OF DEAD MAN'S FINGERS, CORD WEED AND HANGING WRACKFive common
brown intertidal seaweeds of the Cape Peninsula.
by Gavin W Maneveldt, Botany Department, University of the
Western Cape and Rene Frans,International Ocean Institute of
Southern Africa, University of the Western Cape
In this, the second in the series on common intertidal seaweeds
of the Cape Peninsula, we look at the brownseaweeds common to our
rocky shores, with the exception of kelp, which will be covered in
the next issue ofVeld & Flora. Unlike the green seaweeds
mentioned in the previous issue, these brown seaweeds are
generallyslower growing and are less tolerant of salinity and
temperature extremes and thus more prone to desiccationstress. They
therefore tend to occur lower down on the shore than the green
seaweeds.
The usually yellowish-brown colour with grey orblackish tinges
of thebrown seaweeds comes from thepresence of chlorophyll a and
c,as well as an additional xantho-phyll pigment called
fucoxanthin.Some however, are totally blackand are often mistaken
for redseaweeds, but a good way todistinguish between brown andred
seaweeds in the field is toperform a simple test. Take theseaweeds
and rub them onto theback of your hand. If your hand isfull of
mucus after about tenseconds, it is probably a brownseaweed, as
they tend to be gooeyand filled with lots of mucilage.
Economic usesA characteristic feature of brownseaweed is that
their cell walls aremade of cellulose strengthened bycalcium
alginate. Alginate is asubstance of considerableeconomic importance
as it is usedas a gelling and emulsifying agentin a number of
industries. Thismakes brown seaweed economi-cally important as
their alginateextracts are used to make water-based products
thicker, creamierand more stable over extremedifferences in
temperature, pHand time. Products that containalginate extracts
include browniemix, frOZE}ll foods, desserts,relishes, salad
dressing, sauces,gravies and even beer foam.Alginates also prevent
ice crystalsfrom forming in ice cream.
Surprisingly though, at presentit is only the giant
brownseaweeds known as kelp* that areharvested commercially for
alginateextraction. Perhaps it is becauseno other group of brown
seaweedoccurs in large enough densitiesto be commercially
viable.
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CAPE CORD-WEED
Chordariopsis capensis or Cape cord-weed is astringy, limp
seaweed often abundant in shelteredhigh-shore and mid-shore tidal
pools, where it occursin a flaccid untidy mass that easily
becomesdislodged by any significant wave action.
DEAD-MAN'S FINGERS
Splachnidium rugosum, aptly called dead-man's fingers,has
spotted, elongated, cylindrical branches thatresemble withered and
callused fingers. The branches,sometimes also berry-like, are
filled with clear viscousmucilage. It is this mucus that enables S.
rugosum towithstand a high degree of desiccation within the
mid-intertidal zone when it becomes exposed during lowtide of
spring tides. Here Splachnidium rugosum isgrowing with the green
seaweed Ulva.
RALFSIA
RaJfsia verrucosa is anolive brown to khakiencrusting brown
seaweedthat occurs abundantly inlow-shore tidal pools. Hereit often
forms large, flat,smooth expanses giving theimpression that
someonehas accidentally droppedkhaki paint into the pools.RaJfsia
verrucosa can occurin great abundance because of itsrelatively fast
growth rate and alsobecause it produces a chemical whichseems to
deter most grazers. Oddlythough, the periwinkle, OxysteJe
tigrinq, and the long-spined limpet,
I,:::~~:~~~~~jl~~~~Scutellastra Jongicosta, just love eating
LRaJfsia - so much so that Scutellastralongicosta even tends
gardens of thisbrown seaweed, as seen in this photo.
WHAT DOES THATMEAN?
endemic occurringnowhere else in theworld.holdfast the
root-likeattachment of seaweeds.intertidal between thehigh tide and
the lowtide level.kelp giant brownseaweeds that dominatethe
subtidal area.spring tide the tideoccurring every twoweeks during
new andfull moon phases. Atthis time, low tides areat their lowest,
and hightides at their highest.subtidal below thelowest level on
the shorereached by the tides.
HANGING WRACK
The hanging wrack, Bifurcariabrassicaeformis, commonlydominates
that part of the lowershore just above the subtidal zonewhere it
forms extensive dense matsof long, tough, cylindrical
brancheslooking very much like anunmowed lawn. This seaweedachieves
dominance of the lowershore by rhizomatous spreading ofits
holdfast. Some local scientistshave recorded individual
seaweedbases measuring as much as 1 mindiameter. Hanging wrack is
endemicto South Africa, having a veryrestricted distribution that
occursonly from Cape Agulhas to SeaPoint in Cape Town.
The long-leafed sargassum,Anthophyctls Jongifolius, isa somewhat
twisted, robustand knobbled seaweed thatcan easily dominate
largeareas of low-shore tidalpools and the immediatesubtiqal area.
The blades ofthis species are characteris-tically toothy with
thoseblades near the surfacebearing little bladders intheir axils.
These bladderskeep the seaweed afloatenabling it to maximize
itsphotosynthetic abilities. Anthophycuslongifolius is one of the
largest non-kelpbrown seaweeds, often attaining lengths inexcess of
1 m. It is the sale species in thegenus Anthophycus and this genus
isknown only from southern Africa. ®
VeJd &' FJora December 2000Rem
AcknowledgementAll artwork in the seaweedseries has been
adaptedfrom paintings by MargoBranch in The Living Shoresof
Southern Africa byGeorge and Margo Branch.
Further readingBranch, George & Margo.
1981. The Living shores ofsouthern Africa. Struik,Cape Town.
Branch, G.M., C.L. Griffiths,M.L. Branch andL.E. Beckley.
1994.Two oceans: a guide tothe marine Jife ofsouthern Africa.
DavidPhilip, Cape Town.
Contact addresses for theauthors were given in theSeptember
issue of Veld &'Flora on page 125.
'The kelps will be covered inthe next issue.
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