Coin Green Seaweed

Appearance

Thallus prostrate, moderate to strongly calcified; color whitish, becoming grayish on drying; up to 24 cm long; branching di-trichotomous; attachment by multiple holdfasts; segments subcylindrical, cuneate to reniform, with a smooth and shiny surface, margin undulate, about 18 mm wide and 11 mm long (figure 8); segments friable before decalcification, with utricles lightly coalescent after decalcification; cortex with two, rarely three layers of utricles, rounded in outline, 52 (44-67) µm in diameter in surface view (figure 19), and 72 (67-92) µm long in cross section. Every secondary utricle supports up to 8 primary ones; secondary utricles clavate (figure 38), 74 (56-92) µm wide and 170 (166-333) µm long. Medullary siphons interwoven, with extensive paired nodal fusions, eventually with three filaments fused together.

Distribution

Tropical seas.

Behavior

These seaweeds can relocate their chlorophyll (the green pigment used in photosynthesis). At night, chlorophyll might be concentrated in the centre of the segment. As a result, the seaweed may appear white.
Halimeda is also responsible for distinctive circular deposits in various parts of the Great Barrier Reef on the north-east coast of Queensland, Australia. Halimeda beds form in the western or lee side of outer shield reefs where flow of nutrient-rich water from the open sea allows them to flourish, and are the most extensive, actively accumulating Halimeda beds in the world.

Habitat

Intertidal. They are commonly seen growing on coral rubble or among living corals, as well as in sandy areas near reefs. Sometimes forming rather large 'thickets' covering an area of 40-50cm.

Defense

In addition to calcium carbonate, they also have chemicals that protect them from herbivores. Despite this, some slugs like the Halimeda slug (Elysiella pusilla) actually eat the seaweed and incorporates these chemicals into their tissues to protect themselves!