Grazers on Benthic Seaweeds - Seaweed Biology - page 164

controversial and seems to be context-dependent on herbivore and environment
(Amsler and Fairhead 2006 ). Using sensitivity analysis, which determines the
relative contribution of different life processes to overall population growth, Pavia
et al. ( 2002 ) related life processes to different thallus parts in the fucoid Ascophyllum
nodosum , and thus determined fitness values of different thallus parts. Phlorotannin
concentrations and resistance to gastropod grazers were positively correlated to
these fitness values in accordance to the ODT.
8.5 Effects of Seaweed Chemical Defenses on Herbivores
and Communities
Algal chemical defenses can indirectly influence herbivore populations by affecting
their overall food consumption and lead to reduced growth or reproduction.
Metabolites can also directly affect herbivore survival, although practical separa-
tion of indirect and direct effects is often difficult. