ground" with "sea floor" or "substrate"]
Walruses spend significant part of their life resting and sleeping.
Walruses spend
a significant part of their
lives resting and sleeping. ["Walruses" is plural, so "their lives"; if singular you would write "The walrus spends a significant part of its life resting..."]
During the mating season, severe fights between males occur.
This is fine, although the comma is not required.
The female gives birth to one calf, twins are rare.
This is fine, but it looks better with a semi-colon instead of a comma, i.e. "The female gives birth to one calf; twins are rare" or "The female usually gives birth to just one calf; twins are rare" - or even as two sentences, "The female usually gives birth to just one calf. Twins are rare".
Habitat: ocean and ocean shore (seashore?).
"Habitat: ocean and shoreline" or "coastline" or just "coast" are probably the best options for words to use there.