Inalgal anatomy, a pit connection is a hole in the septum between two algal cells, and is found only in multicellular red algae[1] − specifically in the subphylum Eurhodophytina, except haploid Bangiales.[2] They are often stoppered with proteinaceous "pit plugs". By contrast, many fungi (only ascomycetes and basidiomycetes, as most other groups lack septa) contain septal pores − an unrelated phenomenon.[3]
A sieve-like membrane may cover the pit in living algae,[4] but in the majority of algae a plug forms, they likely limit the transfer of metabolites between neighbouring cells.[2][5]
Primary pit connections are formed between cells in the same filament, derived from the same parent cell by its division.[4] Such connections are always single, and usually circular;[4] this is a result of their method of formation. The septum is formed as the walls of a filament grow inwards, dividing the cell; this results in a hole in the middle of the tube where the walls don't quite merge.[2] Thus pit connections are visible in the youngest of septa, widening as the septum thickens, until in some cases they may ultimately occupy the entire septum.[4] Secondary connections, by contrast, occur between unrelated cells, and serve a role in transferring cell contents and nutrients.[2] They may even form between cells of different species, as in the parasite Holmsella.[6]