"Following close at his heels, came Mr. Job Trotter, in the
catalogue of whose vices, want of faith and attachment to his
companion could at all events find no place. He was still ragged
and squalid, but his face was not quite so hollow as on his first
meeting with Mr. Pickwick, a few days before. As he took off his
hat to our benevolent old friend, he murmured some broken
expressions of gratitude, and muttered something about having
been saved from starving.

squalid

SYLLABICATION: squal·id
PRONUNCIATION: skwld
ADJECTIVE: 1. Dirty and wretched, as from poverty or lack of care. See synonyms at dirty. 2. Morally repulsive; sordid: “the squalid atmosphere of intrigue, betrayal, and counterbetrayal” (W. Bruce Lincoln).
ETYMOLOGY: Latin squlidus, from squlre, to be filthy, from squlus, filthy.
OTHER FORMS: squalid·ly —ADVERB
squalid·ness, squa·lidi·ty (skw-ld-t) —NOUN

My question is, why is Latin name of the small shark, the dogfish, "Squalus acanthus"?

According to Lewis and Short, there are two Latin words squalus, one with a long a and the other with a short a. The long a'd squalus is related to squalidus = squalid. The short a'd squalus is the name of some sort of seafish (dunno if it's the dogfish or not).

Bingley