Algae - The Chlorophyta Phylum

Halimeda are calcareous forms of green macroalgae classifed in the Phylum Chlorophyta(or Chlorophycota), under the same Class: Chlorophyceae and Order: Caulerpales that Caulerpa species are under. This makes them related, but Halimeda along with a few other calcareous species like Penicillus and Rhodophyta(Brush forms), and Udotea andAvrainvillea (Fan forms), fall into the Family Udoteaceae rather than into the Family Caulerpaceae with Caulerpa. By calcareous this means these alga are composed of or contain calcium carbonate, calcium, or limestone. They are a segmented algae that are stiffly calcified except for at the joints, which allows for flexibility in currents.

Halimeda calcareous algae grow well in environments with high calcium, medium light, very low levels of nitrates and phosphates, and low amounts of iron in the 0.05 ppm range.  These macroalgae grow upright, many preferring sandy bottoms for attaching their fiberous rootlike holdfast structures, but some such as the H. discoidea and H. optunia may appear on hard bottoms and live rock as well. In nutrient rich reef aquariums they may require thinning.

The fronds can be varied in segmented shapes as tears or beads (H. lacrimosa), irregular cylinders (H. monile), thickly bilobed or trilobed (H. incrassata), and flattened platelike with a central rib (H. copiosa, H. tuna, H. discoidea, H. opuntia, and H. goreaui) making them distinguishable from other varieties by the size and shape of the segments, branching and depth.

From the brown algae group, Phaeophyta Phylum, Padina sanctae-crucis is considered to be the brown macroalgae equivalent to the green Halimeda spp.

Did you know that calcified green algae, particularly the Halimeda spp., are especially important as major contributors of marine sediments? The white sand on many beaches around the world are largely composed of the sun-bleached and eroded calcium-carbonate remnants of calcareous green algae. This calcified sand is deposited from natural die off and consumption by some marine animals, such as the Caribbean sea urchin species Clypeaster rosaceus and Parrotfish.

In Part 4 about the Chlorophyta Phylum we continue on with Bubble Macroalgae, Valonia sp. and others in this group.