Albert Barnes Commentary Job 40:21

"He lieth under the lotus-trees, In the covert of the reed, and the fen." — Job 40:21 (ASV)

He lies under the shady trees - Referring to his usually inactive and lazy life. He is disposed to lie down in the shade, and especially in the vegetable growth in marshy places on the banks of lakes and rivers, rather than to dwell in the open field or in the upland forest. This account agrees well with the habits of the hippopotamus.

The word rendered “shady trees” (צאלים tse'eliym), here and in Job 40:22, is translated by Gesenius, Noyes, Professor Lee, and Schultens as “lotus” and “wild lotus.” The Vulgate, Syriac, Rosenmuller, Aben-Ezra, and others render it “shady trees.” It occurs nowhere else in the Scriptures, and it is difficult, therefore, to determine its meaning.

According to Schultens and Gesenius, it is derived from the obsolete word צאל tsā'al—“to be thin, slender;” and therefore, in Arabic, it is applied to the “wild lotus”—a plant that grows abundantly on the banks of the Nile and often serves as a place of retreat for the wild beasts of the desert.

It is not very important whether it is rendered “lotus” or “shades,” though the probable derivation of the word seems to favor the former.

In the covert of the reed - It is well known that reeds abounded on the banks of the Nile. These would furnish a convenient and natural retreat for the hippopotamus.

And fens - בצה bitstsâh—“marsh, marshy places.” This passage proves that the elephant is not referred to here. He is never found in such places.