Salp

Salps are barrel-shaped, planktonic animals belonging to the family Salpidae. They move by squeezing their gelatinous bodies to push water through themselves, a method considered one of the most efficient forms of jet propulsion in the animal kingdom. To feed, they strain phytoplankton from the water using internal filters. These creatures are found in equatorial, temperate, and cold seas, appearing alone or in long, string-like colonies at the surface. The highest numbers occur in the Southern Ocean near Antarctica, where they sometimes form massive swarms, often in deeper waters, and can even outnumber krill. Since 1910, as krill populations in the Southern Ocean have dropped, salp numbers have risen. They have also been observed in growing numbers off the coast of Washington state. Salps have a complex life cycle that alternates between two distinct generations. Both forms live together in the sea; they look different but are mostly transparent, tubular, and gelatinous, typically 1 to 10 centimeters long. The solitary phase, called an oozooid, is a single barrel-shaped animal that reproduces asexually by producing a chain of tens to hundreds of tiny individuals that break away from the parent. This chain is the aggregate phase, made up of blastozooids that stay attached while swimming and feeding, each growing larger. These blastozooids are sequential hermaphrodites—they mature first as females and are fertilized by male gametes from older chains. A developing embryo (an oozooid) attaches to the parent’s body wall, is later released, and then grows into the solitary asexual phase, completing the cycle. This alternation allows for rapid generation times. When phytoplankton is abundant, it leads to short-lived blooms of salps that can filter out most of the algae. The bloom ends when food runs out. Occasionally, mushroom corals and those in the genus *Heteropsammia* feed on salps during these blooms. In 1920, a large incursion of the species *Salpa fusiformis* into the North Sea caused the Scottish herring fishery to fail. Salps succeed partly because they respond quickly to phytoplankton blooms. When food is plentiful, they bud off clones rapidly, grazing and growing faster than perhaps any other multicellular animal, stripping the sea of phytoplankton. If the algae become too dense, however, salps can clog and sink to the bottom. During blooms, beaches can become slimy with mats of salp bodies, and other plankton species may fluctuate due to competition. The sinking fecal pellets and bodies of salps carry carbon to the seafloor, and because salps are so abundant, they affect the ocean’s biological pump. Large changes in their numbers or distribution could alter the ocean’s carbon cycle and potentially influence climate change. Salps are closely related to other pelagic tunicates like Doliolida and Pyrosoma, as well as to bottom-dwelling tunicates. Though they resemble jellyfish in their simple form and planktonic lifestyle, they are chordates—animals with a dorsal nerve cord, related to vertebrates. Small fish sometimes swim inside salps for protection. The World Register of Marine Species lists the following classification for the order Salpida:

Order Salpida Family Salpidae Subfamily Cyclosalpinae Genus *Cyclosalpa*: *C. affinis*, *C. bakeri*, *C. foxtoni*, *C. ihlei*, *C. pinnata*, *C. polae*, *C. quadriluminis*, *C. sewelli*, *C. strongylenteron* Genus *Helicosalpa*: *H. komaii*, *H. virgula*, *H. younti*

Subfamily Salpinae Genus *Brooksia*: *B. berneri*, *B. rostrata* Genus *Ihlea*: *I. magalhanica*, *I. punctata*, *I. racovitzai* Genus *Metcalfina*: *M. hexagona* Genus *Pegea*: *P. bicaudata*, *P. confederata* Genus *Ritteriella*: *R. amboinensis*, *R. picteti*, *R. retracta* Genus *Salpa*: *S. aspera*, *S. fusiformis*, *S. gerlachei*, *S. maxima*, *S. thompsoni*, *S. tuberculata*, *S. younti* Genus *Soestia* (also accepted as *Iasis*): *S. cylindrica*, *S. zonaria* Genus *Thalia*: *T. cicar*, *T. democratica*, *T. longicauda*, *T. orientalis*, *T. rhinoceros*, *T. rhomboides*, *T.