Marine algae from the genus Caulerpa get most of the attention in the reef aquarium hobby. Caulerpa species are fast-growing, decorative algae that have been mainstay livestock items in the hobby for years. They are an integral component of some current methods of natural filtration such as the mud refugium systems. While it may be hard to argue against Caulerpa's effectiveness as a nutrient exporter, these algae are prone to grow out of control in aquaria and ultimately cause as many headaches as they relieve. A viable alternative for the reefkeeper interested in a group of decorative algae with growth rates matching Caulerpa are the "cactus algae." The common name "Cactus algae" is ascribed to many members of the genus Halimeda.

Halimeda are green algae from the phylum Chlorophyta and are classified in the order Bryopsidales along with Caulerpa. Bryopsidales are somewhat unique in that they are coenocytes; the entire organism lacks individual cells. While a normal plant cell is tiny, enclosed in a cell wall, and contains one nucleus with the genetic material, coenocytic algae can be thought of as a single giant cell with multiple nuclei. Halimeda belong in either the family Halimedaceae or Udoteaceae, along with other close relatives including Udotea and Penicillus (commonly know as shaving brush algae). This family's distinguishing characteristic is the ability of its members to synthesize calcium carbonate from seawater, much like stony corals and the beautiful purple coralline algae do. Roughly thirty modern species of Halimeda are known, and several others are known only from the fossil record.

A typical Halimeda alga is a flexible string of flattened leaf-like structures often referred to as segments. Each segment is a deposit of calcium carbonate covered by the algal protoplasm and connected to its neighbors by a thin strand, giving the plant its flexibility. Some species, such as H. copiosa, take on the form of long necklaces, as if green disk-shaped charms had been fitted on a thin chain. The most common species, H. tuna and H. discoidea, have larger segments and grow in somewhat shorter chains. Another species common to aquaria is H. opuntia. It has very small segments and tends to form dense, shrub-like masses. Halimeda opuntia is named for the Opuntia genus of cactus, which includes Halimeda's terrestrial look-alike, the prickly pear.

Photo by Kirby Adams.

Halimeda inhabit a range of marine habitats from sandy bottom areas to rocky reef structures, and have been reported from depths up to 150 meters (500 feet). The ability to thrive at depths where light radiation is minimal suggests an evolved trait that occupies a niche with few other photosynthetic organisms. Since algae are not vascular plants and do not have true roots, they will grow wherever they settle in an aquarium. A more natural appearance and growth form, however, can be maintained by placing sand-dwelling species on the substrate and reef dwellers on the rocks. The substrate 'rooted' species have holdfasts that can extend 10cm (4in) or deeper into a sandy bottom while the others attach themselves to rocks with a series of short holdfasts similar to those of the commonly kept Caulerpa species. Older aquarium hobby books suggested Halimeda were quite difficult to maintain in a closed system, but today most of the available species are considered very hardy. Lighting is not extremely critical to these algae and any illumination sufficient for a reef aquarium will be more than enough to meet their requirements.

The most stunning characteristic of Halimeda is their rapid growth rate and the accompanying deposits of calcium carbonate. These algae have been found to be the primary reef-building organism of the tropical seas, erasing the notion that stony corals are the champion reef engineers. On the Great Barrier Reef a 2,000 km² (1,250 sq. mile) area covered with coarse gravel from 10-15m (33-50 ft) deep was found to be primarily Halimeda fragments with vast areas comprising as much as 98% algal deposits (Drew 1986). The same study on the Great Barrier Reef showed that huge meadows of Halimeda produced up to 2 kg calcium carbonate per m² every year. That means that a patch of Halimeda the size of an average living room would produce more than 100 pounds of aragonite in a single year. Halimeda accomplish this feat with rapid growth, doubling the biomass of a colony every 15 days (Drew 1983). Considering that 90% of the plant is nothing but aragonite, it's easy to see how it builds up so quickly.

The rapid growth and calcification rates of Halimeda add to their appeal and usefulness in the marine aquarium. The growth indicates these algae could be every bit as effective as an instrument of nutrient export as the more popular Caulerpa. Regular harvesting of any algae will export nitrogen, phosphorus, and other organic compounds, and faster growth rates lead to more frequent harvests. The calcification abilities of Halimeda make them useful as a barometer of calcium and alkalinity parameters in a closed system, but can also render them somewhat dangerous in a system where the calcium demand is already high. Rapidly growing masses of calcareous algae are good indicators that the amounts of available calcium and carbonate in the water are adequate for calcification, a critical consideration for the maintenance of stony corals. Healthy Halimeda that is properly calcifying will have segments that are somewhat rubbery but not spongy. This indicates production and deposit of aragonite crystals and is as reliable as any calcium and alkalinity test kits you can buy at the corner store. The calcification does, however, remove the valuable calcium and carbonate building blocks from the system and can therefore greatly increase the demand for these elements. Halimeda must essentially be treated as a stony coral regarding their demand for proper levels of calcium and carbonate hardness. This isn't much of a concern for the average reef-keeper who already maintains these levels religiously, but it must be kept in mind that large masses of Halimeda can drain a system of both calcium and carbonate hardness rapidly if these materials aren't added regularly.

