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.

Solution 1:

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?

Solution 2:

From the 16c. to the early 19c. a plural form offsprings was current but since then this ancient word (found already in OE) has been invariable in form (Fowler's Modern English Usage).

USAGE EXAMPLES :

• The widows and the offsprings of the poorer, the indigent clergy (1756)
• These are the offspring of Muslim parents; the son tried to become the worthy offspring of his famous father.
• Better to start when the offspring are young and will have no solid memories of particularly rough times.(Washington Times, May 27, 2015)
• The Norway rat has a three-week gestation period and can produce five litters a year, each with four to eight offspring. (The Verge,May 13, 2015)