Salps belong to the Tunicates. They are chordates without ceolom, segmentation or bony tissue. They look a bit like jellyfish, but taxonomically they are actually closer to humans than to 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. Their size ranges from a few millimetres at birth to around 10cm as they grow, although one species is known to reach more than a few meters. Individual salps form a colony during the sexual phase of their lifecycle. In some species the colony is long and chain-like, while in others it appears more like a wheel.
Distribution and abundance
Two species of salp are common in Antarctic waters. One, Salpa thompsoni, is abundant in ice-free areas. The other, Ihlea racovitzai, is distributed exclusively in high-latitude ice edge areas.
There is a belief that Antarctic krill are less abundant in years when Salpa thompsoni are plentiful — and vice versa. Scientists attribute this to yearly variations in the extent of sea ice. Salps appear to be dominant in years of poor ice extent, while krill are dominant in other years.
Salps have a complex life cycle alternating between sexual and asexual forms. Sexual forms are called aggregates because they form a colony. In contrast, 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 and 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. The mesh of their feeding net is efficient enough to catch a variety of different sizes of particles from bacteria to nauplius larva, but 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 eaten by fish. They have also been found in the stomachs of albatrosses and seals. Salps are 95% water — not nutritious enough to sustain a seabirds or marine mammals that require high-energy foods. These species probably only eat salps when their main food supplies are scarce.