Wrigglebob
The Wrigglebob arose from a population of Common Wriggler in the southern end of its species’ range that supplemented their diet of organic detritus and dead fauna with live prey. With many successive generations - and the lack of a proper burrowing predator niche - predatory traits were increasingly selected for, and thus the Wrigglebob split from its ancestral stock.
Physiology
The first of these adaptations is a set of mineralized tooth-like extensions of the ancestral Symbiozoan radula-jaw. These “teeth” are hook-like in structure, curving towards the mouth and ensuring that seized prey do not escape its grasp. The implements the “Wrigglebob” uses to initially capture prey are situated on long, muscular tentacles, which are extended by inflating them with the fluid of its hydrostatic skeleton. Another, parallel set of tentacles above the feeding tentacles serve as sensory organs - lined with a row of setae on each tentacle, they can detect nearby vibrations of prey passing by, and predators too. With suitable prey in its clutches, the Wrigglebob tears apart its meal with a ring of smaller teeth, sending bite-sized chunks of food down its esophagus. Finally, the digestive mesh of its ancestor’s larvae is retained in adulthood, serving to soften up bits of tougher prey such as the Lesser Knightworm by secreting a light concentration of enzymes before inevitably descending deep into the gut.
The second set of adaptations is a form better adapted for burrowing. When a Wrigglebob locates a suitable place to start burrowing into the substrate, it dives in head-segment-first and pushes away sediment with its bristle-lined paddle-like parapodia. Retaining the wriggling locomotion of its aptly-named ancestor, bending a segment of its body to one curve and the segment behind an opposite curve, alternating with each wriggle. Its cuticle has streamlined further to facilitate maneuverability in the substrate and reduce drag, losing the spines of its ancestor; its head segment especially has become a slight wedge-like shape to plow through sediment better as the rest of its body pushes it away. In between each cuticular segment are the parapodia. When it has finally situated in its burrow, the Wrigglebob peeks its head slightly above the substrate and untucks its capturing and sensory tentacles; should it detect a predator approaching, it retreats its head back into the substrate. If the Wrigglebob survives a disturbance from its burrow, it will swim away in a motion similar to its burrowing in search of a comfier location to situate in, anguilliform-like.
Another crucial innovation is its segmentation. While its ancestor only possessed four complete segments besides the head, a mutation of the hox gene somewhere in the Wrigglebob’s ancestry allows it to have up to sixteen repeating body segments, made possible by the relative simplicity of its anatomy. Each segment begins with a soft, muscular subsection bearing the parapodia, and ends with the subsection possessing the rigid cuticle armor. Hemolymph provides each segment with sufficient vector for gas exchange, supplying oxygen and ferrying out carbon dioxide via spiracles behind the armored portions of each segment. Due to the repetitive nature of the Wrigglebob’s segments, each segment also contains identical formations of the hydroskeleton, muscles, gut, and nerves, up until the far posterior segment, which contains the anus and gonads. The increased segmentation gives its gut greater leverage, as this means the Wrigglebob can now extract more nutrients out of its many meals. Besides that, the added segmentation gives its body decent length and facilitates locomotion, as both its burrowing and swimming habits benefit from a long, slender form.
Aside from its setae-lined antennae, the Wrigglebob has made a few other sensory developments. It retains the statocyst from its ancestry, helping it to maintain balance should it be thrown off-kilter due to environmental disturbances; the statocyst itself is located within the cephalon. Most notably however, is the development of rows of tiny, simple eyes on its cephalon - detecting light and focusing via an aragonite-based lens, and conveying the information picked up via a system of nerves. With its newfound optics, the Wrigglebob can react to approaching predators much sooner than the Common Wriggler could, even when these eyes can only detect differences in light and blurred shapes. Each row contains five of these ocelli, with a total of ten eyes in a single organism.
Distribution
The Wrigglebob can be found in great numbers in lower ocean depths, dipping as far down a depth as approximately -450m, well within the twilight zones of southern LadyM and Rhino. To make the most of the lower oxygen content in these depths, its gills have developed branching, feathery structures to filter more oxygen from the water to be ferried by the hemolymph, providing a decent surface area coverage. While maneuvering under the substrate, the gills are mostly retracted in a pair of lateral projections on the cephalon to minimize damage. Most of its body functions have also become better acclimated for the lower, cooler temperatures of the twilight depths, its metabolism now slightly less active than its ancestor. As a consequence of this, populations in the lower-to-mid depths of the Leopard Temperate Coast are more tolerant of the frigid winters of that ecosystem.
Reproduction
Reproduction is somewhat different from the Wrigglebob’s ancestry. As the Wrigglebob spends much of its adult life waiting for prey while partly buried in the substrate, the species has shifted to broadcast spawning. Upon the activation of certain hormonal signals, concentrations of Wrigglebobs convene to a single location to breed. First the females lay tiny, sticky eggs onto the substrate, then the males arrive to spread sperm over them. The eggs do not contrast well against the sediment, so they would remain fairly undetected by sighted predators. After a period of around a few weeks, the eggs hatch into planktonic larvae, beating their parapodia against the water column and gorging on a variety of microbes. Like the larvae of their ancestors, they feed by exuding a sticky, enzyme-covered mesh and retracting that mesh back inside them to digest their catch. Larger, more developed larvae would begin to transition to eating multicellular fauna such as Ciliastars from their earlier diet of simpler microbes as their mouthparts develop and harden, eventually feeding on prey still meatier like Filterbunnies as they approach a benthic adulthood.