Litorincolus flumensis
The Fully-Aquatic Crestworm (Oculolophus flumensis) is a descendant of Oculolophus maritimus that has become fully-aquatic. It was advantageous to stick to the water permanently as water provides safety from land-dwelling predators such as Blattealimax panivore and Muscle-Filled Needleworms (Iniectognathus acidonenum), although their wyvern predators have since gone extinct thanks to the Wyvern Plague (Ventricapillus hemolaimus). Fully-Aquatic Crestworm does not directly compete with its ancestor Oculolophus maritimus as it can swim further into the rivers but has a more difficult time going on land due to its tail fluke, which developed to provide a faster swimming speed.
Without any predators in the water itself, they no longer played a big part in controlling their population. Fully-Aquatic Crestworm populations fluctuate with the amount of food available in the water; those who got food before others were more likely to survive food shortages. They consume Aruraherba submergus from the water on a typical basis and will eat the stems of Aruraherba caesicoles, Aruraherba wetrilli, and Heavy Tuftweed (Aruraherba haberefortis) that drift in the water. They avoid competition with Ciliaurrgs (Primoplanus lavancilia) and Floating Ciliaurrgs (Primoplanus lavantundus) by feeding on Aruraherba species rather than on Carpefloat (Stratunato virenparvus) or Ovalgae (Primophytorus ovalus).
The tail was not the only thing that adapted an aquatic lifestyle. It has also developed a pair of flaps of thin skin on the sides of its head behind the echolocation crest. These flaps are made up of only a layer of skin and veins and they help to collect and diffuse oxygen from the water. The flaps are held up and are able to function in the water because the water reduces their weight, but the flaps collapse if on land because lack muscle.
Fully-Aquatic Crestworms still lower their heartbeat compared to their land-dwelling ancestors to conserve oxygen but they will speed it up if they are after food. If the water is stagnant or there isn't enough oxygen, they will go near the surface and expose their backs to respire cutaneous with a more direct air to water interface.
Fully-Aquatic Crestworm altered their echolocate because they no longer go on land, they instead just let the water go into the echolocation system and adjust to it by accounting for how sound waves travel faster in freshwater than they do in air. This enables them to detect objects in the water clearly from up to a meter (1 m) away. They usually beat their crests once (1) every two seconds (2 sec) if cruising or conserving oxygen, but speed it up to once (1) every half a second (1/2 sec) if they are moving fast to keep alert. Their top swimming speed is five miles per hour (5 mph).
They also lessen the extent the crest goes up to a quarter (1/4) the way up instead of half (1/2) the way up while they actively swim, so the crest creates less water drag from the front. This does reduce how far away they can "echolocate" objects, bringing down their clear detection to half a meter (50 cm) away since the waves emitted are weaker. If they are in an area where they can't find their food then they will heighten the crest the normal height so they can judge where to go to find food from further away.
Their crest a quarter (1/4) up makes roughly a twenty-three degree (22.5°) angle while the crest halfway (1/2) up makes a forty-five degree (45°) angle.
A Fully-Aquatic Crestworm grows to thirty centimeters (30 cm) in length at maturity seven weeks (7 wks) after hatching. Eggs are laid on the bottom of rivers and other sources of freshwater so that the offspring are not stuck on land and can swim from birth. The eggs are translucent and are not covered with a hard shell. This allows the eggs to survive in the water by diffusing oxygen to the embryo. Adults attempt to reproduce each time they come across each other through internal fertilization. The female will only lay her eggs after she has mated. Each female lays twenty-five (25) eggs.