Simplify the following text for a Grade 11 native English-speaking person.
Original: Although only four marine species survive , all of them horseshoe crabs , there are over 77,000 species of air-breathing chelicerates , and there may be about 500,000 unidentified species .
Simplified: On the other hand , there are over 77,000 well-identified species of air-breathing chelicerates , and there may be about 500,000 unidentified species .