Sargassum can be free loating or rooted to a hold fast...read on

Summary of Basic Life Cycle

Fertile branches of adult algae produce gametes . When the ova are squeezed out of the conceptacle, they are not broadcast into the surrounding water like most algal gametes, but remain attached to the receptacle by a mucilaginous stalk made of mesochitin. After fertilization, the zygotes continue to grow on the parent for several days before dropping to ocean floor. The enveloping mucilage protects them from environmental stress, as does their multicellular form. Their large size also allows them to settle rapidly, and the well-developed rhizoids adhere quickly to the substrate. This results in germlings settling near the parent (within 3 meters), where conditions are likely to be favorable.

Alternative Disperal Mechanism

Fertile branches break off from the holdfast and float away. When the germlings are released a long distance from the parent, they do not have to compete with their own relatives and can settle in new territory. The combination of these two dispersal mechanisms proves to be an effective system for global spread.

Interesting Tidbits about Settlement

Gametes are usually released during or just after the spring tide. The timing may be determined by hydrostatic pressure, or more probably by the light of the moon. (Fletcher 1975.)

Gametes are released in cycles of 13 days instead of all simultaneously, increasing the odds that some of them will encounter favorable tides and conditions. (Fletcher 1975.)

Germlings are pear-shaped, enabling the rhizoids to land first.

Rhizoids stick within 48 hours.

Germlings lose their ability to adhere over time, in connection with declining mucopolysaccaride levels. After 18 days, half can still stick, after 49 days, none can. (Deysher and Norton 1982.)

Germlings can grow on kelp, but they fall off before they reach a height of 3 cm and can no longer re-adhere. (Deysher and Norton 1982.)
Individuals that become free-floating can survive in the water column indefinitely.