Salpa maxima, the Big Salp (a.k.a. Salp, plural Salps), is a barrel-shaped, planktonic tunicate. It moves by contracting, thus pumping water through its gelatinous body, one of the most efficient examples of jet propulsion in the animal kingdom. The Big Salp strains the pumped water through its internal feeding filters, feeding on phytoplankton.
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 Salpa maxima are in the Southern Ocean (near Antarctica), where they sometimes form enormous swarms, often in deep water, and are sometimes even more abundant than krill. Its range also include the seas around Malta in the Mediterranean Sea.
Salpa maxima are closely 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 their simple body form and planktonic behaviour, they are chordates: animals with dorsal nerve cords, related to vertebrates, animals with backbones.
Salps 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 1 and 10 cm tall. The solitary life history phase, also known as an oozooid, 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, and then continue to feed and grow as the solitary asexual phase, thus closing the life cycle of Salps.
One reason for the success of Big Salp is how they respond to phytoplankton blooms. When food is plentiful, Salps can quickly bud off clones, which graze the phytoplankton and can grow at a rate which is probably faster than that of 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 Salpa maxima 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.
Photo taken by Brian Azzopardi