Back in the early 1980s I was in Seattle visiting one of my best friends and his wife. Chris is a film buff and wanted to show me a short film he loved, "The Mating Habits of the Pacific Kelp." The movie used time-lapse sequences of two bull kelp, a species we don't have in southern California. They were slithering about on a sandy beach, not exactly their normal trysting place. When these kelp bumped into one another, they wrapped themselves around their mate and did "the wild thing." Both the film and this column are, at worst, PG-rated Mom and Dad, so don't be concerned. It was a funny film for this marine biologist. The mating habits of our own giant kelp are actually far more interesting.
My readers are well aware that "munching" and "mating" in the kelp forest are critical activities for any species. Just as it is important that individual organisms grow and survive, it is critical that they reproduce and maintain the future survival of both the local population and the species as a whole. Although one would never guess it by looking at the large individuals in the kelp forests off our coast, giant kelp has a truly unusual sex life. The massive alga we see is just a small... er large... part of their reproductive cycle. It actually takes two generations of giant kelp to complete the process!
The obvious members of our kelp forests are the spore producing or sporophyte generation. Spores are created in structures known as sporangia on specialized blades known as sporophylls clustered just above the holdfast. The sporangia are often grouped together on the blade in a whitish patch known as a sorus. Spores contain one half the normal compliment of chromosomes. Fifty percent of them are "male" and fifty percent are "female." The large alga may continually release spores from spring to fall. Estimates of spore production by a single plant have been given in various units. One source stated that at maturity, millions of spores are released. Another stated as many as 500,000 spores are produced per hour. Still another reported a single mature sporophyte may have many sporophylls, each containing "billions" of spores... and it wasn't Carl Sagan.
The microscopic spores usually fall relatively close to the adult sporophyte. However their small size and slow rate of sinking may allow them to be dispersed short distances by ocean currents. Large numbers are produced because relatively few survive. They may be consumed by filter feeders as they drift downward, or by grazing animals living on the ocean floor.
The spores grow into male or female gametophytes, which represent the sexual generation in the kelp life cycle. These tiny microscopic algae become reproductively mature within two weeks... very precocious! They may also remain in a vegetative state for up to a year if environmental conditions are not right. For example, light levels may be too low until a storm removes some of their parent's canopy. Female gametophytes produce eggs which release a chemical pheromone known as lamoxirene. The male gametophytes release motile sperm that follow this pheromone's increasing chemical gradient to the egg.
The sperm are so small that the male and female gametophytes must be within 1mm, or 1/25th of an inch, of one another for successful fertilization. Now that's getting up close and personal! Giant kelp may cross fertilize with other species of Macrocystis and even other kelps like the elk kelp, producing fertile hybrids, but despite this they remain morphologically distinct species. The fertilized egg starts a new sporophyte generation. It grows on top of the female gametophyte. The sporophyte first becomes visible with a single, heart-shaped blade. Within a few weeks, at a size of 3-4" it splits into two blades. After 12 to 15 months it is a mature giant sporophyte, ready to start the cycle all over again.
The huge sporophytes we see are perennial, and may live six years or more. An individual frond, composed of a single stem-like stipe and its attached blades, usually has a maximum lifespan of six months. They are constantly being replaced by new ones growing up from the holdfast region. The older fronds break away in a process known as sloughing ("sluffing"). A young kelp forest has many individual sporophytes with only a single stipe growing very close together. Dense mature forests studied on the mainland usually have about one sporophyte every two square meters. However, studies at some locations on Catalina Island show densities about ten times that.
To summarize this complex reproductive strategy, sporophylls at the base of the mature sporophyte generation produce spores asexually. These mature into male and female gametophytes that create sperm and eggs. These gametes fertilize and becoming tiny sporophytes that grow into the 200 foot giants to start the process all over. The gametophytes and very young sporophytes are tiny organisms living near the ocean floor, while the mature sporophytes have most of their biomass in the upper water column near the surface. Thus they must live and grow under markedly different environmental conditions. I guess being an amphibious individual, I'm much the same... living half my life in the ocean and the other half on land!
Image caption: Giant kelp sporophyte, sporophylls, sporangia and spores; gametophyte growing from spore and two mature gametophytes; young kelp sporophyte; and small juvenile sporophytes growing from the bottom. Images courtesy of Drs. Dan Reed, Ray Lewis, Mark Carr and Chuck Kopczak.
