Molte specie vivono nel mare.

Because specie is a feminine noun, so the adjective/quantifier has to agree with it.

• molto = singular masculine
• molta = singular feminine
• molti = plural masculine
• molte = plural feminine

So:

• molte specie = many species

It is plural here, but this can be confusing because specie is one of those Italian nouns that often has the same form in the singular and the plural.

So:

• una specie = one species
• due specie = two species

You often tell whether it is singular or plural from the article, adjective, or verb around it:

• una specie vive... = singular
• molte specie vivono... = plural

Vivono is the 3rd person plural present tense of vivere (to live).

Conjugation of vivere in the present:

• io vivo = I live
• tu vivi = you live
• lui/lei vive = he/she lives
• noi viviamo = we live
• voi vivete = you all live
• loro vivono = they live

So molte specie vivono means many species live.

Nel = in + il, so it means in the.

• nel mare = in the sea

Italian often uses the definite article where English may or may not use the. Here, nel mare refers to the sea as an environment or habitat.

There is also in mare, but it is used differently in many contexts. For example, essere in mare can mean to be at sea/out at sea, rather than specifically in the sea as a habitat.

So in this sentence, nel mare is the natural choice.

Nel is a contraction of:

• in
  • il → nel

This is very common in Italian. Similar combinations are:

• in + lo → nello
• in + la → nella
• in + i → nei
• in + gli → negli
• in + le → nelle

So if the noun changes, the form changes too:

• nel mare
• nella foresta
• nei laghi

Italian uses the definite article more often than English does.

In English, we might say:

• Many species live in the sea
• or sometimes more generally Many species live in sea water depending on context

In Italian, when talking about a natural environment like the sea, the forest, the mountains, it is very common to use the article:

• nel mare
• nella foresta
• in montagna (this one is a special common expression without an article)

So nel mare sounds normal and idiomatic.

Yes. Tante also means many/a lot of and works well here.

• molte specie = many species
• tante specie = many species / lots of species

The difference is mostly one of tone:

• molte can sound a bit more neutral or formal
• tante can sound a bit more conversational or emphatic

Both are correct.

Yes, Italian word order is flexible, though Molte specie vivono nel mare is the most neutral order.

Possible variations include:

• Nel mare vivono molte specie.

This version puts more focus on nel mare and sounds like:

• In the sea, many species live
• or more naturally in English, Many species live in the sea

Both are correct, but the emphasis changes.

It is pronounced roughly SPEH-chee-eh in careful pronunciation.

A few helpful points:

• sp is like English sp
• c before i sounds like English ch
• the ending -ie is not like English -eez; it is two vowel sounds

So specie is approximately:

• SPEH-chee-eh

And the full sentence is roughly:

• MOL-teh SPEH-chee-eh VEE-vo-no nel MA-reh

Mare is masculine.

That is why it takes il in its basic form:

• il mare = the sea

And with in:

• in + il mare → nel mare

So the masculine gender of mare is the reason we get nel, not nella.

Yes. Since specie often looks the same in singular and plural, the other words tell you which one it is.

Compare:

• Una specie vive nel mare. = One species lives in the sea.
• Molte specie vivono nel mare. = Many species live in the sea.

The clue is in:

• una / molte
• vive / vivono

So even if specie itself does not change, the sentence still clearly shows singular or plural.