The reproductive cycle of marine algae is complex and varies greatly between red, brown and green algae. All marine macro algae produce what is known as gametes. A gamete is a cell that fuses with another gamete during fertilization (conception) in organisms that reproduce sexually. Gametes can be either male or female and are released by the development of reproductive spores on the plant. These are normally visible as a bump or node on the fronds in some species. They're known as cystocarps when formed on red algae and are very pronounced. The spores are then released and attach to a surface or substrate. Germination of the algae spores depends upon several conditions, such as temperature, but the germination is never delayed as in terrestrial plants when conditions are not favorable for reproduction.  

Reproduction helps assure each species' survival in the ocean. Note the absence of any reproductive spores on the damaged frond in this species of Halymenia floresia (pictured below). Many aquarists mistake this orange coloration often seen on this species for new growth, but its actually dying tissue. The drying out of cell tissue can induce the algae to produce and release reproductive spores and is used as a means of producing algae seed in the mass culture of commercial seaweeds. 

When marine plants release reproductive spores, or gametes, the result is that the plant dies. This is the cycle that often occurs with species of Caulerpa in the aquarium when conditions are favorable for reproduction. When Caulerpa begin the process, they often turn pale white or yellow with little white spots appearing on the fronds or blades. A white milky substance is then released into the aquarium water. This secretion is composed of the gametes and the cell tissue of the algae. Clear tissue in most species is usually not associated with reproduction, but rather a sign of nutrient deficiency.

What causes reproduction in marine algae?

Marine macro algae reproduce when their environment changes, or when their "biological clock" say it's time. Some of the conditions known to induce a sexual event in nature are: spring tides, lunar phases, temperature and photo period. Some species of algae are even known to release pheromones or organic chemicals to attract male gametes. Several species of marine algae however, are not affected by photo period alone and can reproduce in both light or darkness. In the aquarium the conditions are artificially maintained, so care must be taken to limit any sudden change in water quality that can induce a sexual event. Species of Caulerpa are known to not release reproductive spores if kept under constant illumination. While this may be true, its growth is typically stunted and the algae cease from productively removing nutrients as the process of photosynthesis is slowed. This is known as the 24/7 lighting cycle and is typically employed by aquarists culturing Caulerpa in the refugium. 

Controlling growth

Germinated gametes or spores attach themselves to a surface such as rocks or substrate and then begin their growth. Most live rock, imported or aqua cultured, are covered in both fertile and unfertile algae spores. This is why, after several weeks or months of favorable conditions in the aquarium, up sprouts a new plant, seemingly out of nowhere. It also explains why undesirable algae such as micro algae and invasive turf algae can quickly appear and take over the live rock and eventually an entire system. The proper cycling of live rock is crucial to remove many of the undesirable algae that may reside upon the surface of the rock.

Control of both micro and macro algae growth is usually accomplished by limiting excess nutrients in the aquarium by using the correct spectrum bulbs and maintaining the temperature of a system. The turbidity of the aquarium environment will also affect growth in marine macro algae. Species that grow in protected habitats will not grow at the same rate or share the same characteristics if introduced to strong currents in the aquarium. This is evident with many species of Caulerpa, especially C. mexicana. It will remain short and compact in strong current, while growing tall and wide in a sheltered environment. Overall however, marine micro and macro algae are very resilient and will adapt to almost any artificial conditions including current, lighting, temp, salinity and ph. Another limiting factor of growth in marine algae in the aquarium is the competition for nutrients and sunlight between species. When conditions are favorable for rapid growth, eventually one species will try to dominate the landscape and "choke out" other species or colonies. This can be avoided by diligently pruning the aqua scape so that no colony intrudes or outgrows its biomass or given space.

Reproduction in sea grasses

The marine flowering plants (angiosperms) reproduce by both sexual or asexual (vegetative) methods. In sexual reproduction, the plants produce flowers and transfer pollen from the male flower to the ovary of the female flower. Most sea grass species produce flowers of a single sex on each individual, so there are separate male and female plants. Some species only reproduce through vegetative means. Vegetative reproduction is a type of asexual reproduction for plants in which new plants are formed without the production of seeds or spores. In asexual reproduction, the new plants are formed by the rhizome which serves as an organ of vegetative reproduction. Shown to the left is a picture of a newly formed flower on a species of Thalassia. Flowering plants are rarely seen in the aquarium and are unlikely to reproduce by seed in captivity as conditions are not favorable. Most of the occurrences in the aquarium are from sea grasses collected just before the flowering stage. 

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