By Adam Jenkins © 2009
Caulerpa are fast-growing green algae with fronds (“leaves”) that come in a variety of shapes. The fronds from to between 15-30 cm (6-12 inches) in length and are attached to long runners (“stems”) called rhizomes. Besides simply spreading outwards, Caulerpa can also propagate themselves vegetatively through sections of rhizome that break off the parent plant and become established elsewhere.
Caulerpa rhizomes are attached to the substrate by fine hair-like extensions known as rhizoids. As is the case with all algae, the similarities between rhizoids and the roots of true (vascular) plants such as seagrasses are misleading; rhizoids only anchor the algae in the sediment, and play no role in absorbing mineral nutrients. Caulerpa, like all algae, absorb the minerals they need from the water via their leaves.
Caulerpa are found in a variety of shallow water marine habitats. Substrate types vary from solid rock through to sand and mud, and Caulerpa can be found in both calm and rough water areas. Because of their adaptability and the ease with which they can propagate themselves vegetatively, Caulerpa can be a highly invasive species.
Caulerpa are members of the Chlorophyta, or green algae. More specifically, they are members of the order Bryopsidales, a group that includes some other familiar ornamental algae, including Halimeda and Penicillus. The full taxonomy of the genus is as follows:
The following species are regularly traded and do well under aquarium conditions.
Common names: Feather algae, Fern algae
Lighting: Low to moderate
Flow: Moderate to heavy
Frond description: Dark green segmented fronds
Notes: Grows short fronds in heavy flow ,larger ones in low flow conditions
Common names: Common Caulerpa, Blade algae
Lighting: Moderate to high
Frond description: Flat, blade like leaves with straight edges that taper to a point
Notes: Prefers sandy bottoms and heavy flow
Common names: Green Grape Algae, Grapeweed
Lighting: Low to moderate (different varieties have different preferences)
Flow: Moderate to high
Frond description: Small grape-like clusters; these can be round or disk-like, depending on the variety
Notes: Remains compact under high flow, low light conditions
Common names: Caulerpa, Feather Algae
Lighting: Low to high
Frond description: Flat, evenly spaced segmented frond
Notes: Tolerates a wide range of aquarium conditions
Advantages of Aquarium Use
Caulerpa may be aesthetically pleasing additions to the marine aquarium, but they can also have beneficial applications in terms of nutrient export and pH stabilization.
Caulerpa use nitrate and phosphate, and in doing so rob unwanted nuisance algae of the inorganic nutrients they need, keeping the growth of pest algae to a minimum. As the Caulerpa grows, the aquarist simply trims and removes any excess growth, permanently removing the sequestered nutrients from the aquariums system.
In fact Caulerpa benefit from weekly pruning, but be sure to leave enough to sustain the patch and continue useful nutrient removal. If the algae is being kept in a sump or refugium you can alternate your lighting periods between the display and the sump to minimize the pH drop associated with the night time period of your display, i.e., by having the lights come on over the sump when the lights are out over the display aquarium, and vice versa. Leaving the lights running 24-hours a day has the same effect, but reduces the risk of the algae moving into it sexual phase (of which more will be said shortly).
Caulerpa can also be used as a substitute for vascular plants when creating a lagoon-type setting. Caulerpa prolifera in particular has a similar appearance to seagrasses and turtle grasses, but is much easier to keep under aquarium settings because it requires less intense lighting, doesn’t need a deep substrate, and tolerates new tank conditions much better.
Some species of Caulerpa can also be kept as a food source for herbivorous fish. Because it grows quickly, surplus Caulerpa can be offered to things like surgeonfish, angelfish and sea urchins. However, when used this way fronds should be broken away from rhizomes and the rhizomes discarded; otherwise any uneaten rhizome fragments can become established in the display tank, potentially becoming a nuisance. Be careful when using Caulerpa as a food because some species can be toxic to grazers.
Although Caulerpa can be beneficial it has severe disadvantages. Caulerpa is a very fast grower that can quickly attain plague proportions. It can (and will) overgrow corals and slower growing macroalgae, quickly smothering them. This can also happen in the wild, which is why Caulerpa colonies that appear outside of their natural range are so alarming to marine biologists; without their natural predators to keep them in check, such colonies can smother native algae and corals.
If kept it in a display tank it is best to keep Caulerpa off of any live rock formations, and instead plant your Caulerpa on a sand bed where it can be easily pruned as required. Since any piece of rhizome left in the tank can grow into a whole new patch, trimming Caulerpa has to be done very carefully. Ideally, Caulerpa will be maintained in a sump or refugium away from the slower growing corals and other sessile animals housed in the main display.
When Caulerpa is maintained properly it spreads vegetatively (asexually) by sending out runners that develop new fronds and rhizoids. However, under certain conditions it will reproduce sexually. From the perspective of a marine fishkeeper, this is undesirable, because after reproducing all the algae dies back simultaneously, releasing all their nitrogen and phosphate back into the water column as they decay, ruining water quality. Particulate matter that is released as the algae decays reduces water clarity, turning the water milky white and clogging up mechanical filtration media.
