Pelagic Salps

Jen Howe

Salp chain, South Taranaki. Photo: Bruce Boyd

Salp chain, South Taranaki. Photo: Bruce Boyd

This  critter isn’t particularly attractive, isn’t ‘cute’ and often causes a ‘yuck’ reaction when encountered. You probably bumped into many of them during your swims this year as there was a great abundance of them during warm and sunny summers. We are going to talk about those lumps of jelly that feel like beads against your skin and get stuck in people’s hair. They look like jelly fish but are actually more closely related to you than to the stinging jellies. They are the fascinating pelagic salps.

Salps belong to the tunicate group in the Phylum Chordata, classified as such as there is evidence of a notochord during the larval stage. About 97 % species of the chordates are classified in the subphylum Vertebrata, or animals with backbones, to which we belong. Most of the remaining 3 % chordate species are tunicates (subphylum Urochordata). Most of these are benthic filter feeders in the Class Ascidiacea, such as the sea squirts, meaning that they live on the ocean floor. Salps belong in the small Class Thaliacea which has taken up a pelagic planktonic lifestyle in the open water.

Classic body plan of a salp species (The Natural History of Bodega Head)

Classic body plan of a salp species (The Natural History of Bodega Head)

Salps are gelatinous, are mostly transparent and are cylindrical in shape. They vary in size from a few millimetres at birth to about 10 cm, though one species can grow to a few meters. Salps move by rhythmic contraction of circular muscles within the body wall which pumps water through their body. The bulk of the body consists of a large pharynx. Water enters the pharynx through the large buccal siphon at the front end of the animal. It is then forced through two slits in the pharyngeal wall into the atrial cavity and then ejected out of the atrial siphon at the back end of the animal. The pharynx filters planktonic food from the water with the aid of mucus nets across the slits. By doing so, the salps are able to catch and feed on the oceans’ tiniest organism, which mostly includes phytoplankton but also down to tiny bacteria. Due to filtering for their food salps do not survive well in silty coastal areas due to inorganic particles clogging up their pharyngeal mucous nets. The chains also tend to break up in reef waves, and individuals or part chains predominate.

Though salps look like small bags of jelly they have quite complex systemic features. There is a compact gut nucleus, which is where the stomach and intestine are combined. They have tiny, bilateral groups of nerves equivalent to the brain which scientists speculate evolved into the more complex central nervous system of vertebrates. Salps also have several kinds of sensory nerves, some that respond to light and some that

Chain of salps, Snorkel Trail, Jan 2014. (Photo MH)

Chain of salps, Snorkel Trail, Jan 2014. (Photo MH)

respond to water flowing past their body which regulates the swimming motion via motor neurons to the muscle bands. They also have a simple heart, which alternates the direction blood is pumped around the body. Salps

Luminescent Salps (Youtube), Massachusetts

Luminescent Salps (Youtube), Massachusetts

have a comb like organ on either side of their bodies that produce a bluish white luminescence. When they swarm at night the accumulated luminescence can cause the sea to glow.

Salps have a complex life cycle alternating between sexual colonial and asexual, solitary generations. The colony of cloned individuals are the sexual gonozoids which develop by asexually budding from an elongated stolon within a solitary individual (the oozoid). (The photo below right, by Jen Howe, under a microscope shows a developing chain of gonozoids within a solitary oozoid)

Salp, from Lyall Bay Photo Jen Howe

Salp, from Lyall Bay Photo Jen Howe

The sexual individuals in a chain are sequential hermaphrodites, starting off as females when young and are fertilized by older male individuals in the same chain. The embryos (oozoids) form and develop into the solitary asexual phase inside the individuals of the aggregate. There is no larval stage and even before release, the oozoid

Salpa thompsoni (L solitary form; R chain)

Salpa thompsoni (L solitary form; R chain)

often already has a developing stolon ready to bud into gonozoids. This vastly quick and efficient method of asexual reproduction enables salp populations to explode during periods of abundant phytoplankton blooms and optimal conditions. The whole lifecycle requires only 48 hours to be completed.

At times, the abundance of salps in the water can become so great that the water column is depleted of plankton, a reason given for the collapse of a some local fisheries in the northern hemisphere. Once there is no longer enough food to sustain the enormous numbers of salps the population reduces quickly again. Salps, in turn are predated upon by many species; jellyfish, siphonophores, ctenophores, turtles, marine birds and fish.

Salps are common in equatorial, temperate and cold seas, in shallow and deep waters. The most abundant concentrations of salps are found in the Southern Oceans near Antarctica. In recent years salp populations in the Southern Ocean seem to be increasing as the krill populations have been decreasing. Salps and krill mainly feed on phytoplankton. Krill are reliant on sea ice as the juveniles feed on algae growing underneath it. As temperatures have increased, sea ice has decreased which may have led to the reduction of krill. Salps, on the other hand, are more tolerant of warmer conditions and can increase population numbers tremendously as the phytoplankton blooms. This could be of concern as larger animals, such as whales, penguins and seals that depend on krill in their diet, feed on salps instead but do not obtain the same quality of nutrition from them.

From personal observations, the number of salps in the water during the last few summers seems to have also increased here in Wellington. Will this trend continue if summers do become warmer and less windy? We shall have to wait and see, though as unpleasant as it can be to be bumped by so many bags of jelly, it’s good to know that the salps are harmless and won’t sting you.

Sources:

“Observations upon the Nervous Systems of Pelagic Tunicates” – Quentin Bone, Dept of Zoology, Oxford. 1959

Lacalli, TC, Holland LZ. 1998. The developing dorsal ganglion of the salp Thalia democratic, and the nature of the ancestral chordate brain. Philosophical Transactions of the Royal Society of London Series B-Biological Sciences 353:1943-1967

Bombosch A, 2008. Euphausia superb or Salpa thompsoni – Who is going to win?