Salps are gelatinous, barrel-shaped zooplankton that drift through the world’s oceans. Often mistaken for jellyfish due to their transparent bodies, salps are found in equatorial, temperate, and cold seas. They sometimes form immense swarms, known as blooms, which hint at their outsized importance in marine ecosystems. These creatures play a significant role in ocean processes, from the marine food web to global climate regulation.
Defining Salps
Salps are marine invertebrates belonging to the subphylum Tunicata, placing them closer to vertebrates than to jellyfish. They are planktonic tunicates of the class Thaliacea. Their bodies are largely transparent and cylindrically shaped, typically ranging from a few millimeters up to about 10 centimeters in length.
They possess a muscular, barrel-like body with two siphons, one at the front and one at the back, through which they pump water for movement and feeding. Rhythmic muscle contractions around the body achieve jet propulsion, making them efficient swimmers.
As water is propelled through the body, it is strained through a fine, internal mucus net that captures microscopic plankton for food.
The Unique Salp Life Cycle
Salps exhibit a complex reproductive strategy known as the alternation of generations, which allows for rapid population growth and the formation of massive blooms. The life cycle alternates between a solitary, asexual phase and an aggregate, colonial sexual phase.
The solitary form, called an oozoid, is a single individual that reproduces asexually. The oozoid produces a chain of genetically identical individuals through budding.
This chain, which can contain tens to hundreds of attached individuals, is the aggregate form, where each member is called a blastozooid. The aggregate chain represents the sexual phase of the life cycle, with blastozooids being sequential hermaphrodites.
The blastozooids initially mature as females and are fertilized by male gametes from older chains, which prevents self-fertilization. A single embryo, which develops into the next solitary oozoid, is nurtured inside the parent blastozooid via a placenta-like structure. This rapid alternation allows salp populations to explode.
Salps’ Role in the Marine Food Web
Salps occupy a unique position in the marine food web due to their highly efficient and non-selective feeding behavior. They are continuous filter feeders, utilizing a mucus net to trap a wide spectrum of particles from the water column.
This fine mesh allows them to consume organisms of various sizes, including extremely small phytoplankton and bacteria. This efficiency means that during a bloom, salps exert massive grazing pressure, quickly stripping the water of phytoplankton.
While they serve as a food source for certain pelagic predators, such as sea turtles, seabirds, and fish like the ocean sunfish, they are not universally sought after. An increase in salp populations can lead to a decrease in other zooplankton, such as krill, potentially impacting animals that rely on those more energy-rich sources.
Salps and Global Carbon Cycling
The most significant aspect of salps is their profound influence on the ocean’s biological carbon pump, a process that moves carbon from the surface to the deep ocean. Salps consume enormous amounts of carbon-rich phytoplankton in the sunlit surface waters.
They then package this organic carbon into dense, large fecal pellets. These pellets are structurally robust and sink rapidly through the water column.
This rapid descent means the pellets are less likely to be consumed by other organisms or degraded by microbes in the upper ocean. The quick export of carbon to the deep sea effectively sequesters it, removing it from the atmosphere’s cycle for long periods. Salp blooms significantly increase the efficiency of the biological carbon pump, demonstrating their impact on ocean biogeochemistry. The sheer volume of their waste, combined with the dense nature of their fecal pellets and the sinking of their own carcasses, makes salps a powerful mechanism for carbon sequestration.