Pelagys Anatomy

Meet the Pelagys (Quadrognathia). A group of marine species that act as the dominant alien species on Alladoras. 

At first glance, pelagys appear quite similar to fish. This is a sound conclusion, as the bauplan of fish may have convergently evolved multiple times in the history of the universe in different planets, but pelagys are far from fish, in fact, many of these similarities are superficial and are not truly alike the fish of Earth. 

This variegated grove prober (Cuspirhynchus variegatus) is a textbook pelagys, generalised worm eaters who dig in the sediment for worm-like prey. It possesses a four-part rostrum, or "beak" which are officially termed mandibular rostra, but for simplicity we will be calling them mandibles, despite bearing no similarity to Terran mandibulate arthropods (crustaceans and insects). It opens up to show a complex system of teeth which can retract and extend on will for portability, and a pharyngeal jaw in the case of the grove prober in order to swallow up prey quickly. The anterior part of the pelagys, called a cephalon consists of the skull and gill basket, which protects the sac in which pelagys actively pump in water to respirate utilising dissolved oxygen. This kind of active respiration is sometimes replaced with a more passive and less energy-costly ram-jet ventilation in certain species, using an intake spiracle to suck in water while swimming. This pulmonary sac, which is no way connected to the digestive tract will pump oxygen into the circulatory system. Pelagys possess dorsal and ventral fins placed on the dorsal and ventral sides of the limb girdle, yet two pectoral fins are positioned on the lateral portion of the limb girdle. In the grove prober, these flap up and down to create forward thrust in a method known as paired-fin swimming, which hinders speed, yet is highly affective at steering and controlling pitch and roll. The ophistoma is what is considered the tail, as most of the organs are close to the cephalon. This ophistoma is used in certain pelagic species for thrust utilising the tail, but in the grove prober, it is used in additional thrust at high speeds as the posterior caudal rhachis, analogous to the caudal fin in Terran fish, is used fully extended to help add more thrust, and in certain species which maximised on median/paired fin swimming, they have lost the ability to swim using the ophistoma and use the caudal rhachis for display and signalling.

In their closed circulatory system, pelagys use hemocyanin to transport oxygen to important organs. Some organ analogous to a heart, placed somewhere near the pulmonary sacs (not illustrated here due to forgetfulness- I mean technical issues) helps to pump blood towards the lungs, and a sac helps to pump it elsewhere throughout the body. The blue blood that flows through their body when oxygenated is full of hemocyanin. Since hemocyanin is not stored in a blood cell as hemoglobin is, it simply floats freely through the blood, which is the reason why a pelagys heart pumps at record-breaking rates to maximise circulation of oxygen.  Hemocyanin-based blood operates much better at deeper and/or colder waters, which is why they experience more success in such regions, however, in warmer, shallower water, they simply have to make do with a faster and more efficient heartbeat.

 Their nervous system is not nearly as complex, however, they rely on a brain and two nerve ganglia which help control the movements of specific muscles in the pectoral fins. The nervous system is also responsible for control of the olfactory, gustatory (taste), optical and somatosensory systems (touch). Their most noteworthy sensory system are mechanoreceptors lining the 'nose' above the 4-sided rostra, which are used to detect changes in pressure in the surrounding water, acting as some sort of regionalised 'lateral line' seen in fishes. In certain species, this mechanoreceptor nose is extended to maximise sense around the water, especially in species living in the bathypelagic or benthic zones. Their chemoreception (olfaction) is quite keen, with small tentacles in the mouth extending at irregular intervals to locate chemical signals in the water to determine the location of the source. Predatory species may have feathery olfactory tentacles to maximise surface area to detect prey and/or carrion. Their most keenest sense by far is their optical sense, which has phenomenal light-detection and location in dark areas, despite being colourblind during the day, with many species (predatory ones especially) having a plate of iridescent tissue behind the retina to reflect light out for it to be perceived twice by the eye, similar to a tapetum lucidum. 

Unlike Terran fish, which possess a through-gut (in which food is excreted at another end of the digestive tract), Pelagys possess a blind-gut. The ancestral condition is that they lack any form of an anus, but they have developed a structure analogous to it:  a separate tract that connects back to the mouth, underneath the throat; essentially evolving a through-gut but in a different manner.

Their reproductive systems are quite curious. Located at the very tips of their tails, their intromittent organ is developed for internal reproduction, with the male possessing a thin phallic organ that is designed to be coiled up inside the tail-tip, with testes full of sperm running down the sides of their tails. The females lack the phallic organ and instead have larger tail-tips designed to carry eggs, which are carried in the ovaries. Due to the limited space provided, no pelagys is viviparous: the closest equivalent to that are species that mouth-brood. Most species release a multitude of small, planktonic eggs — r selective species — that develop into larvae. Other species may produce large eggs that they carry in their mouth or create a nest — this is present in many K-selective species — and produce developed young.

While many of these physical features are unorthodox, they have served the pelagys well enough to survive into the modern day, and have earned them the title of one of the most successful marine aliens on the planet.