Salps and doliolids (class Thaliacea) are interesting animals because they are in the phylum Chordata, which includes all animals with a notochord during development (e.g., humans, fish, cats), but thaliaceans have a vastly different appearance and feeding strategy compared to most vertebrates. A salp or doliolid body is essentially a giant pumping muscle that forces water through a mucous net filter that collects phytoplankton and is ingested periodically. Both groups have limited mobility, with salps using muscular contractions to scoot through the water, while doliolids use tiny beating cilia to propel themselves.
The life history of salps and doliolids is remarkable and complex. Similar to plants, their life cycle alternates between sexual and asexual generations. The solitary phase reproduces asexually by budding off clones of itself. On salps, a chain of these clones develops on the solitary animal that is then released and reproduces sexually with other salp chains. The chains first mature as female and then change sexes to become male when they are larger! These chains release small solitary salps that then begin asexual budding once they are a certain size. Doliolids on the other hand produce short-lived tadpole larvae that are not seen in salps. When you consider that a chain of salps contains an average of ~28 individuals, it is no surprise that these organisms are capable of extremely fast reproductive rates and can double their populations in hours (Heron 1972). Some scientists think their remarkable reproductive rates can overwhelm other phytoplankton grazers, which could explain the fact that large salp aggregations are often associated with low biomass of other grazers (Alldredge and Madin 1982).
Because of their ability to reproduce quickly, salps are often very abundant near steady supplies of phytoplankton, such as at ocean fronts (zones where two water masses with differing physical properties meet) and eddies (Deibel and Paffenhöfer 2009). However, these organisms cannot tolerate extremely dense aggregations of phytoplankton because their mucous filters will become clogged with prey, which severely decreases their feeding efficiency. Salps and doliolids can “bloom” like other jellies, and when these blooms die off the dead salp bodies can export a large amount of carbon into deeper waters. Because of salps and doliolids close evolutionary relationship to vertebrates, scientists are also very interested in their developmental biology. Scientists are trying to use salps as a model organism to study the development of complex nervous systems in all vertebrate animals (Lacalli and Holland 1998).
Check out this video from Plankton Chronicles on these remarkable animals!
Plankton Chronicles Project by Christian Sardet, CNRS / Noe Sardet and Sharif Mirshak, Parafilms. See Plankton Chronicles interactive site: planktonchronicles.org
Alldredge AL and Madin LP (1982) Pelagic tunicates: Unique herbivores in the marine plankton. Bioscience 32:655-663
Deibel D and Paffenhöfer GA (2009) Predictability of patches of neritic salps and doliolids (tunicata, thaliacea). J Plankton Res 31:1571-1579
Heron AC (1972) Population ecology of a colonizing species: The pelagic tunicate Thalia democratica – I. individual growth rate and generation time. Oecologia 10:269-293
Lacalli TC and Holland LZ (1998) The developing dorsal ganglion of the salp Thalia democratica, and the nature of the ancestral chordate brain. Philosophical Transactions of the Royal Society B: Biological Sciences 353:1943-1967