Morphology of filamentous thalli of Analipus japonicus (Ralfsiales), temperature and irradiance effects on their formation and growth, and erect thallus formation in the field

Abstract

Free-living filamentous thalli derived from an Analipus japonicus erect thallus were established in laboratory culture. The filamentous thalli were composed of slender filaments and densely pigmented oval or sub-oval cell masses. Temperature (5–20 °C) and irradiance (25–200 μmol photons m⁻² s⁻¹) effects on formation and growth of the thalli from three explant groups (isolated filaments, small cell masses, and large cell masses) were investigated for 15 days. These explants survived over the examined range. The isolated filaments had a high ability to form the thalli, and the isolated and new filaments formed filaments and cell masses. Meanwhile, the isolated and new cell masses formed only filaments, and their ability to grow and form the thalli decreased as they grew in size. Optimum culture conditions for formation and growth of the thalli from the isolated filaments, and the two cell mass growths were 15–20 °C and 100–200 μmol photons m⁻² s⁻¹. To produce erect thalli, the filamentous thalli were cultured on seedling threads for 6 weeks at 15 °C, 60 μmol photons m⁻² s⁻¹ and 20 cm s⁻¹ water flow velocity. Erect axis density increased with increasing coverage of the filamentous thalli, and their cell masses (primordia of erect thallus holdfast) with mean area of about 140 × 10³ μm² began to produce erect axes. After 4 months of field cultivation, they grew into full-grown erect thalli of about 20 cm long. These findings show that the free-living filamentous thalli were applicable for stable seedling production of A. japonicus to be used in aquaculture.