3 The food of fish differs according to their species. Some feed on mud; others eat sea weed; others content themselves with the herbs that grow in water. But the greater part devour each other, and the smaller is food for the larger, and if one which has possessed itself of a fish weaker than itself becomes a prey to another, the conqueror and the conquered are both swallowed up in the belly of the last. And we mortals, do we act otherwise when we press our inferiors? What difference is there between the last fish and the man who, impelled by devouring greed, swallows the weak in the folds of his insatiable avarice? Yon fellow possessed the goods of the poor; you caught him and made him a part of your abundance. You have shown yourself more unjust than the unjust, and more miserly than the miser. Look to it lest you end like the fish, by hook, by weel, or by net. Surely we too, when we have done the deeds of the wicked, shall not escape punishment at the last.
Now see what tricks, what cunning, are to be found in a weak animal, and learn not to imitate wicked doers. The crab loves the flesh of the oyster; but, sheltered by its shell, a solid rampart with which nature has furnished its soft and delicate flesh, it is a difficult prey to seize. Thus they call the oyster “sherd-hide.” Thanks to the two shells with which it is enveloped, and which adapt themselves perfectly the one to the other, the claws of the crab are quite powerless. What does he do? When he sees it, sheltered from the wind, warming itself with pleasure, and half opening its shells to the sun, he secretly throws in a pebble, prevents them from closing, and takes by cunning what force had lost. Such is the malice of these animals, deprived as they are of reason and of speech. But I would that you should at once rival the crab in cunning and industry, and abstain from harming your neighbour; this animal is the image of him who craftily approaches his brother, takes advantage of his neighbour's misfortunes, and finds his delight in other men's troubles. O copy not the damned! Content yourself with your own lot. Poverty, with what is necessary, is of more value in the eyes of the wise than all pleasures.
I will not pass in silence the cunning and trickery of the squid, which takes the colour of the rock to which it attaches itself. Most fish swim idly up to the squid as they might to a rock, and become themselves the prey of the crafty creature. Such are men who court ruling powers, bending themselves to all circumstances and not remaining for a moment in the same purpose; who praise self-restraint in the company of the self-restrained, and license in that of the licentious, accommodating their feelings to the pleasure of each. It is difficult to escape them and to put ourselves on guard against their mischief; because it is under the mask of friendship that they hide their clever wickedness. Men like this are ravening wolves covered with sheep's clothing, as the Lord calls them. Flee then fickleness and pliability; seek truth, sincerity, simplicity. The serpent is shifty; so he has been condemned to crawl. The just is an honest man, like Job. Wherefore God sets the solitary in families. So is this great and wide sea, wherein are things creeping innumerable, both small and great beasts. Yet a wise and marvellous order reigns among these animals. Fish do not always deserve our reproaches; often they offer us useful examples. How is it that each sort of fish, content with the region that has been assigned to it, never travels over its own limits to pass into foreign seas? No surveyor has ever distributed to them their habitations, nor enclosed them in walls, nor assigned limits to them; each kind has been naturally assigned its own home. One gulf nourishes one kind of fish, another other sorts; those which swarm here are absent elsewhere. No mountain raises its sharp peaks between them; no rivers bar the passage to them; it is a law of nature, which according to the needs of each kind, has allotted to them their dwelling places with equality and justice.
Source: Nine Homilies of Hexaemeron (New Advent)