Tummlix
The Tummlix is a commensal that lives in the guts of the Common Siluro (and its descendants, by default), which becomes a mild parasite if the fauna is ill. It “breathes” iron (pure iron or simple iron compounds) floating around in the fauna’s gut from the silt or meat the host eats. It eats hydrogen gas and methane produced as a byproduct of digestion by the host’s digestive symbionts..
Tummlixes often get into the host when larval host species ingest them while sifting through iron-rich silt. Some perish travelling through the acids of the digestive tract. However, unlike its Lilakeuma and Mangalix relatives, in large numbers Tummlixes can produce antacids in great enough numbers to survive the journey to the more hospitable intestines. Once there, they eat and reproduce.
As the host matures, the host ingests less iron, but creates more hydrogen gas from the breakdown of flesh. The exact amount of iron in the guts depends on the host’s diet, and whether it eats organisms with iron-based blood. The Tummlix has genetic contingencies, controlled by epigenetic switches, to adapt to eating different iron compounds. However, due to their short lifespans, this usually happens generation-by-generation rather than within an individual's lifespan.
Normally, Tummlix exist in small numbers in the host's intestines, and don't cause any problems. But if the host is sick and stops eating, the Tummlix in its intestines may creep into the folds of its intestine with mild acids, searching for scraps of iron or hydrogen compounds in chyme or in the tissue itself. Their feeding causes mild damage, easily reversed once the host is well again.
Some Tummlix are excreted along with the host's waste, especially when the host is sick and Tummlix become prone to wandering. The Tummlix end up in the silt again, and take up a free-living existence. However, they live in greater numbers within Siluronian guts, and at any one time most of the population is within guts.
While it occasionally ends up in the gut of Devorator descendants (Sternobrachians), it’s screened out and digested along with typical pathogens on passage through the stomach.
The Tummlix is not quite as UV-resistant as its relatives, due to spending so much time in the dark, safe confines of host guts.