Sea squirt
♦ Scientific name:
: Halocynthia roretzi
♦ Common name:
Ascidians or Sea squirt
♦ Distribution
Ascidian or sea squirt, Halocynthia roretzi, mainly inhabiting the subtidal rocky bottom is a sessile marine animal ubiquitous throughout the world. The wild population of the cultured sea squirts are distributed from northern parts of Japan to Chinese seas, including Korean waters. The eastern and southern coastal waters dominate the species in Korea.
♦ Ecology and habitat
Sea squirts are somewhat barrel-shaped animals and the entire body is invested with a thick covering, the tunic, from which the name tunicate is derived. An individual has two openings: an incurrent branchial (oral) siphon and an out current atrial siphon. Ascidians live by filtering tiny plankton (diatoms, protozoans, copepods, and larvae of various invertebrates) and nutrient materials from seawater, which is taken in through the pharynx. The animals, characterized by hermaphrodite (having both male and female reproductive organs in a body), are mostly found in shallow waters. Mostly, they live in shallow water; usually attach to rocks and artificial structures. Usually, ascidian, Halocynthia roretzi is adapted to cold water. Living water temperature is 2~24℃; optimum temperature near by 12℃.
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A
wild sea squirt living on the rocky shore.
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Wild
sea squirts colonized on the rocky
shore.
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♦ Aquaculture
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Commercially, Halocynthia roretzi is a common species in Korea and Japan. It has been a favored tasty sea food in abundance along the east and south coast of Korea. Most of the cultured sea squirts come from Korea and Japan. It was 1982 when the first cultured production was achieved, producing 39 metric tons in Korea. Thereafter, the farmed production increased sharply with the productions 42,800 tons in 1994 since then decreased 4,500 tons in 2004 by reason of mass mortality during winter season. The sea squirts are cultured in the hanging culture method in Korea. The hatchery-based seeds of the species are attached on the vertical rope hung from the longline (the culture system of the sea squirt is basically same as that of oyster). The attached seeds grow to 22.3㎝ in body height by year 3 via 11.1 cm by year 2 under normal water condition at water temperatures 8∼13℃.
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Mass mortality of sea squirts by the symptom of tunic softening.
Korean aquaculture technology for sea squirt is advanced. However, farmers are still suffering from mass mortality during the growing seasons. Aquaculture environmental changes of global warming might be a culprit for the mass mortality. Studies are also pointing out the ecological unbalance attributed to prolonged intensive culture in a given area for the mass mortality.
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Mass mortality of sea squirts by the symptom of tunic softening. |
♦ Procedure for the hatchery-based seed production of sea squirt
The hermaphroditic spawners become mature in November to March where ambient water temperature remains 8 to 13℃. The mature sea squirt carries about 200,000 eggs which are shedding 6 to 10 times for about 1 hour in the spawning season. The fertilized eggs metamorphose to tadpole larvae in 24 hours. The larvae, 1.75mm in body length, lose their tale part in 6 hours. As soon as the larvae are losing their tail, they are settling on the substrate (palm cord).
Just at the time when the larvae are ready to lose their tail, seed collectors are put into the hatching tanks for the larval attachment. A cord of palm tree is known to be a good substrate for the larval attachment. One of the important things the sea squirt hatchery person should keep in mind is the inserting time of the collectors in the larval culture tank. The duration the larvae lose their tail is about 20 minutes. Therefore the hatchery person should be alert to put the collectors in the tank on time.
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Artificial spawning of sea squirts for larvae collections |
Spawning of gamete(eggs and sperms) |
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The cord of palm tree is good substrate for the larval attachment, but it can contain some chemicals which block the larval attachment on the cord. Therefore, detoxification is essential. For the detoxification,
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Tadpole larvae collection in the hatchery
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Survey of larvae on the collector set in the nursery ground
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The Prepared seed collectors are immersed in the freshwater and seawater for some period of time.
After larval attached on the seed collectors, the collectors are to move into nursery ground. In nursery ground, the collectors should be set in the middle water column where daily water temperature changes are not significant. When the larvae attached are ready to move into the farming ground from the nursery ground, the palm cord with seeds are set in the culture system.
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The sea squirt larvae attached on the cord of
palm tree.
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Setting process of palm cord unfolded
in the culture system
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Immersing
of seed collectors for detoxification
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Unfolding of palm cord before setting the cord
in the culture system. Setting of palm cord in the culture system
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Setting
of palm cord in the culture system
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Cultivation in the culture system
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KOREA-US AQUACULTURE -> Main Species->Others->Sea squirt
Sea squirt