The dominant reef-building organisms of previous times were corals. Algae cells lived symbiotically with the coral, supplying oxygen and carbohydrates used in the production of calcium. With the recent glacial period, in 5 million AD, came massive climatic disruption. The seas filled with mud, depriving the algae of the sunlight they needed for survival. Without the algae and the essential nutrients they provided, the corals also became extinct some time later. Now, tens of millions of years later, there are large areas of warm, shallow water and conditions are right for reefs to develop once more. This time, the reefs are formed not from coral, but from a prolific species of red algae.
Red algae, dating back to the Ectasian period in the Mesoproterozoic era, 1.2 billion BC, were not always prolific. Their reproductive cells cannot swim, so the algae's fertilization technique was a matter of releasing millions of gametes into the water and relying on favorable currents to carry them to the female organs. Now, 100 million AD, the red algae have evolved a sophisticated, symbiotic fertilization process by teaming up with a reef-dwelling animal, like a young reef glider. The algae offers up a protein meal and, as the animal moves from red algae to red algae feeding on this meal, it transports the algae's sticky gametes. This new species of red algae produces fewer gametes and yet enjoys a much better fertilization rate. With such reproductive efficiency, the red algae have built up a successful reef system.
Shafts of sunlight will slant through the clear water of the Shallow Seas, penetrating as far as the jagged spikes of the algal reefs which form much of the seabed. Here and there, cuplike shapes sprout from the red surface of the reef. Swimming animals dart among the reefs and algal flowers. In the base of the cups, the algae produce deposits of protein and carbohydrates that lure in vector animals.
This strategy of recruiting other marine life to help out in the reproductive process has evolved over millions of years. An evolution took its course in the fertile habitat of the Shallow Seas, red algae split into two broad types. Those algae which released gametes into the open sea placed their survival at the mercy of the currents and fate. Meanwhile, algae that began to rely on unwitting feeding animals to distribute their sticky reproductive cells were soon able to breed more efficiently. Eventually, the sticky-gamete trait became predominant, and algae that did not have it died out. Evolution does not always follow the most straightforward route, but it always rewards reproductive efficiency, as it has in the case of red algae.
The reef-forming species of red algae went through a long period of evolutionary experimentation to perfect its reproductive strategy. Different kinds of swimming animals were used as fertilizing carriers, known as vectors, and the algae developed different forms of marine "nectar" to act as bait for them. Finally, the flowerlike structures were developed to tempt and feed the vector animals, and reproductive cells were produced within these. The female part of the "flower" became more sophisticated too, evolving parts to remove the gametes from a visiting animal. Over time, juvenile reef gliders became the ultimate vector animal, and have been for millions of years.
Above the waterline, an ocean phantom's exposed surface is covered by red algae. Like the reef algae, these organisms build structures from calcium, forming small trunks upon which the algal strands cling, streaming in the wind. The algae are a kind of farm, providing much of the nutrition for the siphonophore colony. Carbohydrates generated by the algae through photosynthesis are carried throughout the colony by a vascular system which takes food to every individual member. In return, the red algae are provided with a safe base and even a supply of fresh water. When rainfall does not provide this, specialist individuals in the ocean phantom colony can desalinate seawater and pump it to the upper surface. Algae-generated sugars and starches do not provide all the nutrition the ocean phantom colony needs. For protein it must hunt other animals in the waters beneath. Nitrates from consumed prey are partly delivered to the algal meadow on the exposed surface of the ocean phantom, providing additional nutrients for the algae.