Formation and early development of the Monostroma nitidum Wittrock

•Morphological and developmental characteristics, combined with ITS sequences were used to identify the Monostroma strains.•The sac-like microthallus was observed on the early ontogeny of Monostroma nitidum.•The early ontogeny of M. nitidum was firstly described in detail. The monostromatic green macroalga Monostroma nitidum Wittrock is attracting global attention due to its commercial importance. In this study, field Monostroma strains were collected from Dalang, Naozhou Island, on the South China Sea Coast of Zhanjiang City. They were identified as M. nitidum using morphological and developmental characteristics combined with nuclear-encoded internal transcribed spacer sequences. Moreover, the early ontogeny of M. nitidum was observed for the first time in detail under a photomicroscope. The active zoospores had a spherical shape, were 6.88 μm in diameter, and completely settled on the Petri dish 1 day after release. The first transverse division of settled zoospores occurred on the 3rd day; the length of the elongate cells was 15.94 μm. Longitudinal divisions started to occur in the middle and at the top of erect filaments (61.27 μm in length) on the 12th day, and rhizoids began to appear at the posterior end of the microscopic filamentous thallus. The sac-like microthallus (113.79 μm in length) that developed from erect filaments was observed on the 18th day and divided longitudinally towards the rhizoids to produce expanded monostromatic blades (178.96 μm in length) on the 24th day. These data increase our understanding of the ontogeny of M. nitidum.