Miniwhorls

In the vast blue depths of LadyM and Jujubee, the niche of mesofaunal zooplankton seemed surprisingly empty, the only organisms filling it being some smaller species of Miniswarmer and Krillpede; this statement can only be true for so long, as the transient fauna of Sagan 4 will eventually fill these empty niches. An errant population of Marine Whorl from the crowded Krakow Polar Coast followed an upwelling of nutrients and microbial plankton circulating to the east. From there, the descendants of this population spread out further and reached more of the planet’s oceans, attracted by the abundant nutrients; as the populations continued to spread, they diverged and speciated as their numbers increased throughout many environments. In only a little more than a million-years' time, from that ancestral stock came a vast genus of planktonic fauna - species numbering in the hundreds and can be found in practically all marine biomes above the midnight zone.

The typical Miniwhorl possesses a four-point radial symmetry. On the base of a Miniwhorl is a wide orifice which leads to an extensive internal cavity which handles gas exchange. This cavity radiates into a fine network of canals where oxygen is diffused into the whorl’s cells; carbon dioxide is released into these same canals and is expelled through tubes on the underside of the whorl’s arms. Due to this, Miniwhorls completely lack blood.

Miniwhorls acquire food using feathery tendrils trailing from their arms. Food captured by the tendrils’ fine bristles is digested externally, and the remaining slurry of foodstuffs is transported inside them through many small pores before being circulated throughout a primitive intestinal network, where nutrients will be absorbed directly by the whorl’s cells. Food waste will be sent down the intestinal network to a ring of pores under the base, so as not to contaminate the feeding tendrils.

As free-swimming plankton, Miniwhorls locomote by vigorously beating their tails with rapid alternating contractions of simple muscles, which often appears to be a corkscrew-like motion to outside observers. The tail is located on the center of the underside of the base to ensure efficient maneuverability and was the ancestral condition of the greater whorl clade. On occasion, a whorl may fold its radial arms inwards, reducing drag - and with a strong motion of its tail, can achieve a momentary burst of speed to escape some precarious situations, though this strategy costs energy.

Miniwhorls lack a truly centralized nervous system; signals are instead communicated throughout the body via a nerve net. Sensory organs are similarly simplistic: the apical “thorns” of their ancestor - as they lacked any hard, mineralized reinforcement - were not efficient at defending from enemy attack, thus they became chemoreceptive setae. These mainly serve to relay information on where the nearest concentrations of prey are, and the information processed by its crude nerves propel it to its meal.

A major factor in how the Miniwhorl genus achieved such diversity is the innovation of sexual reproduction. In conditions of notable abundance, mature Miniwhorls start producing copious quantities of gametes to be expelled into the water; the gonads are located near the anal pores. Usually, female whorls release many thousands of oocytes into the water, then the males bathe the oocytes in clouds of their own sperm - each spermatozoon atavistically resembling the Ur-Mancerxian, Protomancerxia Binarflagella. The oocytes and sperm conjugate, mixing genes from each parent, and developing into eggs; with a seemingly almost-infinite possibility of any combination of parent genes, greater genetic variation within the genus is ensured, compared to the exclusivity of chance mutations in the ancestral stock. Within a timeframe approximate to a week (Sagan 4 does not have days strictly as long as those on Earth), the eggs hatch into tiny larvae tangentially resembling the Miniwhorls’ ancient ancestor - the Radia Primus - but with a tail. Recent larvae are typically around 500μm long on average and few will survive long enough to grow and develop into adulthood - typical for an r-strategist. It usually takes little under a Terran month for a larva to develop into a mature Miniwhorl.

Due to the massive diversity of the Miniwhorl genus, individual species can often be hard to distinguish from morphology and behavior alone, although trends shared among subgenera can be less difficult to observe. Species with ranges in the tropics are migratory, following nutrient-rich currents into and eventually out of the otherwise desert-like equatorial oceans. Likewise, there is a greater richness of species in temperate and polar waters, thanks to the consistent abundance of nutrients and oxygenated water there. Larger species are typically found in the lower photic layers, but never deeper than the twilight zone; while they mainly gorge on marine snow particles, on many nights they rise to waters closer to the surface to stuff themselves on the abundant plankton of the sunlight zone, as most of their would-be predators are inactive. Due to their unsophisticated bodyplans, certain morphological features like the number of feeding tendrils on each arm can easily reduplicate among different species of Miniwhorl; larger species such as R. vexilloaster (not pictured) typically have more feeding tendrils - up to four on each arm.

Certain species such as R. stylomorpha (top left) and R. conicus (top right) are bullet-shaped and have comparatively longer tails to facilitate a speedier morphology and active behavior, although the majority of species have wide arms and are apically-flattened to suit a more placid nature, like the 1.6 centimeter-wide R. plumosus. (bottom left) Given a couple million years of genetic drift, these subgeneral morphs could diverge into distinct genera of their own.