Sea Squirts and Salps
Classified within the same phylum (Chordata), sea squirts and salps belong to separate classes, the Ascidiacea and Thaliacea, respectively. Both groups are also known as tunicates, a group of primitive chordates which have a primitive feature known as the notochordthe earliest and simplest equivalent to the vertebrae of more developed animals. In appearance adult sea squirts and salps are barrel-shaped animals, resembling a small open bag with a tough surrounding "tunic" that has two openings through which water passes. Water enters the body through one of these openings through the buccal siphon, passing into a large and highly perforated sac where it is strained for food particles before passing out through a second opening, the atrial or cloacal siphon. Food particles such as plankton that have been retained in the sac pass directly into the stomach where they are digested. When the animal is not feeding, the buccal siphon is closed, thereby stopping the water flow. All adult sea squirts are sessile, being attached to rocks, shells, piers, wood pilings, ships, and even the sea bed where this provides a firm base.
One of the most obvious differences between sea squirts and salps is that the latter group have their openings at opposite ends of the body, whereas these are both arranged on the upper part of a sea squirt's body. The flow of water directly through a salp's body may therefore also be exploited as a simple means of moving from one place to another, although most salps rely on the larval phase of development for dispersal and long-distance movement.
The notochord, which distinguishes these animals from other soft-bodied marine organisms, is not visible in adult sea squirts or salps. Instead it makes an appearance in the larval stage, which resembles a tadpole. The larvae are free-living, and when they settle, they undergo a state of change known as metamorphosis in which the notochord and nerve cord are lost and a simplified adult structure develops.
Sea squirts and salps are among the most successful colonizing marine animals and are commonly found on most seashores, with their range extending down to moderate depths. Sea squirts are often solitary, but some species may form colonies with the individuals united at the base, while others may form a gelatinous encrustation on the surface of rocks or on weeds. In colonial species, each individual has its own mouth opening but the second, or atrial opening, is common to the group.