Chlorophyta, Green Algae
Halimeda tuna (Ellis and Solander) Lamouroux
Chlorophyta, Order: Caulerpales; Family: Codiaceae. Some consider the Halimeda and the Flabellia a separate group in the
Codiaceae family, and some consider them a separate family altogether (Udoteaceae.(
Halimeda, is Greek for ‘marine’. The species’ name, tuna, refers to the ancient name for a species of cactus (Opuntia), due to the shape of the thallus, which resembles a small cactus in shape.
The Halimeda’s body is comprised of flat joints that are laid one on top of the other. The alga is calcified and feels hard. The entire bush resembles a sort of small cactus. The joints are connected to one another through internal fibers.
They are triangular in shape and tend towards rectangular with rounded corners.
The alga reaches 10 cm in height, usually less, and the diameter of the joints ranges from 1 to 1.5 cm. The thallus’ width is usually larger than 1 mm, and mature joints are thicker than younger ones.
The Halimeda tuna is dark-green. Upon maturing, the white calcium and the alga turn greenish. Often the alga is covered with epiphytes, which endow the alga with their colour.
The Halimeda has a characteristic shape, and it is not easily confused with other algae.
Halimeda tuna -general view of the thallus. Halimeda tuna -a cluster of sporangia.
The Halimeda in this region may be found mostly in potholes and in deep water. In the open sea, individuals have been found at depths of over 30 m. In potholes, Halimeda bushes are found mostly on the sides of the hole in its deeper parts. Halimeda individuals are nearly never exposed to air.
Biology and reproduction
The Halimeda genus calcifies in its cell walls, a trait that provides it with strength and makes it popular with various epiphytes. A mature cell evinces two organisms (plastids), a chloroplast that performs photosynthesis and an amyloplast that has the ability to store storage substances, starches. Under favorable conditions of active growth, the alga can sprout an entire joint in a single day. On its first day, the young joint is white, since it as yet contains only amyloplasts and no chloroplasts, which are the source of the green colour. The calcification process takes 36 to 48
hours, and it is performed in two stages: at first, ions of calcium and carbon dioxide develop on the wall, and then calcium carbonate (CaCO3) combines them.
Tests that examined the depth to which the holdfast penetrates showed that the Halimeda tuna penetrates up to 5 cm in rock, a fact that grants it stability and enables regeneration after being damaged or eaten.
Seasonality and distribution
The Halimeda tuna can be found along the coast all year round. Its global distribution includes warm seas, the Mediterranean, the Atlantic Ocean, the Indian Ocean and the Pacific Ocean.
Three species of Halimeda have been reported in the world. The Halimeda tuna is apparently the only species that appears in this part of the Eastern Mediterranean.