Monterey Bay Aquarium Research Institute
Marine Botany


Life History

Above is an animated story of the life history of Macrocystis pyrifera.  The beautiful giant kelp that you see in the kelp forest is actually the sporophyte stage of the life history.  The sporophyte is diploid, 2N, which means it has paired chromosomes.  At the base of the kelp, thousands of spores are produced in reproductive blades.  These spores become the male and female gametophytes, each of which is haploid, 1N.  The female gametophyte produces an egg.  The  motile sperm of the male gametophyte fertilizes the egg by following a pheromone concentration gradient to locate the egg.

This fertilized egg then divides.  Half of it grows down to form the holdfast, while the other half grows up to form the blade.  This juvenile sporophyte formed grows atop the female gametophyte and eventually overtakes it.  At the base of the blades a tear forms, which splits the blade into two, then four, then eight and so forth blades.  A small pneumatocyst forms at the base of the blades.  

Eventually, the magnificent giant kelp sporophyte is formed to produce more spores, and start the life history again.  The bull kelp, Nereocystis luetkeana, does not form a perennial holdfast, and must complete a similar life history annually to maintain its population.

© 2001 James Lopez              For Educational and Private Use Only