Salps belong to the Tunicates. They are chordates without ceolom, segmentation and bony tissue. They are taxonomicaly closer to humans than jellyfish even though they look like jellyfish. Unlike other Tunicates, salps are planktonic.
Scientific names: Two species are common in Antarctic waters: Salpa thompsoni and Ihlea racovitzai.
Salps are gelatinous, mostly transparent, and cylindrically shaped, so look like jellyfish. They vary in size from a few millimetres at their birth and grow up to about 10 cm. One species is known to reach more than a few meters. Individual salps form a colony during the sexual phase of their lifecycle. The colony is long and chain-like in some species and wheel-like in others.
Distribution and abundance
Of the two salp species, Salpa thompsoni is abundant in ice-free areas and Ihlea racovitzai is distributed exclusively in high-latitude ice edge areas. There is a belief that Antarctic krill, a key species in the Antarctic food web, are less abundant in a year when S. thompsoni is abundant, and vice versa.
Scientists attribute this to yearly variation of sea-ice extent: salps are dominant in years of poor ice extent while krill is dominant in other years. Details of this mechanism remain unknown. If sea-ice condition is important, what will happen to Antarctic ecosystem if global warming is a reality?
Salps have a complex lifecycle alternating between a sexual and asexual form. Sexual forms are called aggregates because they form a colony while asexual forms are solitary. All females have one or two eggs when released by a solitary parent. Mating occurs with larger male aggregates and embryos grow inside an aggregate body by being nurtured through a placenta. When embryos mature, they are released and the mother aggregate becomes male. Released embryos grow to be mature solitaries that asexually reproduce 'stolons' - buds of young aggregates.
In an optimal environment, salps grow very quickly while large swarms form mainly by asexual reproduction. Individual growth of a temperate species is as fast as 10% body length increase per hour. This species requires only 48 hours to complete their whole lifecycle. Subarctic species are slower growing due to low ambient temperatures. No data exists on growth rates of Antarctic species.
Diet and feeding
Salps are non-selective filter feeders eating everything that they trap in their feeding net. Although the mesh of their feeding net is efficient enough to catch a variety of different sizes of particles from bacteria to nauplius larva, their main food is phytoplankton. Salps are well adapted to offshore environments where phytoplankton concentration is moderate. They cannot survive in coastal areas because the high concentration of inorganic particles causes their feeding net to become clogged and they die.
Salps filter food particles by pumping seawater in from the mouth opening and out through the atrial opening using muscle contractions. This pumping action gives animals propulsion, therefore, swimming and feeding occur at the same time.
Salps are the main diet of some fish species, but have also been found in the stomach of albatrosses and seals. Since 95% of salps are water, they are not nutritious enough to sustain seabirds and marine mammals that require high energy foods. Such species probably eat salps only when their main food supplies are scarce.