Undaria Life Cycle

The large blades of kelp that we see are just one part of the life cycle of Undaria. Like many other algae, Undaria has two distinct life phases: a microscopic filamentous stage and the large, macroalgal stage that we see. Both phases live attached to substrate. The microscopic stage can remain dormant, attached to the substrate, for up to 2.5 years (Hewitt et al 2005), before producing the large kelp stage. For management purposes, the microscopic stage can be thought of as a “seed bank.”

Here are some further details on the life cycle of Undaria. The large kelp that we find attached to docks and other substrate is a sporophyte; it produces spores. These microscopic “zoospores” swim in the water column (for up to 3 days in the laboratory, Thornber et al. 2004). They settle on hard substrate and develop into a microscopic stage called a gametophyte (by 8 days in the lab, Thornber et al. 2004). “Male” gametophytes release swimming sperm called antherozoids that fertilize eggs retained by the “female” gametophyte. Fertilized eggs grow up into the sporophyte, reaching reproductive maturity in 6 to 8 weeks in the lab (Thornber et al 2004), thus continuing the cycle.

Each sphorophyte is capable of producing 10s of millions of spores, but many of these will not survive to maturity. Undaria can disperse naturally, traveling on currents during the swimming zoospore stage. However, both lab experiments and field experiences suggest that on its own, Undaria appears to spread only 10s of meters during this phase. The longer-range and more rapid dispersal of Undaria in places where it has invaded is likely human-mediated.