[Video link: Follow this link to see Caulerpa going into its sexual phase]
Clearly, the conditions that trigger the sexual phase of the life cycle must be avoided. Of the three types discussed in this article, Caulerpa racemosa (Green Grape Algae) is the one most likely to “go sexual” under aquarium conditions, and it should be watched particularly carefully when grown in large quantities. But all Caulerpa have the potential to switch into their sexual stage of life, and to prevent this, regular pruning is important. If grown in a sump, illuminating the algae for 24 hours per day will also prevent the sexual phase from occurring.
How can you tell if your Caulerpa are about to being sexual reproduction? Sexually reproducing Caulerpa develop small thorn- or whisker-like appendages on their fronds known as gametangia. These structures are where the spores will be formed and eventually released into the water. Should you see these on your Caulerpa, prompt removal will be necessary.
Killer Alga: the aquarium/Mediterranean strain of Caulerpa taxifolia
The coldwater strain of C. taxifolia is a fast growing and invasive variety originally raised for use in the aquaria. Since accidentally being introduced into the Mediterranean sea in the early to mid 80s the alga quickly spread to waterways worldwide, smothering the indigenous plants, algae and sessile animals, reducing biodiversity, and eventually causing major biological and economic problems in the affected areas.
The strain of C. taxifolia that ultimately became known as the Mediterranean or aquarium strain was first noticed in saltwater aquariums at the Wilhelmina Zoo in Stuttgart, Germany. This strain was carefully bred by the the staff for its beauty, quick growth and ability to not only survive but thrive in relatively cool waters. For these reasons it was soon acquired by aquariums around the world, including the Oceanographic Museum in Monaco.
Two years after being acquired by the Oceanographic Museum a clump of C. taxifolia was found growing in the sea close to the museum. This clump was about a square yard in size, and had apparently got into the Mediterranean Sea via waste water from the museums. Within 5 years the square yard had become 2.5 acres, and by 2001 more than 50 square miles of the coast within a 120 mile stretch was infested with the alga.
Although the spread of C. taxifolia has become associated with improper disposal of aquarium water, most outbreaks elsewhere in the world are centered around port and mooring facilities. This has lead to the thought that C. taxifolia is usually spread by cuttings inadvertently carried by ships’ anchors and ballast water. In this way C. taxifolia has found its way into waterways around the world including recent discoveries off coasts of California and New South Wales.
The Mediterranean strain is morphologically very similar to the tropical strain having flat regularly spaced fronds that are opposite to their attachment at the midrib (as apposed to alternating). Physiological differences between typical C. taxifolia and the Mediterranean strain includes the latter ability to thrive at lower temperatures, down to 10 degrees C/50 degrees F. The Mediterranean strain of C. taxifolia also lacks the ability to reproduce sexually, producing only male gametes.
In terms of ecology, the Mediterranean strain of C. taxifolia has been found at depths down to 100 m, much deeper than the standard variety of C. taxifolia. It can also grow on almost any type of substrate, from deep rocky overhangs to polluted muddy harbors. Like other Caulerpa species, it is somewhat toxic, and consequently not eaten by most marine herbivores. This, combined with its ability to thrive in a wide variety of environments means that the Mediterranean strain of C. taxifolia can quickly become established in new habitats and grow into plague-like populations.
Little wonder then that the Mediterranean strain of C. taxifolia has been nominated as one of the world’s most invasive species!
Given its potential to cause problems, getting rid of Caulerpa trimmings can be problematic in regions where there is a risk of these algae becoming established in local waters.
In some areas Caulerpa have been banned from the fishkeeping trade for exactly this reason. There are nine banned species of Caulerpa in the State of California, including Caulerpa racemosa and Caulerpa mexicana, two ornamental species described elsewhere in this article. Other American states the ban the sale of Caulerpa include Alabama, Massachusetts, North Carolina, Oregon, South Carolina, and Vermont. Similar laws exist in states where Caulerpa has already become a nuisance or is believed to have the potential to do so, including New South Wales in Australia, France, and New Zealand. With its cooler climate, cold temperate countries like the UK are at less of a risk, but with global warming already having raised average sea temperatures slightly, Plantlife International considers Caulerpa a potential threat and believes they should be banned from sale.
The Southern California Caulerpa Action Team (SCCAT) recommend that clippings be placed inside a sealed plastic bag and frozen for 24 hours before being disposed of with the household waste.
Caulerpa are beautiful algae that are best kept in a sump or refugium where the aquarist use them to improve water quality without exposing organisms in the display tank to the risk over overgrowth. If kept in a display tank, constant vigilance is needed to keep their invasive tendencies in check.
Algae: A Problem Solvers Guide by Julian Sprung
Marine Aquarium Handbook by Robert Goldstein
A Warning Sign of Impending Caulerpa Sporulation by Gene Schwartz at reefkeeping.com
Caulerpa by John Cunningham