| Salp | |
|---|---|
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| A chain of salps near the surface in theRed Sea | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Subphylum: | Tunicata |
| Class: | Thaliacea |
| Order: | Salpida |
| Family: | Salpidae Lahille, 1888 |
| Subfamilies, genera and species[1] | |
See text | |
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Asalp (pl.:salps) orsalpa (pl.:salpae orsalpas[2]) is a barrel-shaped,planktonictunicate in the familySalpidae. The salp moves by contracting its gelatinous body in order to pump water through it; it is one of the most efficient examples ofjet propulsion in the animal kingdom.[3] The salp feeds onphytoplankton, which it collects by straining water through its internal feeding filters.
Salps are common inequatorial, temperate, and cold seas, where they can be seen at the surface, singly or in long, stringycolonies. The most abundant concentrations of salps are in theSouthern Ocean[4] (nearAntarctica), where they sometimes form enormous swarms, often in deep water, and are sometimes even more abundant thankrill.[5] Since 1910, while krill populations in the Southern Ocean have declined, salp populations appear to be increasing. Salps have been seen in increasing numbers along the coast ofWashington, United States.[6]
Salps have a complex life cycle, with an obligatoryalternation of generations. Both portions of the life cycle exist together in the seas—they look quite different, but both are mostly transparent, tubular, gelatinous animals that are typically between 1 and 10 cm (0.4 and 3.9 in) long. Thesolitary life history phase, also known as an oozooid, is a single, barrel-shaped animal thatreproduces asexually by producing a chain of tens to hundreds of individuals, which are released from the parent at a small size.
The chain of salps is the 'aggregate' portion of the life cycle. The aggregate individuals are also known as blastozooids; they remain attached together while swimming and feeding, and each individual grows in size. Each blastozooid in the chain reproduces sexually (the blastozooids are sequentialhermaphrodites, first maturing as females, and are fertilized by male gametes produced by older chains), with a growing embryo oozooid attached to the body wall of the parent.
The growing oozooids are eventually released from the parent blastozooids, and then continue to feed and grow as the solitary asexual phase, closing the life cycle of salps. The alternation of generations allows for a fast generation time, with both solitary individuals and aggregate chains living and feeding together in the sea. When phytoplankton is abundant, this rapid reproduction leads to fairly short-lived blooms of salps, which eventually filter out most of the phytoplankton. The bloom ends when enough food is no longer available to sustain the enormous population of salps. Occasionally,mushroom corals and those of the genusHeteropsammia are known to feed on salps during blooms.[7]
The incursion of a large number of salps (Salpa fusiformis) into theNorth Sea in 1920 led to a failure of the Scottish herring fishery.[8]
A reason for the success of salps is how they respond tophytoplankton blooms. When food is plentiful, salps can quickly bud offclones, which graze on the phytoplankton and can grow at a rate that is probably faster than that of any othermulticellular animal, quickly stripping the phytoplankton from the sea. But if the phytoplankton is too dense, the salps can clog and sink to the bottom. During these blooms, beaches can become slimy with mats of salp bodies, and otherplanktonic species can experience fluctuations in their numbers due to competition with the salps.
Sinkingfecal pellets and bodies of salps carrycarbon to the seafloor, and salps are abundant enough to have an effect on the ocean'sbiological pump. Consequently, large changes in their abundance or distribution may alter the ocean'scarbon cycle, and potentially play a role inclimate change.[9][10]
Salps are closely related to the pelagic tunicate groupsDoliolida andPyrosoma, as well as to other bottom-living (benthic)tunicates.
Although salps appear similar tojellyfish because of their simple body form and planktonic behavior, they arechordates: animals withdorsal nerve cords, related tovertebrates (animals withbackbones).
Small fish swim inside salps as protection from predators.[11]
TheWorld Register of Marine Species lists the followinggenera andspecies in the order Salpida:[12]
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