Salps or salpa (also known colloquially as ‘sea grape’) are small, barrel-shaped marine invertebrates that belong to the family Salpidae within the order Salpida. Although they resemble jellyfish, salps are actually tunicates and thus are taxonomically closer to vertebrates.
Their semi-transparent bodies are equipped with a highly efficient jet propulsion system, allowing them to move by contracting muscle bands that ring their transparent bodies. As they propel themselves through the water, they filter feed on microscopic plankton, straining their food through internal filters.
Salps are most commonly found in warm seas, particularly in the Southern Ocean, either singly or in elongated bead-like colonies. They are known for their complex life cycle, which alternates between sexual and asexual phases. During the asexual phase, they often form long, luminescent chains, creating a mesmerizing sight in the ocean depths.
These chordates are semi-transparent and barrel-shaped, with a dorsal nerve cord running down their backs. They are typically between 0.4 and 3.9 in (1 and 10 cm) long and possess bands of contracting muscles that form rings throughout their bodies. Similar to tunicates, they have two openings, an oral and an atrial siphon, through which water enters and leaves the body.
In their larval stage, salps possess a notochord, a flexible, rod-like structure that runs along their backs. However, as they mature into adults, they lose this notochord.
According to the World Register of Marine Species, 42 species of salps are classified under two subfamilies and 13 genera.
These tunicates are closely related to other pelagic groups, like doliolids (order Doliolida) and pyrosomes (genus Pyrosoma).
Salps are commonly found in offshore marine environments, particularly in equatorial, temperate, and cold seas, with a significant concentration in the Southern Ocean around Antarctica. The species Salpa thompsoni and Ihlea racovitzai are the most frequently encountered in the icy waters of the Antarctic. Recently, the salp population has been found to increase along the Washington coast in the United States.
They primarily feed on phytoplankton, but they are non-selective feeders and will consume almost anything that gets caught in their feeding trap.
These tunicates often engage in vertical migration, rising to the surface at night and descending to depths of around 1,600 ft during the day.
Salps create a feeding net made of mucus that hangs inside their bodies. As they swim, they pump seawater in through their oral siphon and expel it through the atrial siphon. During this process, food particles in the water are captured by the mucus net, allowing the salp to feed as it moves.
Salps move through the water using a method called jet propulsion. In this process, they contract the muscular bands around their bodies, drawing water in through the oral siphon. As they feed, they simultaneously pump the water out through the atrial siphon, creating a thrust that propels them in the opposite direction. This mechanism allows salps to combine feeding and locomotion in a single motion, much like the function of a jet engine.
Salps have a complex life cycle characterized by an obligatory alternation of generations, switching between a solitary asexual phase and a colonial sexual phase. In the solitary phase, a single barrel-shaped oozooid produces a chain of individuals asexually. This chain represents the ‘aggregate phase’ of their life cycle, where each individual, known as a blastozooid, feeds and grows while remaining part of the cluster.
The blastozooids, which are hermaphroditic, reproduce sexually. Initially, they mature as females and are fertilized by male gametes from older chains. The resulting embryo, a new oozooid, remains attached to the parent blastozooid’s body wall, along with 20 to 80 other identical embryos. The parent nourishes it through a placenta-like structure until it is ready to be released into the water, where it begins a new solitary, asexual phase.
This cyclic alternation of generations allows for rapid population growth, enabling both sexually and asexually reproducing individuals to coexist. When resources are plentiful, this can lead to short-lived salp blooms, which eventually decline as food becomes scarce.
They are usually preyed upon by fish, seals, and sea birds, like albatrosses.
Salps are considered the fastest-growing multicellular animals on Earth, being able to increase their body length by about 10% per hour. Thus, they can easily attain maturity in about 48 hours.
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