Sea Squirt

Sea Squirt

Ascidiacea, commonly known as ascidians or sea squirts, are sac-like marine invertebrates belonging to the subphylum Tunicata within the phylum Chordata. Found in oceans across the globe, these animals derive their name from their distinctive outer body covering, known as a tunic or test. Unlike the calcareous shells of molluscs, this tunic is composed of living tissue reinforced with proteins, calcium salts, and cellulose—a complex polysaccharide rarely produced by animals. Adult sea squirts exhibit three distinct growth forms: solitary individuals, social species that grow in clumped clusters attached at their bases, and compound (colonial) ascidians. In compound species, numerous tiny individuals called zooids are embedded within a shared gelatinous test, sometimes sharing a centralized exhalent opening while maintaining separate inhalent siphons. Individual sea squirts range in size from 0.5 to 10 cm, displaying a wide variety of shapes including cylindrical, rounded, vase-like, or encrusting forms. Sessile in their adult stage, sea squirts attach firmly to hard surfaces such as rocks, shells, corals, and artificial harbour structures. At the unattached upper end of the animal lie two openings called siphons. The larger oral siphon draws seawater into an internal pharyngeal chamber, while the atrial siphon expels water, digestive waste, and gametes. When removed from the water or squeezed at low tide, the sudden contraction of their body wall forcefully ejects water jets from these siphons, giving the animals their common name. Though adult sea squirts appear simple, they represent a critical evolutionary link between invertebrates and vertebrates. Their short-lived, free-swimming larvae are lecithotrophic (non-feeding) tadpoles that clearly demonstrate chordate traits, including a dorsal tubular nerve cord, a notochord, a post-anal tail, and pharyngeal gill slits. Upon attaching to a suitable surface, the larva undergoes metamorphosis, absorbing its tail and notochord while developing into a sessile filter feeder. For scuba divers, sea squirts add remarkable texture and vibrant colour to underwater landscapes. They frequently blanket vertical cave walls, shaded reef overhangs, and wharf pilings in rich growth, serving both as beautiful photographic subjects and key bioindicators that accumulate environmental pollutants through continuous water filtration.