Sea squirts are marine invertebrates that appear as simple, sac-like organisms, often mistaken for sponges. They belong to the subphylum Tunicata, a group that holds a connection to vertebrates. Their common name is derived from their habit of forcefully ejecting a stream of water when disturbed. Found in oceans worldwide, these animals are more biologically complex than their appearance suggests.
Anatomy and Habitat
The adult sea squirt’s body is barrel-shaped and protected by a thick outer layer called a tunic, made of a substance similar to cellulose. The body has two openings, or siphons: an incurrent siphon that draws water and food into the body, and an excurrent siphon that expels water and waste. As adults, sea squirts are sessile, permanently attached to a single spot.
They are found in a wide range of marine environments, from shallow intertidal zones to great depths, and attach to hard surfaces like rocks, pier pilings, and ship hulls. While some species are solitary, others form large, interconnected colonies that can vary widely in color and shape.
The Tadpole Transformation
A key aspect of the sea squirt is its life cycle, which reveals its relationship to more complex animals. It begins life as a free-swimming larva that resembles a tadpole. This larval form possesses advanced features, including a supportive notochord and a dorsal nerve cord, which are characteristic structures of all chordates, the phylum that includes vertebrates. The larva also has a primitive eye and a basic brain.
This larval stage is brief, as the tadpole does not feed; its purpose is to find a suitable surface for its transformation. Once a location is selected, the larva attaches itself head-first and undergoes a metamorphosis where it absorbs its own tail, notochord, and most of its nervous system, becoming the simple, immobile adult.
Feeding and Reproduction
Adult sea squirts are filter feeders, consuming microscopic plankton and organic debris from the water. Water is drawn in through the incurrent siphon and passes through a mucus-covered pharynx that traps food particles before being expelled.
Most species are hermaphrodites, possessing both male and female reproductive organs, though they generally avoid self-fertilization. Sexual reproduction occurs when sperm and eggs are released into the water column for fertilization. Many species also reproduce asexually through budding, where new individuals grow from an adult’s base to form new colonies.
Ecological and Scientific Relevance
The sea squirt’s tadpole-like larva provides an evolutionary link between invertebrates and vertebrates. Ecologically, they act as filter feeders that improve water clarity, but their hardiness has also allowed some species to become invasive.
These animals are also of interest to medical science, as some produce unique chemical compounds; one from Ecteinascidia turbinata has been developed into an anticancer drug. Their ability to accumulate substances also makes them useful indicators for studying marine pollution.