The Sea Hydra

Hydrozoans have long been a staple of microfaunal communities in both freshwater and saltwater ecosystems. Hydra were, in fact, one of the first few multicellular animals introduced to Apterra's lakes and rivers, helping pave the way for larger organisms that would arrive later. Today, the handful of species originally seeded on the planet have diversified into nearly a thousand, and they can be found in every imaginable source of water. From the deepest ocean trenches to the shortest-lived vernal pools, these cnidarian predators make their living anywhere and everywhere. Most are so small that they would go unnoticed by any but the most attentive human eye, but a handful have managed to expand out of their ancestral niche. In the plankton-rich waters of the central Medithalassic, a relative giant drifts on the current, its sticky tentacles gradually bringing food towards its mouth. This is the Sea Hydra (Epipelagiomedusa), a genus descended from the subalgal Tubulomedusa. 

The sea hydra no longer has a sessile polyp stage; instead, it spends its entire life as a free-floating medusa. Composed of over 90% water, the adult stage is neutrally buoyant and goes wherever the waves take it. It is not, however, entirely devoid of hard parts. Its mouth is lined with "teeth" made of silica that serve to lacerate prey as it enters its digestive tract. This is a common trait among seagoing Apterran life-forms, as the planet's lack of diatoms has resulted in a buildup of dissolved silicon dioxide throughout the planet's oceans. This makes it easy for sea creatures to absorb the mineral and incorporate it into their bodies. This feature has emerged independently among many sessile hydrozoans, but the sea hydra has an especially complex setup, with four rings of tooth-like barbs lining the hypostome. 

Food is caught passively with twelve sticky tentacles; instead of venomous nematocysts, their arms use a combination of glue-like mucus and claw-like silica shards to ensnare fish, tailtube worms, and Pelagarthrid isopods in the water column. Though the adult sea hydra can't actively swim, its tentacle muscles are powerful enough to grip struggling prey, bringing it towards the mouth while inflicting hundreds of tiny cuts. 

Though feared by various small nektonic organisms, sea hydras don't grow larger than a dinner plate, as their teeth place an upper size limit on the prey they can eat. The scales and exoskeletons of marine animals larger than about 20cm are generally much harder to pierce, so Epipelagiomedusa remains specialized for small game. Other drifting hydrozoans may reach sizes of up to half a meter in diameter by feeding on detritus and planktonic prey, or by growing on the seabed in clusters of sessile, genetically-identical individuals. However, none of these possess the same extent of siliceous "dentition" seen in the sea hydra. Though such structures are simple for now, they have the potential to specialize for a variety of food sources in the future.