Marine Animal Encyclopedia
Giant Kelp Macrocystis pyrifera
Giant kelp is the largest seaweed on Earth. It can grow at the phenomenal rate of 24 in (60 cm) per day in ideal conditions, and reaches lengths of over 100 ft (30 m) in a year. Giant kelp normally grows at a depth of 30–100 ft (10–30 m) but can grow much deeper in very clear water. The huge branched holdfast, which is about 24 in (60 cm) high and wide after three years, is firmly attached to the seabed. From it, a number of long, flexible stalks stretch toward the surface, bearing many straplike fronds, each buoyed by a gas-filled bladder. The giant kelp's fronds continue to grow on reaching the surface, floating as a dense canopy.
Giant kelp has a two-phase life cycle. Fronds (sporophylls) at the base of the kelp produce spores that develop into tiny creeping filaments. The filaments produce eggs and sperm, which combine to produce embryonic kelp plants.