Saturday, October 17, 2009

Salps in the bay

[Originally posted to MBA Student Oceanography Club (SOC)]
During our kayak trip, we saw some pretty cool organisms. Among the more unique were salps, the gelatinous creatures that were swimming everywhere. Salps are tunicates, marine chordates identified by their rigid tunic (also known as a test). Most tunicates, commonly known as sea squirts, are sessile, meaning they live permanently in one place firmly attached to the bottom. The 24 species of salps, however, have adopted a free-swimming, pelagic lifestyle. All are transparent, tubular, gelatinous animals that swim via jet propulsion as they pass water through their bodies.

Salps feed continuously on plankton and detritus as they swim. A mucus net captures particles suspended in the water that passes through the salp’s body. The net is constantly renewed, and cilia move the mucus like a conveyor belt into the gut. They are very efficient feeders, and some of the fastest growing of all multicellular animals; one species can increase its body size by as much as 20% per hour! Salps also reproduce rapidly, with generation times of several days. Because of their rapid reproduction and incredible growth rates, when phytoplankton is abundant, salps can form massive blooms. They can swarm by the millions or even billions until they filter out most of the plankton in the surrounding water, at which point the populations crash.

We likely witnessed such a bloom during our kayak trip. The species that we saw in the bay was probably Cyclosalpa affinis. Cyclosalpa can consume approximately half of their body mass in 24 hours, increase in body size by about 25% a day, and reproduce quickly, explaining the huge number that were present in the bay for several days. Like other salps, Cyclosalpa have a complex life cycle that alternates between solitary, asexually reproducing generations, and aggregate, sexually reproducing generations. The solitary phase, called an oozoid (shown in photo 1), buds chains of aggregate salps, called blastozooids (the arrow in photo 2 shows the budding chain coming from an oozoid). In some species, this chain can be up to 15 feet long. In Cyclosalpa, however, the aggregate salps form rings of individuals (shown in photo 3). The aggregate individuals remain attached while swimming and feeding, growing and reproducing as a unit. The aggregate salps are sequential hermaphrodites, starting life as females, and becoming males when they grow larger. Chains of small female salps will therefore mate with chains of larger male salps, and embryos are gestated within the female’s body wall. The embryos develop into solitary salps and are released, beginning the cycle anew.

Salps are an important part of the pelagic ecosystem. They are food for many predators, including jellyfish, fish, sea turtles and birds, and host a number of other organisms, including parasitic crustaceans and fish (the arrow in photo 3 shows a parasitic amphipod residing in a blastozooid). The fecal pellets formed from their prodigious feeding are also a major source of food for mid-water and deep-sea creatures.

Salp Oozoid Budding Oozoid Blastozoid with parasitic amphipod