The Bryopsidales (also known as the Caulerpales or Siphonales)1 are a fascinating order of green macroalgae found in oceans across the globe. They are well known to marine aquarists since the group includes several genera that thrive in the aquarium environment. Some of the most common ornamentals such as Bryopsis, Derbesia, Caulerpa, Codium, Penicillus and Halimeda all belong to this order. Why is it that although only a small proportion of the world's macroalgal species seem to take readily to home aquarium culture, this particular group is represented so well ? Presumably, members of the Bryopsidales possess adaptations that promote survival in captivity. Many occur naturally in shallow, coastal waters where they have developed tolerance to elevated nutrient levels and fluctuations in temperature, salinity, and light. The Bryopsidales' rapid, flexible growth, and ability to propagate vegetatively from small fragments probably also contribute to their aquarium success.
At The Ohio State University we use Instant Ocean Reef Crystals™ and Sea Garden™ nutrient supplement to maintain long-term cultures of several species of Bryopsidalean algae. These are used for (among other things) my own PhD dissertation research, which considers the unique cell biology of these algae. I suspect that the growth characteristics that make this group successful in the aquarium are linked to adaptations at the cellular level.
Typically, the Bryopsidales are filamentous and consist of a mass of continuous cytoplasm held inside a hollow, cylindrical cell wall. There are usually no cross-walls and therefore no physical separation between nuclei or other internal structures. In some genera, such as Halimeda, the filaments are highly branched and packed into a dense mass called a thallus.
In Caulerpa the filaments are massively enlarged, strengthened with internal support struts, and shaped into organs resembling the stems, leaves and roots of land plants. Unlike land plants, however, these organs are not made up of many separate cells packed together. Instead they are filled with uninterrupted cytoplasm held in place by a single, continuous cell wall.
In essence, any piece of Bryopsidalean algae could be considered a single, enormous, multinucleate cell. Such a cell is said to be coenocytic. This condition appears elsewhere among living things, but never so strikingly. Caulerpa paspaloides, for example, can form a network of rhizomes filled with a mass of cytoplasm 0.25 cm in diameter and several meters long. In some species of Halimeda there is evidence that whole meadows of natural growth may consist of a single individual connected by fine filaments running through the substrate.2 This suggests that some of the largest and most morphologically complex cells of the living world belong to this algal order.
The cytoplasm of these giant cells is highly mobile. Nuclei, chloroplasts and other organelles are free to move throughout the organism. This phenomenon is well known in Caulerpa, where streams of chloroplasts are visible to the naked eye as fine green lines just under the cell wall.3 The internal transportation of materials is facilitated by a network of fibrous proteins that act as "highways". These carry a continuous traffic of cell organelles and packages of raw materials from wherever they are abundant to wherever they are needed most.
It is my contention that the coenocytic structure and extensive internal transportation promote survival in captivity by allowing a rapid, flexible response to the conditions of a particular aquarium. For example, photosynthetic organelles can be deployed wherever light falls on the organism most optimally. If an alga is tipped over or inverted, materials can be re-routed to allow new growth with the correct orientation relative to gravity. If a growing tip is damaged, unused raw materials can be diverted elsewhere. Even a small fragment of a damaged individual can quickly re-organize its internal structure so that photosynthesis and growth may resume with the minimum of delay.
By allowing rapid movement of materials and messenger substances throughout the alga, the coenocytic cell structure may also be the basis of some species' ability to grow as different morphological forms. For example, when a stock of Caulerpa racemosa is separated and cultured in tanks with different lighting, circulation or nutrient regimes, it is often observed that the two portions quickly develop different growth patterns. Presumably, these different "varieties" or "morphs" optimize shape and size in response to local conditions. Aquarists may even observe several different varieties of Caulerpa growing in different parts of the same tank, all vegetative descendants of a single piece of algae.
The coenocytic structure, streaming of cellular material and morphological plasticity of the Bryopsidales not only contribute to the group's success in marine aquaria, but also present a unique challenge to biologists. By studying an organism that seems to defy established cell theory, it may be possible to gain entirely new insights into the mechanics of all living cells, including our own.
References:
1) Hillis-Colinvaux, L.(1984) Systematics of the Siphonales. In "The Systematics of the Green Algae."Eds. Irvine, E.G., John,D.M. 271-296.
2) Hillis-Colinvaux, L.(1972) Reproduction in the Calcareous Green Algae of Coral Reefs. J. Mar. Bio. Assoc. of India 14, 328-334.
3) Jacobs, W. P. (1994) Caulerpa. Sci. Am. 271, 100-105
Slide captions:
1) Filamentous form of Codium fragile. Special microscopy allows us to see the nuclei as light blue and the chloroplasts as red. Note that the nuclei are evenly distributed, not packaged into separate cells. (250x)
2) Cross-section of Caulerpa prolifera rhizome.The
cell wall forms a hollow tube strengthened by finger-like internal struts.
The dark spots are cross sections through huge streams of chloroplasts
moving along the rhizome. (40x)
