‘Certain amphipods cannot move around like many sea creatures’. Citing this sentence from an article in LA Times (reprinted in DH), Venkatesh wants to know whether it is ambiguous. If it is ambiguous the two interpretations should be: (a) Certain amphipods cannot move around in the same way that many other sea creatures cannot; (b) Certain amphipods cannot move around unlike many other sea creatures which can. In the first interpretations both the amphipods and some sea creatures cannot move around.  In the second interpretation the amphipods cannot move around while some other sea creatures can.
Let’s see now how natural these interpretations are. What is one’s normal expectation about a sea animal?  How do we get these interpretations? In the case of interpretation (a) we read the sentence in this way: Certain amphipods cannot move around (like many sea creatures). The point would become clearer if we put a comma before like. Certain amphipods cannot move around, like many sea creatures. We get the second interpretation if we read: Certain amphipods (cannot move around like many sea creatures).  In the first case what is predicated about amphipods is: they cannot move around. In the second case what is posited is: they cannot move around like many sea creatures.
It should be clear by now that both interpretations are possible. But while reading a sentence we interpret in terms of not merely the structure but also in terms of our knowledge of the world.  This is not always possible. The structure may override all other considerations.  We know as a matter of fact that sea creatures normally move around. But what the author is saying here is that many of them don’t and certain amphipods belong to this group. If that were not the case he would have said ‘Certain amphipods do not move around, unlike many sea creatures.’  (=Many sea creatures move around. Unlike them certain amphipods don’t move around,)
What I have done is to show the two meanings of the cited sentence; to choose the intended meaning on the basis of real world knowledge and, finally, to show how the meanings could have been conveyed unambiguously.
Some other ways of conveying the intended meaning unambiguously would have been: Like many sea creatures which don’t move around, amphipods, too, don’t move around.  / Amphipods cannot move around, like many other sea creatures.
The ambiguity we noticed in the LA sentence is present in all {?} sentences which contain a negative element (not) and it is possible to break up the sentence in a way which may or may not include the negative element. Sambaji was not a fighter like Shivaji. This has two meanings. 1. Sambaji was not a fighter, like Shivaji.  This means that neither Sambaji nor Shivaji was a fighter.  2. Sambaji was (not a fighter like Shivaji).  This means that Shivaji was a fighter but not Sambaji. As in the case of the amphipods sentence we can eliminate all ambiguity by writing: Unlike Shivaji, Sambaji was not a fighter.
The ambiguity in the sentence about Sambaji is only of theoretical interest.  For anyone who has read about (or heard of) Shivaji, there is no ambiguity in Sambaji was not a fighter like Shivaji. To most Indians it means only one thing.  Shivaji was a fighter but not Sambaji.  Another case where prior knowledge of the world neutralises a potential ambiguity in the sentence.
A larger question here is:  How to characterise, rigorously, the sentences which result in the sort of ambiguity discussed above? More specifically, with sentences containing a clause in the negative, under what conditions does ambiguity arise?
A satisfactory answer to this question would go far beyond the limits of a column like this.  Rather than attempt an answer, I will note one more sentence which is also ambiguous.  He did not oppose the motion because he feared public opinion. This has two readings. (1) He was afraid of public opinion. Therefore he did not oppose the motion.  (2) He opposed the motion. But it was not because he feared public opinion (=what people would say if he didn’t oppose.) The sentence is from Fowler: Modern English Usage, second ed. 19. For a discussion of this and another similar sentence noted by me, see my Current English, 2003, . OUP. 

The writer can be contacted at ksyadurajan@yahoo.com