What’s the singular form of the word species? Let’s say for example, that I want to say the following:
There are many different species of snake around the world.
That’s OK, I’m referring to multiple species so of course it has to be plural. But what if I want to be more specific, and say something like:
The deadlies species of snake is possibly the boa constrictor.
Here I’m talking about one specific species of snake, so it feels a little odd to say species.
But, it is correct. Species is one of those rare nouns that only exists in a plural form. And the main reason for that is that it’s always going to be used to refer to multiple animals. Even when I’m talking about the boa constrictor, I’m talking about all boa constrictors in the world, and not one specific individual snake. It’s a little similar to how we can refer to groups like sports teams as plural words.
There are a few quite common words that are always plural: clothes, pants, trousers, scissors, and stairs are probably the ones we use most often. And we generally never have to worry about what the single form is, as they only really exist as pairs of multiple forms. When have you ever seen a scissor, or pant (though pants/trousers were originally two separate parts, one for each leg, which is why the words are plural)?
Clothes does cause learners some problems, as often when they’re talking about a lot of clothes, they feel like there must be a plural form of the word to convey the sheer amount of clothes they want to talk about. They’ll usually reach for something likes clotheses (often with the th pronounced). This may also be due to the fact that in other languages it can be more common to use singular words to refer to clothes. I know that in French for example, there’s the word vêtements, which is plural and a direct equivalent to clothes in English. But it seems to be more common to use the word costume, meaning outfit, with vêtements being used more to refer to clothes in a very general sense.
Species though, doesn’t cause too many problems. It’s not a word someone learning English is likely to use often, and for native speakers, it’s usually clear from the context if we’re referring to one or more species.
All that being said though, specie is a legitimate singular noun in English. It just has nothing to do with animals. It refers to coins, as distinct from banknotes. You could say, for example, I’m sorry, we cannot accept specie payments at this desk. We can only take banknotes. It comes from the Latin in specie, meaning in the real or actual form, derived in turn from the Latin species meaning kind or type.
So you can use the word specie; just not to refer to a boa constrictor.