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This large and sinister hydrozoan jellyfish can be seen slowly gliding over a red alga reef. It casts a long shadow which creeps across the irregular surface of the reef.
The looming shape of the creature has a peculiar translucent quality when seen from below. It trails a forest of streamer-like organs and tentacles that brush across the surface of the reef, probing into crevices, questing around the red cups of the algae. The creeping darkness overhead will throw smaller swimmers (like young reef gliders that cruise and gyrate among the red algae) into confusion.
The ocean phantom is a floating menace. It is no different from its relatives - an order of communal jellyfish, delicate and transparent in appearance but predatory in behavior. The most well-known of these was the Portuguese man o' war of the Quaternary, which the ocean phantom is descended from. Each siphonophore specimen, although appearing to be a single organism, is actually a colony composed of many individuals (one provides a floating chamber while others adapt as feeding organs, stringing organs or reproductive organs).
Now, 100 million AD, the Portuguese man o' war's descendants have grown to a huge size. A typical ocean phantom is over 30 feet (10 meters) long, 13 feet (4 meters) wide and consists of many thousands of individuals. The largest part of it is the float. This looks like a giant mattress, made up of an assemblage of small air sacs. On its surface are a number of sails, turning and catching the wind to drive the animal along. These sails are gas-filled, but their walls contain a network of tubes which can be filled with water to control their shape. By filling different tubes, the ocean phantom can turn the sail to face any direction, catching the wind from whichever quarter it comes. When the water is withdrawn, the sails collapse.
The ocean phantom does not just drift downwind. When necessary it can tack like a yacht, moving its bulk against the wind. The force of the wind is counterbalanced by the pressure of the water against the submerged "keels". There are also individuals in the colony that produce water jets, driving the phantom along when winds and currents are insufficient to do so. Other individuals act as rudders.
This sophisticated setup carries the ocean phantom from one reef to another on its quest for food supplies. It can sense wind direction and bottom depth, and so avoids dangerous shallows, beaches and exposed rocks where it might become trapped or damaged. When not feeding or avoiding hazards, the phantom simply drifts along with its sails collapsed and its underwater appendages retracted. It traverses the Shallow Seas like an enormous piece of lifeless flotsam. Only when it is hungry, or when it drifts towards the reefs, does the whole colony spring into action.
A complex sensory array is also used to detect feeding grounds and potential danger. This system can analyze wind strength and the position of the sun, information which allows the animal to navigate. Other siphonophores possess a simple neural network, connecting individuals of the colony to one another. The sheer size of the ocean phantom and the degree of communication required for so many individuals to function in harmony mean that, in many ways, this mass of living matter resembles a giant, floating brain.
Above the waterline, the 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 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. This is why the ocean phantom presents such a threat to the reef-living creatures. When the dark shape drifts overhead, all except the younger and most inexperienced of the Shallow Sea swimmers make themselves scarce.
The ocean phantom uses suction bells to hunt. At the end of each hunting tentacle there is a spherical structure with a downward pointing mouth. In the tentacles above, muscle fibers operate like bellows, expanding and contracting to open the bells. Small water jets are used for more accurate, localized movement. Around the mouth of each bell, there is a ring of stalked eyes and feathery sensors. Such a structure allows each tentacles to function and hunt independently.
This apparatus is dragged across the surface of the reef until it detects prey of the right size. The bells then opens and closes like an umbrella, its pulsing action driving the whole unit into an attack position. With a sudden dilation, the prey is drawn into the bell and trapped, to be digested at the ocean phantom's leisure. The nutrients extracted are then pumped up the tentacle and distributed to the whole colony through its vascular system. Nitrates from the prey are also delivered to the algal meadow on the exposed surface of the ocean phantom, providing additional nutrition for the red algae.
Underpinning the ocean phantom's sophistication and the astounding specialization of its constituent parts is an inherent flexibility. The whole structure of the creature is modular. It can exist as a huge mass or as smaller units, provided each unit has the full range of individuals it needs to survive. After a heavy storm, a large ocean phantom may have broken into many smaller parts, each of which will eventually regenerate. The ocean phantom is not invulnerable, though. Drifting along in the open water, even this huge creature is open to attack.
Adult reef gliders which, as juveniles, were prey to the ocean phantom, are now its chief predator. The ocean phantom is at the top of an adult reef glider's menu. The large gastropod will bite off chunks from an ocean phantom from below. In the face of such an assault, the phantom has evolved a brilliant symbiotic defense mechanism.
Some of the suction bells have ceased to function as hunters. Instead, they are troop carriers. Should a reef glider be unlucky enough to brush against one of these modified bells, an army of spindletroopers emerges and comes to the rescue of its host.
Before forming their symbiotic relationship with the ocean phantoms, spindletroopers live on the algal reefs, occasionally raiding an ocean phantom's suction bells for food. Soon some begin to remain safely inside the bells for longer periods of time. Eventually those particular spindletroopers adapt to fold up and fit nearly inside a suction bell, which in turn modifies itself to house and feed the spindletrooper.
When hungry, an ocean phantom-guarding spindletrooper scratches at the walls of its home bell, stimulating it to regurgitate food from the rest of the colony. In return, the spindletrooper provides defense, coming out to fight when its ocean phantom colony is threatened. With their large chelicerae and long chelae, they slash at anything that attacks their ocean phantom home.