What mystery mollusk puzzled scientists for over two decades?

The answer

A bizarre deep-sea creature first spotted over two decades ago was finally identified in 2025 as Bathydevius caudactylus - a nudibranch (sea slug) from an entirely new family previously unknown to science. For years, the translucent, finger-tailed organism had been recorded by deep-sea cameras but defied classification because its body plan didn't match any known group. Advanced DNA analysis and detailed anatomical studies finally revealed it belonged to an entirely new evolutionary lineage. The discovery highlights how the deep ocean remains Earth's last true frontier of discovery - organisms photographed decades ago can still await formal scientific description, and entirely new families of animals continue to be found.