A salp is a barrel-shaped, free-floating tunicate (any living organism which has a saclike body enclosed in a thick membrane or tunic with two openings or siphons for the ingress and egress of water). It moves by contracting which pumps water through its body. The salp strains the water with internal feeding filters as it goes through the body. It consumes phytoplankton that are strained from the water.
Salps are common throughout equatorial, temperate, and colder seas. They are most often seen at the surface, singly or sometimes in long, strung-together colonies. The most abundant population of salps is concentrated in the Southern Ocean near Antarctica. Here they often form giant colonies, sometimes in deeper water. Salps in this area are often more abundant than krill. Since the early 1900s, krill populations have been declining and salp populations seem to be increasing.
The life cycle of the salp is quite complex. There are two portions of the life cycle that exist together in the seas. Both cycles are very different from one another, but both are transparent, tubular, and gelatinous. The solitary life phase of the salp (known as oozoid), is a single animal that reproduces asexually and produces chains of up to hundreds of individuals, which are released from the parent in small size. The chain of salps is the aggregate part of the life cycle. These individuals are known as blastozooids. Blastozooids remain chained together while swimming and feeding; however, each individual grows separately.
Blastozooids reproduce sexually. The younger chains mature as females, and are fertilized by older chains of males. The blastozooid females carry an embryonic oozoid that remains attached to the wall of the body. The oozoids are eventually released from the parent blastozooids, then they continue to feed and grow as a solitary asexual salp. Reproduction is based upon the abundance of phytoplankton. With an over-abundant source of phytoplankton, the salp reproduces more rapidly and shorter-lived chains of salp emerge to feed on the phytoplankton. When there is not enough food to sustain this enormous population, the rapid reproduction process ends.
In areas where food is abundant, the salp can produce clones, which consume the mass quantities of phytoplankton and can grow at exponential rates, much faster than any other multicultural animal. Sometimes, if the phytoplankton is too dense, the salp can clog up and sink to the bottom. During these times when phytoplankton is over-abundant, it is common for beaches to become ridden with hundreds of slimy salp bodies. Other species that feed on plankton are affected by this process and fluctuations in populations are observed where competition for food is severe.
The salp may play a big role in climate changes. Fecal matter and dead salp bodies that sink to the ocean floor carry carbon, and in regions where salps are abundant this can have a major effect on the ocean’s biological pump. Changes in the distribution of salp in large numbers may alter the carbon cycle of the ocean as well.
Salps may seem closely similar to jellyfish because of their simple form and free-floating way of life. They are actually more closely related to vertebrates, animals with true backbones. Salps have a preliminary form similar to vertebrates, and are used as a model of how vertebrates evolved. The tiny groups of nerves found in salps may be one of the first instances of a primitive nervous system. These may have evolved into more complex central nervous systems found in true vertebrates.