Effects of irradiance and water flow on formation and growth of spongy and filamentous thalli of Codium fragile

Effects of irradiance and water flow on formation and growth of filamentous and spongy thalli of Codium fragile (Suringar) Hariot growing on vinylon threads were investigated at the laboratory culture. They showed clear differences in their irradiance and water flow requirements for their formation and growth. Spongy thalli were formed from the cultured filamentous thalli only at the high water flow velocity (10 cm s−1). Number of the spongy thalli remarkably increased with increasing irradiance because those at 10, 50 and 100 μmol m−2 s−1 reached 0, 2 and 76 thalli m−1, respectively, by 10 weeks of culture. In contrast, filamentous thalli were formed from the cultured spongy thalli at 0 and 3 cm s−1, and difference in irradiance had no effect on their formation. Growth of the spongy thalli greatly accelerated under the combination of the high irradiance and high water velocity (200 μmol m−2 s−1 and 10 cm s−1) because their relative growth rate in wet weight under the condition was two-four times higher than those at the other examined irradiances and water velocities. On the other hand, difference in water velocity had no effect on growth of the filamentous thalli under flowing water, and their growth decelerated at the high irradiance (200 μmol m−2 s−1). This demonstrates that water flow is a major factor controlling the formation of the spongy and filamentous thalli. The formation and growth of the spongy thalli surely occur under the combination of the high irradiance and fast flowing water. In contrast, the formation of the filamentous thalli occurs in the calm water, and their growth is inhibited under the high irradiance.