Stercor
The Stercor (Initiasterus fluminis) is a radially symmetric organism found in the sediment of zone 15. This tiny (~500 ÎŒm armspan) generalist crawls along searching for anything edible, tasting the water with chemoreceptors on the end of each arm and âglidingâ in the direction of food using cilia along its underside. Its small size allows it to find sustenance in things larger organisms cannot, such as microscopic cells and the tiniest particles of detritus.
The Stercor reproduces very quickly both sexually and asexually, allowing it to quickly populate any body of water, even isolated ponds, as long as thereâs food and at least one individual has made its way there. Its rapid reproduction also means that it adapts to its environment very quickly, allowing it to thrive everywhere in the watershed, from the frigid northern tributaries in winter (typically deeper underwater it doesn't freeze) to the warmer brackish regions to the east where the river flows out into the Pulchran Ocean.
Anatomy
The Stercor, despite its simple appearance, has 3 germ cell layers--an endoderm (internal, consisting of the digestive system), a mesoderm (lying in between, containing the muscles and nervous system), and an ectoderm (the outer layer including the skin and cilia). It is completely soft-bodied. Each arm contains its own abdominal cavity and a hydrostatic skeleton.
The Stercor has 6 arms, each one consisting of 8 segments, though mutants with more or fewer arms or segments are common. The terminating segment of each arm contains chemoreceptive patches on the ventral side and a photoreceptive dot, or eyespot, upon the dorsal side which detects light intensity. The terminal segment of each arm also contains a mouth, and beating cilia draw any and all edible particles inwards.
The gut of the stercor is multidirectional. Every arm has a mouth, but depending on its direction of movement, any one of them--typically the one currently trailing behind--will temporarily be the anus. A standard 6-armed individual could be said to have 5 heads at any given time. As the organism is translucent, food particles may be visible in its gut when one looks closely. All food passes through the center of the organism at some point, where the most enzymes are dispensed, but digestion and absorption of nutrients occurs throughout the entire gut. The gut lining is protected by mucus, particularly in the center where hydrochloric acid is produced which would eat through the organismâs tissues otherwise. Nutrients are carried throughout its body by clear hemolymph.
The Stercor has no true blood, but it does have blood vessels which pump nutrients through its body in the form of hemolymph. This does incidentally carry dissolved oxygen as well, but there is no blood pigment. It has a closed circulatory system and 6 hearts--one for each arm--in its center.
Paired major nerves run down each arm and connect ganglia at the end of each arm to a ring-shaped âbrainâ (really more like a coil of nerves) in the center. This allows sensory information gathered by the tips of the arms to be processed, so that the organism may coordinate its movement direction. It is not very intelligent.
Externally, the stercorâs upper surface is smooth, while the underside is covered in cilia. Its skin is permeable and made of living cells, allowing oxygen to freely diffuse into its body and hemolymph. Likewise, carbon dioxide and cellular waste readily diffuses out. The organism is unpigmented and, due to it also lacking a blood pigment, it appears transparent or white as a result, apart from the black eyespot on the end of each arm. It avoids intense light when it can, as with its small size and lack of pigment it has little defense against ultraviolet radiation.
Reproduction
The Stercor has two methods of reproduction: sexual and asexual. In asexual reproduction, the organism will fragment--a muscle spasm tears it asunder along lines of weakness, or fracture planes, which lie between each arm, separating each one into a new individual. Each fragment includes part of the central part, which includes the heart. New arms are able to regenerate fairly quickly as long as food is available, with little or no sign of scarring. When well-fed in perfect conditions, they can do this every 5 days, but as many as 15 days can pass between divisions.
Sexual reproduction occurs when environmental conditions become unfavorable, such as when resources are scarce. The stercor has many gonads which each produce a number of eggs or sperm. There are two gonads on each segment, which on most stercors means 16 gonads per arm and 96 gonads total. Each gonadâs opening is held closed by a sphincter when not in use so that foreign objects cannot enter and clog it up. Stercors can be either male or female, but are capable of changing sex based on pheromonal cues, such as if a male senses no female pheromones in his environment; this is fairly simple to do, as the only difference between a male and a female is the type of gamete produced. Eggs contain yolk and lack any shell, only having a cell membrane. The Stercor broadcast spawns, releasing millions of gametes into the water in hopes that some will combine with those of other Stercors of the opposite sex.
Development
Stercor embryos begin as balls of cells with a mouth on the underside. As previously mentioned, the Stercor has 6 arms each with 8 segments. During embryonic development, these are determined by production of a morphogen analogous to the sonic hedgehog protein found in Earth animals.
The number of arms is determined first, and over-production of the morphogen such as from a mutation can cause there to be fewer arms while under-production causes there to be more, due to pattern densities respectively resulting in fewer or more defined groups from which the embryo determines where to grow an arm. The arms grow from the underside around the âlipâ of the mouth, causing the embryo to briefly resemble a rose bud, before the mouth is split between each arm. Now bearing a mouth at the tip of each arm-nub, it then unfurls and the arms begin to grow their segments.
The number of segments per arm is determined here. The segments grow from the end of the arm, extending the gut with them, and the terminal segment and sensory organs develop last. This usually takes mere minutes, producing a fully developed but very tiny arm. The morphogenâs production serves more as a timer for segment development, as once it reaches a certain concentration the embryo will end off the arm with the terminal segment. Overproduction causes segment growth to terminate early, making fewer segments, while underproduction results in more segments due to it taking longer to reach the morphogen levels necessary to tell the embryo to produce the terminal segment, thus more segments are produced. Sometimes, the number of segments may be different on each arm in an individual created by asexual reproduction, due to environmental factors affecting morphogen production during regeneration. Once the arms have all of their segments, they enlarge to their adult proportions.
The morphogen affects many other parts of development, including but not limited to the size and shape of the eye spots, the number and distribution of chemoreceptive cells, and the distribution of enzyme- and mucus-producing glands in the gut. Morphogen mutations can also cause the organism to grow strangely, such as having an extremely long, oval-shaped center with arms all along its edge, though they rarely survive such mutations as so many signals coming from all those arms causes trouble for determining movement direction.
Stercors take about a day to be ready to hatch after fertilization, but will wait until they have settled somewhere sheltered in the sediment. In optimal conditions they reach full size in about 7 days, but may take as long as a month if conditions are poor.
Diet
The Stercor will eat anything that can fit into its mouths, which usually means microscopic particles. This includes tiny pieces of dead muscals and polyphs, microbes such as B. breviflagella, the sperm or milt of organisms such as R. deminutivus, the spores of organisms such as B. caulimuscos, and zoospores and broken-off fragments of decomposers such as D. occidenflumen.
The Stercor can sometimes consume its own gametes, eggs, or juveniles. It has no defense against this other than the redundancy of its own reproduction, however it does not usually consume its own spawn as its gonad openings are on its dorsal side and thus spawn is released upwards rather than directly onto its path through the sediment.
The Stercor is mostly eaten by M. nocivisquilla at the time of its appearance, but it will also often be incidentally consumed by detritivores such as M. diegoii, R. deminutivus, and M. lutumanguis, or by any organism having a drink which happens to disturb it from the sediment.
