Sea Urchin Aquaculture For Beginners
Sea urchins also known as echinoderms are spiny, globular animals falling under the class of Echinoid which inhabits in all oceans in a depth zone from intertidal to 5,000 meter (16,000 feet) and can also be raised in tanks.
They have hard shells that are round and spiny, which are about 3 – 10 centimeters (1 – 4 inches) across. They move slowly, crawling with their transparent adhesive tube feet; sometimes pushing themselves with their spines.
Types of sea urchin can be categorized as;
- Full life cycle grow-out urchins whereby the adults are collected and their larvae are raised in hatchery until they are gonads reach the marketable size and quality.
- Gonads enhanced from wild populations as the adults are collected from wild populations then maintained in captivity; they are artificially fed to increase the gonads mass and quality.
Not All Sea Urchins Can Be Consumed, Below Are A Few Common Types;
- Common heart urchin (Spatangoida) also known as sea potatoes – they have an oval shape; mouth is on the bottom and anus on the opposite side which makes them appear to be heart like hence their name. This urchin type can reach a length of around 5 – 9 cm (1.97 – 3.5 inches); reproduction is by external fertilization and after, their larva needs 20- 40 days to grow up until juvenile.
- Common sea urchin (Echinus Esculentus) is the largest urchin with a diameter reaching 16 cm (6.3 inches) and their largest recorded at 17.6 cm (6.9 inches); can be found in 1,200 feet (30.5 m) deep and they are commonly found in Atlantic Ocean (North East).
- This type of urchins need around 40-60 days for their larva to grow up to become an adult. Their beautiful appearance makes humans to collect and make them souvenirs.
- The Rock boring urchin (Echinometra Lucunter) also known as Tropical Sea Urchin – has an oval shape and have two long tiny pipes. They use their spine to enlarge holes in rocks making them to fit their size as they dig.
- Pencil sea urchin (Phyllacanthus Imperiallis) also known as mine urchin – are urchins that hide in holes during the day in any corals and come out to hunt for food during the night like invertebrates and sponges. They can reach a size of 15 cm (5.9 feet) and usually found in Caribbean.
- Red sea urchin (Mesocentrotus Franciscanus) – this type of urchin is usually found in Pacific Ocean in a depth of 100 m (328 feet); with a common size ranging from 8 – 19 cm (3.15 – 7.5 inches) long. Their colour (red and dark burgundy) distinguishes them from the other urchins.
The edible sea urchin and uni
Edible sea urchins sought for are the large ones with shot; strong spines and are mostly pink in colour and sometimes white. These edible large echinoids in the North Sea are located in littoral zone in the depth of up to 40 m (131 feet).
The urchins are also sought for their roe (eggs or sperm of a fish) which is the male and female gonad which is edible and it is considered the most valuable product especially in the sushi industry as they extract uni from the gonad.
In Japan, there is so much demand for sea urchin especially as it is over exploited in the fishery industry and it became a substitute for caviar. Green sea urchins are also great for consumption though harvesting them is costly because they are smaller but they keep in much large numbers.
The urchins species have varying days needed for their larva to be grown up to adults, with some ranging from 20 – 60 days ; thereafter reproduction, the urchins become orphans because their parents die after fertilization.
Juvenile Grow-Out And Gonad Enhancement
Juvenile grow-outs can be done through land-based rearing system and sea-based rearing system.
Sea urchins are cultured in cages that are 80 cm x 80 cm x 30 cm deep with 5-mm mesh. The cages are suspended in the sea in March or April with 1,000 urchins per cage. Fresh cultured kelp is added to the cages once or twice a month, except during the winter when salted or boiled kelp is used.
Sea urchins can be found in shallow coral reef at the depth of less than 160 feet (4.9 m) or in the hidden deepest part of the coral about 1,500 feet (457.2 m) depending on the type of sea urchin. Individuals by the beach may be lucky to stumble across sea urchins by the shores.
Some urchins usually hide behind algae which can be found in 1,200 feet (30.5 m) depth; they can also be found behind limestone, under rock slabs, broken coral or among sea weeds. How To Raise Sea Urchin Though other urchins usually live in stones to protect themselves from being washed by big waves, they cannot survive in places with too much sand or mud.
Some urchins go out at night to hunt for food, which is usually algae and the other type of feed they is sea weed, sea grass, and other seas particles such as dead fish, sponges, shells. Once they finish eating, they return to the same hole they habit. The common sea urchin is an omnivore; they feed on worms, barnacles, hydroids, and tunicates.
Harvesting Sea Urchin And Gonads
Sea urchin roe is harvested and when marketed, it can be available in several forms; fresh, frozen, baked, steamed and baked frozen then salted. The mostly sought after roe is the chilled fresh roe which is quite popular in the sushi making industry.
The sea urchins are harvested to improve their taste, colour and size; before they are processed for export in other countries like Japan and Asia. Sea urchins are harvested twice a year which is in spring and summer.
Grazing activity of the urchins is critical for the maintenance of enlarged gonads. Hence, moderate exchanges of water from the open sea to the cultured grounds are required to avoid the de cline in water temperature in the winter that can result from low air temperatures.