Unlike their trisymbiomat cousins, the ironmat did not simply add ferrumcarcer to its repertoire of symbiotes. Instead, it entirely replaces the krakowhexian component, so the ironmat is no longer photosynthetic.
The overall shape of the mat is somewhat flatter than its ancestor. The amount of ferrumcarcer cells is liable to vary, based on amount of iron in the sand as well as a population oscillation. At times the ferrumcarcer cells thrive and reproduce like mad, filling up the mats, but as they exhaust their resources, they'll die back to much smaller quantities. Therefore, ironmats vary in appearance from a bright red through a rusty color to a dull gray.
As before, the Laminanimbus component consists of clumps of four cells, where each cell has a tentacle that it uses to attach to its neighbors, all held together in very diffuse layers, with large gaps. The ferrumcarcer cells fit within these gaps, generally sinking to the mat's lowest layers. As before, the upper layers of the Laminanimbus filter-feeds food particles that pass above, while the lower layers decompose detritus, and the entire mat shares nutrients throughout. Laminanimbus also extracts iron from its food, which it passes to ferrumcarcer cells to metabolize. These ferrumcarcer cells also feed on any iron in the sand where the mat has settled. Then, Laminanimbus is further able to feed on the ferrumcarcer cells' waste products and their dead.
These mats also grant ferrumcarcer cells some mobility, as the cells are unable to move on their own. The Laminanimbus tentacles are able to push the cells around into different parts of the mat, so if they've exhausted the iron in one area, they can spread to another one, which is especially useful as the mats grow. Indeed, this can lead to a mat growing in one direction as it dies back in the other direction, effectively allowing it to creep across the ocean floor over the course of months. This also brings ferrumcarcer cells into more frequent contact with each other, so they are now adapted to exchange genetic material with one another.