Complex sentence: The giant squid is the largest invertebrate, and, except for their larvae and some recently captured juveniles, has never been observed alive, although the colossal squid is likely to be even larger.
Split into simpler sentences: The giant squid is the largest invertebrate, and, until recently (except for larvae and some recently captured juveniles) had never been observed alive. The colossal squid is likely to be even larger.