Fine structure of deep‐layer sloughing and epithallial regeneration in Lithophyllum neoatalayense (Corallinales, Rhodophyta)

Abstract

SUMMARYTransmission electron microscopic study of Lithophyllum neoatalayense Masaki demonstrated that this alga used two mechanisms by which it sloughed cells from its surface. Epithallial filaments, produced by a layer of underlying initial cells, were two to four cells long. Terminal epithallial cells, those closest to the thallus surface, underwent senescence and shedding as previously described for other coralline algae. In addition, deep‐layer sloughing occurred. During this process, which is known for only one other coralline alga, cell death occurred uniformly within a surface layer 40‐50 μm thick. No pathogens or evidence of mechanical damage was detected. The zone of dead cells included the epithallial layer, the layer of initial cells, and some cortex cells beneath the initials. Pit plugs connecting living and dead cells at the edge of the necrotic zone were sealed by wall material deposited by the surviving cells, but cells within the necrotic zone showed no evidence of sealed pit plugs. The outermost surviving cells became new initial cells, which then divided to form new epithallial cells. Regeneration of the epithallial layer occurred before the thick layer of dead cells peeled away. The extensive deep shedding of both epithallial and initial cells in L. neoatalayense and the effectiveness of regeneration from cortex cells challenge the assumption that protection of initial cells is an important function of epithallial cells.