A Closer Look at Caulerpa: Common Aquarium Species and Their Care

Author: Adam Jenkins

Used responsibly, various macroalgae species of the genus Caulerpa can be ornamental additions to a display tank or help control the nutrient load of a large reef tank.

Caulerpa Basics

Caulerpa are fast-growing green algae with fronds (leaf-like structures) that come in a variety of shapes. The fronds are between 6 to 12 inches in length and are attached to long runners (stem-like structures) called rhizomes. Besides simply spreading outward, Caulerpa can also propagate itself 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 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.

Species Roster

The following species are regularly traded and do well under aquarium conditions.

Caulerpa mexicana

Common Names: Feather algae, fern algae

Lighting: Low to moderate

Flow: Moderate to heavy

Fronds: Dark-green, segmented

Notes: Grows short fronds in heavy flow, larger ones in low-flow conditions

Caulerpa prolifera

Common Names: Common Caulerpa, blade algae

Lighting: Moderate to high

Flow: High

Fronds: Flat, blade-like leaves with straight edges that taper to a point

Notes: Prefers sandy bottoms and heavy flow; a useful alternative to seagrasses in lagoon-style tanks

Caulerpa racemosa

Common Names: Green grape algae, grape weed

Lighting: Low to moderate (different varieties have different preferences)

Flow: Moderate to high

Fronds: Small grape-like clusters that can be round or disk-like, depending on the variety

Notes: Remains compact under high-flow, low-light conditions

Caulerpa taxifolia

Common Names: Caulerpa, feather algae

Lighting: Low to high

Flow: High

Fronds: Flat, evenly spaced segmented frond

Notes: Tolerates a wide range of aquarium conditions

Advantages in the Aquarium

Caulerpa may be aesthetically pleasing additions to the marine aquarium, and 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 increases in size, the aquarist simply trims and removes any excess growth, permanently removing the sequestered nutrients from the aquarium’s 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, alternating the lighting periods between the display and the sump (in other words, having the lights come on over the sump when the lights are out over the display aquarium and vice versa) will help minimize the pH drop associated with the lights-off period in the display tank. Leaving the lights running 24 hours a day has a similar effect and reduces the risk of the algae moving into the sexual phase (of which more will be said shortly).

C. racemosa and many of its congeners can rapidly overgrow and smother corals and slower-growing macroalgae. Caulerpa, however, can also be used as a substitute for vascular plants when creating a lagoon-type setting. C. 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.

Taxonomy

Division: Chlorophyta

Class: Bryopsidophyceae

Order: Bryopsidales

Family: Caulerpaceae

Some species of Caulerpa can also be kept as a food source for herbivorous fish, but be careful when doing so because some species can be toxic to grazers. Since it grows quickly, surplus Caulerpa can be offered to surgeonfish, angelfish, sea urchins, and other herbivores. However, unless your fish can eat every bit of it in one sitting, be sure to break the fronds from the rhizomes and discard the rhizomes. Otherwise, any uneaten rhizome fragments can become established in the display tank and potentially be a nuisance.

Disadvantages in the Aquarium

Overgrowth

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 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 should be maintained in a sump or refugium away from the slower-growing corals and other sessile animals housed in the main display. C. taxifolia is notoriously invasive, both in the wild and in aquaria.

Sexual Reproduction

When Caulerpa is maintained properly, it spreads vegetatively 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; after reproducing, all the algae die 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.

Clearly, the conditions that trigger the sexual phase of the life cycle must be avoided. Of the three types discussed in this article, green grape algae C. racemosa is the species 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 begin 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.

A Killer Alga

The coldwater strain of C. taxifolia is a fast-growing, invasive variety. Since being accidentally introduced into the Mediterranean Sea in the early to mid-1980s, 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.

Most C. taxifolia outbreaks subsequent to the original Mediterranean invasion are centered around port and mooring facilities. This has led 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 the 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 opposed to alternating). Physiological differences between typical C. taxifolia and the Mediterranean strain include the latter’s ability to thrive at lower temperatures, down to 50°F. The Mediterranean strain of C. taxifolia also lacks the ability to reproduce sexually, producing only male gametes.

Use (and Dispose of) Properly

To sum up the proper aquarium use of Caulerpa, when keeping the algae, never release any of it into wild waterways. Be careful to make sure no toxic species are fed to herbivorous fish. And watch the algae closely for signs of going sexual. When properly taken care of and responsibly disposed of, Caulerpa can be an excellent macroalgae to use for nutrient export in marine aquaria.

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