|A diver swims with a chain-like colony of salps|
I love salps, and all of the other weird creatures out there with unexpected phylogenetic relationships. Salps are grouped in with Tunicates, or sea squirts as they are more commonly known. Salps and their relatives look rather a lot like jellyfish due to their soft, see-through bodies, but a closer examination shows numerous major differences. Where as jellies are very simple animals with few specialized tissues, salps are far more complex. In fact, as larvae they look remarkably like tadpoles. It is during the transition to adulthood that they loose their tail and other features and become specialized filter feeders. As hard as it may seem, larval traits such as a postanal tail, dorsal hollow nerve cord, notochord, and pharyngeal gill slits firmly sets these strange creatures into the Phylum Chordata. Chordata, of course, also includes such animals as fish, reptiles, birds, and mammals like us. However, salps are non-vertebrate chordates, and thus distinctly different from the fish, reptiles, birds, mammals, and other vertebrates. About fifty species of salps are known.
As adults, salps are quite efficient filter feeders, taking in water with one of the two siphons and expelling it through the other after filtering out food particles. They are usually found in colonies, and are able to reproduce either through sexual or asexual reproduction. Salps are able to swim by forcing water out of one of their siphons. Though they can be rather large in size, their free-floating, weak-swimming nature makes them plankton. In certain areas, salps can be so abundant that they become the second most commonly seen planktonic animals. Despite the fact that most people associate the term "plankton" with small living things, it is very possible for plankton to be a meter or more in length. By definition, this category encompasses any organism that is unable to swim against a strong current, which includes such things as jellyfish (including some truly massive species). Plankton that is over twenty millimeters in length is called megaplankton.
Sources are Woods Hole Oceanographic Institution and Biology of the Invertebrates. Image is from Wikimedia Commons under a Creative Commons license.