Salps are free-swimming marine invertebrates found drifting in warm, temperate, and equatorial waters. These gelatinous animals are often mistaken for jellyfish because they appear as transparent, barrel-shaped organisms. They sometimes wash ashore in large numbers or form long, chain-like colonies in the water column. The sudden appearance of these translucent creatures leads many to question their safety and potential hazard to humans. This misconception stems from their resemblance to stinging cnidarians, but salps are fascinating members of the planktonic community with a unique life history and a significant function in the marine ecosystem.
Physical Characteristics and Classification
Salps are classified as tunicates (subphylum Tunicata), placing them closer to vertebrates than to jellyfish. This classification stems from the fact that larval salps possess a notochord, a flexible support rod shared with all chordates. The adult salp retains a barrel-shaped body enclosed in a transparent, gelatinous outer layer.
Their bodies are equipped with muscle bands that encircle the translucent casing, allowing them to move using jet propulsion. By contracting these bands, they pump water through their body, which facilitates both locomotion and feeding. Water enters through an oral siphon and exits through an atrial siphon, pushing the animal forward.
Inside the body, they deploy a fine mucous net that acts as a filter, straining microscopic plankton from the water. This filter-feeding mechanism is visible as a small, dense internal structure within the clear organism. Individual salps typically range from a few millimeters to 10 centimeters, though their colonial forms can extend for several meters.
Assessing the Danger to Humans
The primary concern for people encountering salps is whether they pose a stinging threat similar to jellyfish. Salps are completely harmless to humans because they do not possess nematocysts, the stinging cells used by true jellyfish. These gelatinous creatures are non-toxic and lack the ability to inject venom.
Salps are filter feeders that rely on a mucous net for sustenance, unlike cnidarians, which use tentacles to sting. Contact with a salp, whether solitary or in a chain, is not dangerous and will not result in a venomous sting. When salps are found massed on beaches, they are simply dead or dying organisms pushed ashore by currents and tides.
These large aggregations often indicate a recent phytoplankton bloom, which serves as their food source. This abundance leads to rapid reproduction and subsequent mass strandings when the food supply diminishes or ocean conditions change. Salps are not a marine hazard, and their presence is not a cause for alarm.
Remarkable Life Cycle and Reproduction
Salps exhibit a complex life cycle characterized by an obligatory alternation of generations. This cycle allows them to switch between two distinct forms: a solitary, asexual phase and a colonial, sexual phase. The solitary individual, called an oozooid, is a single, barrel-shaped animal that initiates the reproductive cycle asexually.
The oozooid produces a chain of genetically identical buds, known as blastozooids, on a structure called a stolon. This chain, which can contain dozens to hundreds of individuals, is released to drift freely as an aggregate colony. Rapid asexual budding allows for explosive population growth when food sources, like phytoplankton blooms, are abundant.
Once released, each blastozooid in the aggregate chain matures into a sexual form. These blastozooids are sequential hermaphrodites, maturing first as females and then being fertilized by sperm released from older chains. The resulting embryo is nourished by the parent blastozooid until it develops into a new solitary oozooid, which is then released to begin the cycle.
Role in Ocean Ecology
Salps play a large role in the global ocean ecosystem, particularly in carbon sequestration. As efficient filter feeders, they consume phytoplankton, which have absorbed carbon dioxide from the atmosphere. They are capable of clearing vast volumes of water, grazing on particles as small as bacteria.
Their ecological function is tied to their fecal pellets, which are large, dense, and sink rapidly through the water column. This rapid sinking allows the particulate organic carbon within the waste to bypass microbial decomposition in the surface waters. The material is quickly transported to the deep ocean floor, effectively removing carbon from the upper layers.
This process is a fundamental component of the ocean’s “biological pump,” which transfers carbon from the surface to the deep sea. When salp populations bloom, the massive amount of rapidly sinking organic material they produce increases the efficiency of this pump. Salps also serve as a food source for numerous marine animals, including various fish, turtles, and seabirds, connecting the planktonic food web to larger ocean predators.