http://www.fao.org/DOCREP/003/X6878E/X6878E33.htm

Caulerpa racemosa (dried): 14.9 mcg. pr 100g

More about sea grapes / "green caviar" here:
http://www.fao.org/docrep/006/y4765e/y4765e0b.htm
8.8 Sea grapes or green caviar (Caulerpa lentillifera)

There are many species of the genus Caulerpa, but Caulerpa lentillifera and C. racemosa are the two most popular edible ones. Both have a grape-like appearance and are used in fresh salads. They are commonly found on sandy or muddy sea bottoms in shallow protected areas. The pond cultivation of C. lentillifera has been very successful on Mactan Island, Cebu, in the central Philippines, with markets in Cebu and Manila and some exports to Japan. About 400 ha of ponds are under cultivation, producing 12-15 tonnes of fresh seaweed per hectare per year.

C. lentillifera (Figure 61) is the species best adapted to pond culture, although some strains of C. racemosa also give good yields. C. lentillifera is sensitive to changes of salinity, so pond areas must be placed away from any freshwater sources, and in the wet season in the Philippines surface drains are placed around the ponds to remove freshwater. The seaweed can tolerate a salinity range of 30-35 parts per thousand. Successful cultivation depends on good water management and the ponds must be designed so that tidal flows can be used to change the water in the ponds every second day. Water temperature can range between 25 and 30C. Pond depth should be about 0.5 m and areas of about 0.5 ha are usual.

Planting is done by hand; about 100 g lots are pushed into the soft bottom at 0.5-1 m intervals. Sometimes broadcasting is used but this is not as efficient, the plants are loose on the bottom and can be moved by water motion induced by wind action on the surface. Key factors to control during growth are water exchange, weeding of other species of seaweed that would otherwise compete with the Caulerpa, and fertilization if the plants appear unhealthy or pale green to yellow in colour. Harvesting can commence about two months after the first planting; the seaweed is pulled out of the muddy bottom, but about 25 percent of the plants are left as seed for the next harvest. Depending on growth rates, harvesting can then be done every two weeks. The harvested plants are washed thoroughly in seawater to remove all sand and mud, then inspected, sorted and placed in 100-200 g packages; these will stay fresh for 7 days if chilled and kept moist. For local consumption, or air freight to Metro Manila, the seaweed may be packed in baskets lined with banana leaves; 20-30 kg of seaweed is placed on the leaves, the basket is topped with more banana leaves and covered with a plastic sack that is then fixed to the basket.

As always - and this is true for both vegan and non-vegan products, but more important for vegans than non-vegans - a B12 level alone doesn't tell the whole truth about the value of the B12 found in food, because age, it's human-activity and many other factors are important. Re. seaweed, the time of the year is also important, because the amount of B12 in seaweed varies from season to season.