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 share a feature known as the notochord– an ancestral characteristic to the vertebrae. 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, stopping the water flow. All adult sea squirts are sessile, found attached to rocks, shells, piers, wood pilings, ships, and the sea floor.

Sea squirts are among the most successful colonizing marine animals and are 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.

Salps have their openings at opposite ends of the body, whereas 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 and salps are commonly planktonic animals. They may occur in large swarms in the water column, especially in the Southern Ocean, where they feed on blooms of phytoplankton.

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 change known as metamorphosis in which the notochord and nerve cord are lost and a simplified adult structure develops.