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Ciona robusta
Ciona robusta is a member of the phylum Chordata and the subphylum Tunicata. The name for the subphylum comes from the soft, gelatinous tunics that encase the animal. These animals are solitary tunicates, commonly known as sea squirts. They are filter-feeders, specifically feeding on forms of phytoplankton. Ciona robusta is often confused with Ciona intestinalis, another member of the subphylum Tunicata, due to extremely similar appearances. Genetic sequencing revealed a divergence range in genealogies of six nuclear gene loci in Type A vs. Type B of 0.035 to 0.124, respectively. Organisms of Type A and B from allopatric populations were able to breed and reproduce, however the hybrid offspring were found to be infertile. For these reasons, scientists began to evaluate the similarities and differences between Type A and Type B, leading to the reclassification of Type B as Ciona robusta.

Characteristics

C. robusta is a cylindrical sea squirt that is encased by a gelatinous, translucent tunic. This organism is made up of two siphons, the oral siphon and the atrial siphon. The oral siphon is much larger and is composed of eight lobes, whereas the atrial siphon is only composed of 6 lobes. The average length of C. robusta is 150 mm, but can reach lengths of 210 mm. C. robusta is typically confused with many other ciona species such as C. intestinalis and C. savignyii due to the similar characteristics they share, such as similar physical characteristics like the oral and atrial siphons.

Ecology
Ciona robusta is non-native to North America, and is thought to have originated in Northeastern Japan/Northwestern Pacific. C. robusta is often considered invasive due to its ability to filter and live in polluted water, allowing it to out-compete native species. This animal is commonly found on the pillings of docks, boats, buoys, aquaculture facilities, in the endobenthic zone. C. robusta tends to live in higher salinity and temperatures, and appear in the Northeast of America during the months of May to September, when the water is warmest.

C. robusta are filter feeders, meaning they take in water from their surroundings and filter out what they do not digest. Phytoplankton and detritus are moved to their stomach on mucus strings. Predators of C. robusta include species of crab and fish. Although C. robusta are not typically found on rocky shores due to high predation. With the removal of predators biologically through other predators and by changes in the environment, C. robusta acclimates to the rocky shore.

Early Life
C. robusta begins life as a tadpole, leading to the classification of phylum Chordata. However, once the tadpole settles on a spot, the organism becomes non-motile, with no bones. Fertilization has been found to be both self-fertilizing and inter-fertilizing. Once the egg has become fertilized, the embryo floats in the water until the tadpole is grown. Settling of the organism usually occurs within 3 days of birth.

C. robusta is a hermaphroditic species, but is usually unable to self-fertilize. In order to reproduce, sperm and eggs are ejected into the water through the atrial syphon. Once in the water, the eggs and sperm mix creating a fertilized egg.

Control Methods

The preferred control methods for this invasive species typically involves preventative biological control and/or identifying and exploiting the activity of natural predators. Small gastropods, such as Mitrella lunata are known to be natural predators that feed on various Ciona species. If the number of M. lunata in a given area are increased, they can eradicate/maintain most of the C. robusta species from increasing at an alarming rate. Another method that is being considered in order to limit the growth rate of C. robusta is the use of biological management strategies. These strategies include the statistics obtained by researchers in order to understand the ecology and physical characteristics that allow these organisms to thrive in its environment. These statistics are collected based on the observations of population numbers in areas where C. robusta are found. An example of biological strategies includes the use of a control agents to decrease the abundance of the species. These include, chemicals, trapping mechanisms, or biological agents.

Impact Summary

The C. robusta species tend to flourish in environments which typically have conditions that are unsuitable for other species to survive in. With that being said, there is limited predation in these areas which allows for this species to thrive. C. robusta are found within harbours and bays with extensive aquaculture operations, this allows them to rapidly occupy all of the available space, monopolize the food resources, and potentially even displace or inhibit the growth of other competing species. It has been observed that this species does not pose a significant threat outside of these areas. This is likely due to predation pressures are higher and the increased competition with other species for food resources and habitats are limited.