Sea squirts start out life as a small fish-like tadpole stage, with a nerve cord very much like our spinal cord. Because of this (and some other characteristics) they are distinctly related to the the bony vertebrates: fish, birds, reptiles and amphibians - and mammals (that's us). But the sea squirts have succeeded evolutionarily by diverging from the vertebrate plan in a very dramatic fashion. The adult stages have body plans more similar to a typical invertebrate than any vertebrate relative. Glued to the bottom, they simply filter water - no eyes, no fins, legs, or jaws.
We often see the sea squirt larva in our plankton tows (similar to the image at the top). We often see adult sea squirts in our algae labs (the image below).
Evolution is all about survival - it is not about increasing complexity, or going anywhere. What works survives, simple or complex. Millions of years of natural selection has produced a wondrous range of effective survivors in every environment - from all sorts of source organisms.
No comments:
Post a Comment