Although the slime secreted by a threatened hagfish is small

[MdAbuAsad]

Although the slime secreted by a threatened hagfish is small in quantity, it expands several hundred times as it absorbs seawater, forming a slime ball that can coat the gills of predatory fish and either suffocate them or distress them enough to make them flee.
^^
In this correct sentence, 'and' is the parallel marker. EITHER vs OR is perfectly fine. But, we know that 'parallelism is nothing but a beauty contest . So, how it makes beauty when we think of can coat and either. The 'can coat' is the verb whereas 'either' is not a verb at all.
Thanks in advance...

[Whit Garner]

Great question! The second item in the list created by "and" is actually two different things, so you need a weird structure here. What we're saying is essentially the following:

The hagfish forms a slimeball that can:
(A) coat the gills of predatory fish
(B) suffocate them (predatory fish)
(C) distress them enough to make them flee.

However, it isn't simply a list of 3 things because A and B can happen together, A and C can happen together, but B and C cannot happen together. That means that I could connect A-C with an "and" or A-C with an "and," but B-C would need the connector "or" to make sense. For example, "the hurricane damaged and destroyed many buildings" is incorrect because you can't damage AND destroy - you can damage OR destroy.

So how does the author make sense of the fact that A-B and A-C are "and" while B-C is "or"? They use a form that says that A happens and then B OR C would happen next. To make this parallel, you need to make sure that the pieces of the AND and OR construction are all parallel (which they are).

It is also worth noting for folks reading this post that the issue in question was NOT underlined (meaning that you weren't asked to evaluate whether this strange construction was correct).

[MdAbuAsad]

Great question! The second item in the list created by "and" is actually two different things, so you need a weird structure here. What we're saying is essentially the following:

The hagfish forms a slimeball that can:
(A) coat the gills of predatory fish
(B) suffocate them (predatory fish)
(C) distress them enough to make them flee.

However, it isn't simply a list of 3 things because A and B can happen together, A and C can happen together, but B and C cannot happen together. That means that I could connect A-C with an "and" or A-C with an "and," but B-C would need the connector "or" to make sense. For example, "the hurricane damaged and destroyed many buildings" is incorrect because you can't damage AND destroy - you can damage OR destroy.

So how does the author make sense of the fact that A-B and A-C are "and" while B-C is "or"? They use a form that says that A happens and then B OR C would happen next. To make this parallel, you need to make sure that the pieces of the AND and OR construction are all parallel (which they are).

It is also worth noting for folks reading this post that the issue in question was NOT underlined (meaning that you weren't asked to evaluate whether this strange construction was correct).

Thanks for the explanation. Did you mean A-B in the colored part?

[Whit Garner]

The hagfish forms a slimeball that can:
(A) coat the gills of predatory fish
(B) suffocate them (predatory fish)
(C) distress them enough to make them flee.

However, it isn't simply a list of 3 things because A and B can happen together, A and C can happen together, but B and C cannot happen together. That means that I could connect A-C with an "and" or A-C with an "and," but B-C would need the connector "or" to make sense. For example, "the hurricane damaged and destroyed many buildings" is incorrect because you can't damage AND destroy - you can damage OR destroy.

Thanks for the explanation. Did you mean A-B in the colored part?

I did! Thank you for catching the typo!