grammatical number - What is the correct plural form of "offspring"?

What is the correct plural form of the word "offspring"?

From the dictionaries I have looked at, I am guessing it is just "offspring", but Dictionary.com and Merriam-Webster say "offsprings" is also acceptable.

offspring noun (plural offspring, offsprings)

Dictionary.com

offspring noun (plural offspring, offsprings)

Merriam-Webster

offspring noun (plural offspring)

Cambridge Dictionaries Online

offspring noun (plural offspring)

Oxford Dictionaries

Macquarie Dictionary and OED have no mention of the plural form.

The sentence I want to use the word in is:

...more long-legged frogs exist to generate long-legged [offspring(s)].

In this context "offspring" as a plural (without the "S") doesn't sound right.

Answer

From Google Books Ngram Viewer, Offspring is more common, in the past 200 years, so the negligible usage of Offsprings might be attributed to a few ignorant authors.

Very common usage of Offspring vs negligible usage of Offsprings

Very common usage of Fish vs smaller (but not negligible) usage of fishes

Possible Explanation:
Fish and Fishes are both correct, but the singular is generally more common, and the ratio looks good. So we can consider that both are used normally.
Offspring is correct and is used commonly, while Offspings, being "wrong" in some sense, has negligible usage, and the ratio is "abnormally low". So we can consider that Offsprings should be avoided?