Evolutionary Significance Of Retrogressive Metamorphosis

Retrogressive metamorphosis is a powerful evolutionary strategy with two major implications:

• Ecological "Division of Labor": It allows a single species to have two distinct body forms, each perfectly adapted for a different job.
  • The larva is a motile, "advanced" organism designed for dispersal and habitat selection. Its tail, notochord, and sensory organs are essential for swimming and finding a safe place to live.
  • The adult is a sessile organism designed for feeding and reproduction. It ditches the costly locomotive and sensory gear and invests all its energy into developing a highly efficient filter-feeding basket (pharynx) and large gonads.
• Revealing Phylogenetic Relationships: This is its most profound significance. The adult sea squirt looks nothing like a chordate. Based on the adult alone, zoologists in the 1800s classified them with mollusks. It was the discovery of the "tadpole" larva, with its clear chordate features (notochord, dorsal nerve cord, etc.), that proved tunicates are close relatives of vertebrates. The larva retains the ancestral body plan, providing the crucial evolutionary link.