Trisymbiomats
When symbiomats extended southward, they came in contact with another colonial photosynthesizer: the violet mat. The violet mat had long competed with carpetesta for space on the ocean floor, but with the symbiomat, such competition was unnecessary. The symbiomats' carpetestan component was accustomed to letting photosynthesizers settle within the gaps between its cell clumps and layers and could easily accommodate violet mat cells growing within it. The violet mat readily grew in this sheltered cradle.
Thus, three symbiotic cell lines developed: the carpetestan host, Laminanimbus triumviratus, the blocky layers of the photosagnian Etiamphotonae triumviratae, and the krakowhexian cells that filled in the gaps, Krakowhexia triumvirata.
The Laminanimbus host grows in four-cell clumps in which each cell has a tentacle that connects it to neighboring cells, thus forming a flat and diffuse layer. However, the tentacles switch from time to time to connect with layers above and below, thus linking the mat as a whole. These tentacles distribute nutrition throughout the mat and run around the two other cell types, where they secrete nutrients for them to absorb as well. Upper layers also reach their tentacles out to ensnare passing cells for filter-feeding and defense, while the lower layers are solely decomposers, which includes decomposing dead cells from the krakowhexian and photosagnian components.
Etiamphotonae already formed extensive mats on its own. It continued this structure within its new host. The blocky cells clump together complete chunks that rest among the upper Laminanimbus layers, with new cells migrating upward when the mat as a whole grows. These chunks benefit from the host's nutrition and defense, and it provides them with a guaranteed platform to grow on.
Krakowhexia cells have become fewer than in the original symbiomat, as the newcomers occupy much of the photosynthetic space. However, as the only non-colonial member of the assemblage, they're primed for filling in the smaller gaps. And they're able to ensnare larger cells, which the rest of the colony can then absorb for nourishment. They're much shorter-lived than the rest of the mat, and will quickly be absorbed by the decomposition layers, but they reproduce quickly and spread out.
Trisymbiomats grow most readily in shallow water, where all components can easily reproduce. They all reproduce by binary fission (each clump simultaneously in the case of Laminanimbus, where new cells can swim out and colonize a new site or join another mat. But within a mat, each component can share genetic material when touching cells of its own species. The mats also grow within the tidal zone and can grow past it. In the original symbiomats, Laminanimbus formed a protective outer layer on its own, but in trisymbiomats, the photosagnians can form a shield too, as their sturdy cell walls are resistant to desiccation. So these join together to form a hard shell whenever the tide recedes or when a mat extends past the shoreline. The krakowhexian part recedes into the moist insides.