Examples Of Animals That Show Retrogressive Metamorphosis

 

The most prominent and classic examples of retrogressive metamorphosis are found within the phylum Chordata, subphylum Tunicata (also known as Urochordata).

• Ascidians (Sea Squirts): These are the textbook examples. Common genera include Herdmania, Ciona, and Molgula. Their free-swimming "tadpole larva" possesses all the key chordate features (a notochord, a dorsal hollow nerve cord, pharyngeal gill slits, and a post-anal tail), which are then lost as it settles and transforms into a sessile, bag-like, filter-feeding adult.
• Thaliaceans (Salps): These are free-floating, pelagic tunicates. While their life cycle is different from ascidians, they also undergo a metamorphosis where the notochord and tail are lost, and the nervous system is reduced to a simple ganglion in the adult stage.

While not always termed "retrogressive metamorphosis" in the same chordate context, some parasitic animals, like the barnacle Sacculina, exhibit a similar 'degenerate' transformation. A complex, free-swimming crustacean larva settles on a host (a crab) and metamorphoses into a simple, sac-like parasitic form, losing its limbs, segmentation, and most sensory organs.