Interspecific fastening tissue between Hypnea pannosa and Hypnea spinella (Cystocloniaceae, Rhodophyta)

Abstract

ABSTRACT There have been frequent reports in several macroalgae of fusions between algal structures. However, most of these reports have documented the (intraspecific) adhesion of portions of the same thallus. The different adhesion structures have been termed anastomoses, and include ring-shaped cells, adherent discs, tentacle cells and pseudo-laterals. Hypnea pannosa and H. spinella are seasonally highly persistent and abundant species in the Mexican Tropical Pacific, with a wide distribution in the region. When the species grow together in the rocky intertidal zone, they produce contact points between their thalli, which hold them together. The fusions between the branches and axes of these species result in a fastening tissue that is distinct from all those previously described. Here we describe the anatomical changes that occur at the contact points between the branches and axes of the two species using optical microscopy, scanning electron microscopy and transmission electron microscopy. The adhesion structures resulted in significant morphological and anatomical changes to the superficial cortical cells, with a new type of interspecific fastening tissue growing outside of the contact points. We characterize this new type of interspecific fastening tissue, consider its origin and structural complexity, and compare it to other connecting structures reported. Additionally, we discuss the consequences of this new type of tissue and its possible ecological significance.