Salpida are free swimming pelagic tunicates belonging to the Class Thaliacea. They are marine and holoplanktonic. There is an alternation of generations, with a solitary form (the asexual oozooid, with a stolon) and an aggregated form (the sexual blastozooid). Because these forms are so dissimilar —besides the absence or presence of gonads— in the early days they were sometimes described as different species.
The body is cylindrical, spindle or prism shaped, often with various patterns of posterior projections. The test is gelatinous and transparent. There is an anteriorly branchial and posteriorly atrial opening, both provided with special muscles and a sort of closing valve.
Muscle bands mostly open at the ventral side only; rarely open at both ventral and dorsal side, or not open at all. Rhythmic contractions of the muscle bands produce a water current through the tubular body cavity, in which the flow direction is ensured in a backward direction by the alternating contractions of the oral and anal muscles. Due to the strong muscles, salps are active swimmers. The water flow enables gas exchange and food uptake by filtering of microflagellates and small diatoms; salps are non-selective feeders.
The dorsal and posterior side of the pharynx has two very large and long gill slits, leaving room for only a single, narrow median gill bar that is attached dorsally and anteriorly, extending obliquely backwards to its ventral and posterior attachment.
There is a ventral endostyle along almost the whole length of the body. Dorsally of the pharynx is the nerve ganglion, with direct to it an accessory eye (without optical function). In the solitary forms, the eye is horseshoe shaped; in the aggregate forms, the shape of the eye is various and consists of a forward, dorsal protuberance of the ganglion. (The eye is of taxonomic value. However, the reddish pigment is quickly destroyed in formalin).
Dorsally behind the oral aperture is a ciliated funnel, aiding in the collection of food particles. The passage from the funnel leads to the oesophagus, and then to the stomach (nucleus), that is located posteriorly. Exceptionally, light organs occur in the subfamily Cyclosalpinae only (not represented in the North Sea).
In the sexual form (blastozooid) the ovary is on the right side of the body, midway of the intestine and the atrial siphon; the testis lies at the ventral side of the stomach.
Reproduction
Inside the blastozooid, the fertilised egg develops into an embryo, which is attached to the integument (placenta) in the cloacal cavity. When the embryo is big enough, they are released as free swimming asexual forms, the oozooids. In the posterior part of the oozooid, a stolon is formed, consisting of a chain of budded blastozooids but still attached to each other. As the chain grows, it breaks up into groups of aggregated individuals.
Distribution
Salpida are known from all oceans but the number of species is highest in the tropical and subtropical waters. The warm water forms are cosmopolitan but are not equally distributed over all oceans. The salps in our region are entering there with warm northerly (Lusitanian) currents. Only a few species are known from the Antarctic and Arctic Oceans. As filter feeders, most of the salps live predominantly in the epipelagic; especially the warm water species are not found deeper than ca. 400 m. The eurytherm Salpa fusiformis aspersa (not occurring in our area), however, is found to 1500 m depth. Diurnal vertical migration occurs in various species.
Systematics
There is only one family, the Salpidae, with two subfamilies, Cyclosalpinae and Salpinae. In total, some 40 salpid species are known. The pattern of the muscle bands is one of the main characters for identification.
Identification of stages
— Animals with a stolon emerging ventrally or posteriorly, gonads always absent -> oozooid or solitary form (proles solitaria)
— Animals with one or several eggs or embryos on the posterior right side, stolon absent -> blastozooid or aggregate form (proles gregata)
The key to the salps of the North Sea starts at Page 470: Salpida. The following species are included:
Order Salpida
Family Salpidae
Salpa fusiformis
Thalia democratica
Pegea confoederata
Thetys vagina
Ihlea punctata
Soestia zonaria