The term “marine plants” is often used loosely in the aquarium trade to describe both macro algae and sea grass. Macro algae, unlike sea grass are not true plants but are actually large celled algae. They absorb nutrients through their cell structure instead of the root system as terrestrial plants do. All macro algae are photosynthetic and thus rely on the sun's energy for food. Marine plants serve as the base of our ocean's food chain, providing food, oxygen and habitat for thousands of marine inhabitants. Unlike marine plants (sea grass), macro algae lack conductive tissue, true roots, stems and flowers. Instead of roots they have holdfasts or rhizomes in which individual runners support the growth between blades or stalks depending on the species.

Caulerpa TaxifoliaMarine plants grown in the aquarium will thrive under the right conditions and offer a replenishing food source to tank inhabitants.  In addition to converting sunlight to energy they also require nutrients to grow. It is generally accepted that all marine plants require the same essential nutrients as aquatic and terrestrial plants do. The major nutrients required by marine plants are nitrogen, phosphorus, potassium, sulfur, calcium & magnesium. The minor nutrients are iron, manganese, copper, zinc & molybdenum.       

Marine plants are distributed worldwide and are found in some form in almost every ocean on the Earth. There are two distinct habitats where macro algae are found, temperate or cold water algae, and tropical varieties. The tropical and sub tropical species are of most interest to hobbyists to culture, as the cold water species have special needs requiring a climate controlled environment (chiller). Throughout the world, marine algae grow at varying depths and environments so some are more suitable for culture in the aquarium than others. Several species can even be successfully kept with lower light as they are found and collected from deep water habitats, while others need a brightly illuminated habitat to thrive.

There has been much debate among marine aquarists over which individual species are best suited for culture in a refugium or display tank and which should be avoided all together. Some species are fast growing, invasive and can be a nuisance in any aquarium, especially reef tanks. 

Photo by Chau HoDespite their invasive qualities, there are many beautiful species of macro algae that are unlikely to be introduced to non native regions but are easy to grow in a marine aquarium. For the aquarist interested in the culture of macro algae there are hundreds to choose from, but only a few tropical species that are really suitable for aquarium use. It is these species that will be covered in detail. 

The three main classifications of marine macro algae are Chlorophyta (green algae), Phaeophyta (brown algae), and Rhodophyta (red algae). They derive their names from the dominant pigments associated with each species. All of these classes also contain slightly calcified or heavily calcified algae, which depend upon properly maintained calcium levels to grow and reproduce. Sea grasses have their own classification (Magnoliophytae or Angiospermae) and will be covered separately. Mangrove plants are also discussed, as they are often used by hobbyists in the marine aquarium. Although their growth is slow and overall nutrient uptake limited, they remain popular additions, especially in a refugium.

Photo by Chau HoBeginning in the early 1980’s a strain of Caulerpa known as Taxifolia was used as a tank decoration in several marine aquariums in Germany and was then successfully propagated and distributed for use in commercial aquariums across the globe. This extremely hardy and more temperature tolerant strain was soon discovered to have escaped into the waters of the Mediterranean and was found growing in large patches near shore. This highly invasive species has since been discovered worldwide, most notably off the coast of California, USA and New South Wales, Australia. The introduction of the hybrid Taxifolia is blamed on the ornamental aquarium industry, reportably being released only feet from the famous Monaco Aquarium where it was being used as tank decorations. Because of its invasive qualities and toxicity to herbivorous fish, Caulerpa Taxifolia is now banned for sale or distribution in several U.S. states as well as many countries. Needless to say, this particular type of macro algae should be avoided and its popularity and availability has decreased in the United States considerably in the last decade.  

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The Refugium