Halimeda is a calcareous algae which requires good calcium and alkalinity levels to grow well. In hobbyist parlance, it can "go sexual", which means that it forms sexual organs (gametangia) and losses pigmentation, appearing predominately white with small green spots. After going through this process, the original plant dies, and becomes totally white.
Thallus constructed of articulated sequences of flattened calcified segments (plates) of various shapes, alternating with non-calcareous joints (nodes). Growth form erect, pendant, or sprawling, achieving a height (length) of a few centimeters to a meter or more. Attachment to substratum by 1) a single large bulbous holdfast, typically 1 cm. to about 13 cm. long, which can represent up to ca. 40% total biomass and consists of fine siphons with adhering sand particles, 2) by a single small, discrete holdfast of matted filaments, ca. a few mm. to 1 cm. long, or 3) by several diffuse and inconspicuous patches of rhizoids arising from segments or nodes. New segments are added apically, with new ones generally completed in 24-48 hours. Deposition of aragonitic calcium carbonate begins after about 36 hours of segment development, and occurs first in external fibrous matting of siphon walls; crystals subsequently grow into spaces between siphons. In old segments secondary crystal formation may occur around aragonite needles. Extent of calcification varies with age, taxonomy and environment. Asexual reproduction is by fragmentation, or by development of new thalli at ends of uncorticated siphons growing out either from segments or from filaments of the holdfast. In sexual reproduction biflagellated gametes are produced in specialized globular to pyriform gametangia on simple or branched uncalcified stalks, which develop as outgrowths from nodes or the surface of segments.
The Halimeda genus consists of forty four current species. Some of the common ones encountered are: