A salp (plural salps; also salpa, plural salpae or salpas) is a barrel-shaped, free-floating tunicate. It moves by contracting, thus pumping water through its gelatinous body. The salp strains the pumped water through its internal feeding filters, feeding on phytoplankton that it sieves out of the water.
Salps are common in equatorial, temperate, and cold seas, where they can be seen at the surface, singly or in long, stringy colonies. The most abundant concentrations of salps are in the Southern Ocean (near Antarctica). Here they sometimes form enormous swarms, often in deep water, and are sometimes even more abundant than krill. Over the last century, while krill populations in the Southern Ocean have declined, salp populations appear to be increasing.
Life historySalps have a complex life cycle, with an obligatory alternation 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 one and ten cm tall. The solitary life history phase, also known as an oozoid, is a single barrel-shaped animal that reproduces 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 sequential hermaphrodites, first maturing as females, and are fertilized by male gametes produced by older chains), with a growing embryo oozoid attached to the body wall of the parent. The growing oozoids are eventually released from the parent blastozooids, then they continue to feed and grow as the solitary asexual phase, thus closing the life cycle of salps.
Oceanographic importanceOne reason for the success of salps is how they respond to phytoplankton blooms. When there is plenty of food, salps can quickly bud off clones, which graze the phytoplankton and can grow at a rate which is probably faster than any other multicellular 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 other planktonic species can experience fluctuations in their numbers due to competition with the salps.
Sinking fecal pellets and bodies of salps carry carbon to the sea floor, and salps are abundant enough to have an effect on the ocean's biological pump. Consequently, large changes in their abundance or distribution may alter the ocean's carbon cycle, and potentially play a role in climate change.
Salp pellets take carbon to the floor of the ocean.
Two of Britain's leading environmental thinkers say it is time to develop a quick technical fix for climate change.
Writing in the journal Nature, Science Museum head Chris Rapley and Gaia theorist James Lovelock suggest looking at boosting ocean take-up of CO2.
Their idea, already being investigated by a US firm, involves huge flotillas of vertical pipes in the tropical seas.
The two scientists say they doubt that existing plans for curbing carbon emissions can work quickly enough.
"We are taking the very strong line that we are not going to save the planet by the regular approaches like the Kyoto Protocol or renewable energy," Professor Lovelock told BBC News.
"What we have to do is to look at it in a systems sense, or a Gaian sense, and see if it's curable by direct action."
Professor Rapley, who has just moved to head up the Science Museum from a similar post at the British Antarctic survey, said the two men developed the ocean pipes concept during country walks in James Lovelock's beloved Devon.
Unbeknown to them, a US company, Atmocean, had already begun trials of a very similar technology.
Floating pipes reaching down from the top of the ocean into colder water below move up and down with the swell.
As the pipe moves down, cold water flows up and out onto the ocean surface. A simple valve blocks any downward flow when the pipe is moving upwards.
1. Buoy: Helps hold the pump in position
2. Pump: James Lovelock believes the tubes would be about 100m long to access deep cold water, and 10m wide; Phil Kithil thinks 200m long and 3m wide could be optimum
3. Valve: Could be at the top or bottom of the pipe; top perhaps preferable for maintenance. Water is drawn through the open valve on wave down slopes; no external power needed
4. Cold water: On wave up slopes, cool water spills out of the pump
5. Pump sites: Locations could also be chosen to reduce hurricane risk by cooling surface waters
Colder water is more "productive" - it contains more life, and so in principle can absorb more carbon.
One of the life-forms that might benefit, Atmocean believes, is the salp, a tiny tube which excretes carbon in its solid faecal pellets, which descend to the ocean floor, perhaps storing the carbon away for millennia.
Atmocean CEO Phil Kithil has calculated that deploying about 134 million pipes could potentially sequester about one-third of the carbon dioxide produced by human activities each year. But he acknowledges that research is in the early stages.
Nervous systems and relationships to other animalsSalps are related to the pelagic tunicate groups doliolida and pyrosoma, as well as to other bottom-living (benthic) tunicates.
Although salps appear similar to jellyfish because of the simple form of their bodies and their free-floating way of life, they are structurally most closely related to vertebrates, animals with true backbones.
Salps appear to have a form preliminary to vertebrates, and are used as a starting point in models of how vertebrates evolved. Scientists speculate that the tiny groups of nerves in salps are one of the first instances of a primitive nervous system, which eventually evolved into the more complex central nervous systems of vertebrates.
Studies on salp brains have been done by Thurston Lacalli and Linda Holland and published in Philosophical Transactions of the Royal Society of London.
See them move here : BBC Real Player video
Salps are interesting for us, inspiration for the shape, flocking behavior toward a function (reduce energy expenses by reducing friction surface). What they do : assimilating CO2 and sending to the bottom of the oceans where they become nutriment again... very nice.
Download the .flv video 2.8mb
Stream a longer BBC explanation here in a.mp4 video 10mb.
Thanks to Usman Haque, we also got interested into Siphonophora, an order of the Hydrozoa, a class of marine invertebrates belonging to the phylum Cnidaria. They are colonial, but the colonies can superficially resemble jellyfish; although they appear to be a single organism, each specimen is actually a colony of Siphonophora. The best known species is the dangerous Portuguese Man o' War (Physalia physalis, below).