Photo by Kirby Adams.

The presence of gigantic unmolested meadows of Halimeda in the Great Barrier Reef indicates that they are distasteful to potential grazers, or perhaps repel herbivores in some fashion. Both of those presumptions have been proven true. The plants' calcareous nature makes them a less appetizing meal to grazing fish such as surgeonfishes than more succulent algae. Halimeda go a step further to ward off aragonite-munching herbivores, such as parrotfish, by synthesizing noxious and potentially toxic secondary metabolites. The aptly named halimedatrial and halimedatetraacetate are diterpenoid compounds that appear to give Halimeda an extremely noxious taste and could prove toxic in large quantities (Paul and vanAlstyne 1988). Some of the few predators that threaten Halimeda are the chloroplast-thieving sacoglossan slugs. Lettuce slugs steal chloroplasts from algae, killing or damaging the algae and rendering themselves photosynthetic. Halimeda are vulnerable to this robbery, but have developed, perhaps inadvertently, a defense against even this type of grazing. The chloroplasts of the green tissue, which are normally clustered near the surface of the thallus, migrate more deeply into the tissue at night, leaving little for a marauding slug to pilfer (Drew 1990).

Caulerpa fanciers are usually familiar with those algae's method of sexual reproduction. A colony will expel its gametes along with all of its cytoplasm, leaving a snow white or transparent (and very dead) clump of algae and a pea soup green aquarium in its wake. Halimeda's sexual reproduction is similar, but with the added benefit of a known warning indicator. Hours before releasing gametes, the algae will turn pale white with dots of very dark green or almost black along the edges of the thalli. The dots are called gametangia and contain all of the contents of the living plant, concentrated in tiny capsules. This creation of the gametangia is called sporulation. Shortly thereafter, the gametes are released in a fashion similar to Caulerpa's. Plants that reproduce in this fashion, with the entire plant becoming reproductive, are said to be holocarpic. These sexual events have been blamed for sudden deaths of tank inhabitants, and the secondary metabolites of the algae are often fingered as the cause. While this is certainly possible, it seems more likely that fish and invertebrates succumb to oxygen deprivation during these gamete-releasing events. The entire content of large masses of algae is concentrated in millions of short-lived gametes, putting an incredible oxygen demand on the system. Under these circumstances, immediate partial water exchanges combined with increased aeration and protein skimming are called for. If an ORP meter is used, the severity of the situation can be assessed more easily, and oxidizers such as ozone and potassium permanganate can be used. This is a worst-case scenario, however, and most sexual reproduction occurs without the destruction of the entire system, or any ill effects at all. It has been suggested that a lack of pruning and a deficiency of iron lead to sporulation (Tullock 1997). Personal observations indicate regular pruning to be a deterrent to sporulation, but the lack of an essential nutrient such as iron triggering sexual reproduction seems questionable. After sporulation, the remaining Halimeda can be left in the aquarium, as they are essentially nothing but aragonite and can become part of the natural substrate. If you choose to remove a dead clump of Halimeda, be certain that it is actually dead. The chloroplast migration mentioned above leaves the plant noticeably pale at night and can mimic the look of a spent plant following gamete release. It is also important to note that white tips on the terminal segments indicate new growth and are not harbingers of death or reproduction.

Halimeda's resistance to grazing and their tendency not to overrun and damage sessile invertebrates make them a far better choice as a decorative and functional alga than Caulerpa. Factor in their nutrient export capabilities and service as an alkalinity test and you have the perfect marine plant for the home aquarium!

Print References and Further Reading:

Adey, W.H. & K. Loveland. 1998. Dynamic Aquaria 2^nd ed. Academic Press. San Diego, CA.

Drew, E.A. 1983. Halimeda biomass, growth rates, and sediment generation on reefs in the Great Barrier Reef Province. Coral Reefs 2:101-110.

Drew, E.A. & K.M. Abel. 1986. Sediment generation by Halimeda meadows throughout the northern Great Barrier Reef Province. Recent Sediments of Australia, edit E Frankel, J B Keene & A E Waltho, Geol Soc Aust Symp, 13, 53-67.

Drew, E.A. & K.M. Abel. 1990. Studies on Halimeda III, A daily cycle of chloroplast migration within segments. Botanica Marina, 33, 31-45.

Littler, D.S., M.M. Littler, K.E. Bucher, & J.N. Norris. 1989. Marine Plants of the Caribbean. Smithsonian Institution Press. Washington, DC.

Littler, D.S., M.M. Littler. 2000. Caribbean Reef Plants. OffShore Graphics, Inc. Washington, DC.

Littler, D.S., M.M. Littler. 2003. South Pacific Reef Plants. OffShore Graphics, Inc. Washington, DC.

Lobban, C.S. & M.J. Wynne ed. 1981. The Biology of Seaweeds. University of California Press. Berkeley, CA.

Paul, V.J. & K.L. vanAlstyne. 1988. Chemical defense and chemical variation in some tropical Pacific species of Halimeda. Coral Reefs 6:263-269

Tullock, J.H. 1997. Natural Reef Aquariums. Microcosm Ltd. Shelburne, VT.