Ciona intestinalis, commonly known as the “sea squirt,” is a marine invertebrate important in coastal ecosystems and scientific research. It belongs to the subphylum Tunicata, a group within the phylum Chordata, which also includes vertebrates. This organism is widely distributed in temperate coastal waters, attaching to submerged surfaces. Its biological characteristics make it an important subject for understanding fundamental biological processes.
Physical Characteristics and Habitat
Ciona intestinalis has a distinctive, translucent, cylindrical body, often pale greenish-yellow, that can reach 15 centimeters. Its body is covered by a flexible outer layer, the tunic, through which internal organs are visible. It typically attaches at its posterior end to a substrate, often hanging vertically.
At the opposite, anterior end are two prominent tubular openings called siphons. The larger, terminal siphon is the incurrent (buccal) siphon, through which water is drawn into the animal. The smaller, subterminal siphon is the excurrent (atrial) siphon, where filtered water and waste are expelled. Inside, Ciona intestinalis is a filter feeder, using a large pharynx with gill slits to capture microscopic particles like phytoplankton and detritus from the water.
As an adult, this species is sessile, remaining attached to a surface. Its preferred habitats include submerged hard substrates like docks, pilings, ropes, chains, boat hulls, bedrock, and rocky areas in shallow coastal waters. While widely distributed in temperate regions, its natural range is debated due to its ability to spread, often through human activities like hull fouling. Ciona intestinalis tolerates a wide range of environmental conditions and is abundant in harbors and marinas worldwide.
Life Cycle and Reproduction
The life cycle of Ciona intestinalis is characterized by a transformation from a free-swimming larva to a sessile adult. Reproduction is external, with individuals releasing sperm and eggs into the surrounding seawater. Although it is a hermaphrodite, each individual possesses both male and female reproductive organs, but it is generally self-sterile, preventing self-fertilization. This mechanism promotes genetic diversity within the population.
The fertilized eggs develop into a distinctive “tadpole-like” larval stage. This larval form is free-swimming and possesses several chordate characteristics, including a dorsal nerve cord and a notochord, a flexible rod that provides support. These features are significant as they highlight the evolutionary link between tunicates and vertebrates. The larval stage typically lasts for a short period, ranging from two to ten days, during which the larva seeks a suitable substrate for settlement.
Upon finding a suitable surface, the larva undergoes rapid metamorphosis. It attaches head-first, and its tail, including the notochord and neural tube, is reabsorbed. The larval body then reorganizes and transforms into the sessile, filter-feeding adult. This change from a mobile, chordate-like larva to a stationary, sac-like adult defines tunicates.
Significance in Biological Research
Ciona intestinalis is a model organism in biological research due to its characteristics. Its transparent embryos allow scientists to observe developmental processes in real-time, providing insights into embryogenesis. The organism also exhibits rapid development, enabling researchers to study changes over a short period.
Its evolutionary position is valuable; as an invertebrate chordate, Ciona intestinalis shares ancestry with vertebrates. This makes it an ideal system for understanding the genetic and developmental origins of vertebrate traits, such as the nervous system and body plan. For example, while the adult loses its notochord, its larval stage demonstrates this chordate feature.
The genome of Ciona intestinalis is compact, containing approximately 16,000 genes, and has been fully sequenced. This compact genome, coupled with the presence of genes corresponding to nearly every gene family found in vertebrates, facilitates genetic studies and comparative genomics. Researchers can manipulate its genes and observe the effects on development and function, contributing to our understanding of gene regulation.
Ciona intestinalis has contributed to developmental biology, where its transparent embryos and rapid development aid in studying cell lineage and organ formation. It is also a model for neurobiology, despite its simple nervous system, providing insights into fundamental neural processes. Ciona intestinalis exhibits regenerative capacities, particularly during juvenile and young adult stages, making it a model for studying regeneration and its relationship with aging.