Tough, bushy, erect thallus, 4 - 70 cm tall. Plant consists
of a primary axes, rounded or slightly compressed below first or second
blade, 0.5 - 4 mm diam., with spines. Plants usually highly branched, with
secondary branches variable, some short compared to primary branch on some
plants, and as long as primary branch on others. Blades narrow, oblong, and
either flat or spirally twisted, 1 - 6 cm long, 0.1 - 0.8 cm wide, with
short flattened petiole, petiole usually with spines. Blade margins are
often spiny, occasionally smooth, wavy or straight, with spines or wings
developing on upper or lower surface of blade midrib.
S. polyphyllum varies greatly in different habitats.
Plants from high and mid intertidal are usually shorter, bushier (less than
20 cm tall), blades shorter with smooth or tooth-like margins, and with no
vesicles. Subtidal plants are usually larger and longer, with blades more
narrowly elliptical or oblong, and with vesicles.
Light golden brown to
Thalli dioecious, fertile branches have receptacles;
elongate hermaphroditic structures that house the male conceptacles with the
male antheridia, which produce sperm, and the and female conceptacles, with
female oogonia, which hold the ova. Nonreproductive conceptacles are visible
on the leaf surface as dark spots.
Sargassum polyphyllum is found on wave-swept benches,
tidepools and on reef flats. It is often heavily epiphytized by Hypnea
sp.,and Ulva reticulata on shallow, calm reef flats, and is found
with S. echinocarpum or S. obtusifolium on high energy
Hawai‘i: Kaua‘i, Moloka‘i, Oah‘ u, Maui,
Lana‘i, and Hawai‘i Island.
Worldwide: Endemic to Hawai‘i.
Like most other endemic species, little research has been
done on the endemic Sargassum polyphyllum. This common brown alga is
often found in communities with S. obtusifolium and S.
echinocarpum on wave swept benches with high energy and subtidally on
In mature algal communties, Sargassum spp. can
account for a large part of the biomass. In a 1971-1973 study, approximately
70 percent of the total biomass at Waikiki was identified as S.
polyphyllum, which was highly correlated with antecendent water
tempertures. These plants can grow quite large, overcoming competitors for
space by overgrowth and shading, and may form dense communities, thus
inhibiting recruitment by other algae. They are often epiphytized and it is
not uncommon to find Hypnea spp. and Ulva reticulata attached
to the upper branches and blades.
Sargassum polyphyllum has great variation in its morphology.
This alga can withstand a wide range of environmental conditions: it is
found in areas of high salinity, high wave action, and changing
temperatures. Reproduction is by vegetative fragmentation or sexual