Sea salps are widespread and abundant gelatinous marine invertebrates. These free-swimming, planktonic organisms are characterized by their transparent, hollow, and barrel-shaped bodies. Salps hold a surprising ecological significance, particularly in the deep-ocean carbon cycle. Despite their superficial resemblance to jellyfish, salps possess a complex biology that places them in a unique position on the tree of life.
Defining the Salp
Salps belong to the phylum Chordata, making them closer biological relatives to humans than to the jellyfish they are often mistaken for. They are part of the subphylum Tunicata. This classification stems from the fact that salp larvae possess a notochord, a flexible, rod-like structure that is a defining feature of the phylum.
The adult salp body is essentially a transparent, gelatinous tunic that surrounds a large internal cavity. This barrel-like structure, typically ranging from 1 to 10 centimeters in length, is girdled by bands of circular muscles. Water enters through the oral siphon at the front and exits through the atrial siphon at the rear. These muscle bands contract rhythmically to pump water through the body, simultaneously facilitating movement and feeding.
Unique Life Cycles
Salps are defined by an alternation of generations, which allows for rapid population growth and enables them to form massive, short-lived blooms when food is plentiful. The solitary form, known as the oozooid, is an asexual individual.
The oozooid reproduces asexually by budding hundreds of genetically identical clones from a structure called a stolon. These clones remain attached, forming long, linear colonies (blastozooid chains) that can stretch for many meters. Each individual blastozooid is a sexual form.
The blastozooids are sequential hermaphrodites, maturing as females first and being fertilized by male gametes from older chains. An embryo develops within the blastozooid and is released as a new, solitary oozooid once mature, completing the cycle.
Feeding and Movement Mechanisms
Salps are recognized as one of the most energetically efficient examples of jet propulsion, combining locomotion and feeding. The rhythmic contraction of the body’s muscle bands draws water in through the oral siphon and expels it as a forceful jet through the atrial siphon. This provides the thrust necessary to propel the salp forward.
As water passes through the body, it is strained through a fine, internal mucus net secreted by a structure called the endostyle. This net acts as a non-selective filter feeder, capturing a wide range of particles, including the tiniest bacteria and microscopic phytoplankton. The entire mucus net, along with the captured food, is then rolled up and ingested, allowing the salp to graze on the ocean’s smallest organic matter.
Ecological Importance
The salp’s exceptional feeding efficiency makes them primary grazers that can rapidly clear the water column of phytoplankton during bloom events. Their ability to reproduce quickly and form dense swarms means they can exert significant top-down control on primary producers. This grazing activity helps regulate the base of the food web.
The most significant ecological function of salps lies in their contribution to the ocean’s biological carbon pump. Salps produce large, dense fecal pellets packed with the carbon they consume from surface waters. These pellets are significantly larger and heavier than the waste products of most other zooplankton, allowing them to sink at extremely high velocities. This rapid sinking transports carbon to the deep ocean floor, effectively removing it from circulation with the atmosphere. Research has shown that during a dense salp bloom, their fecal pellets can account for over 80% of the total sinking particulate organic carbon.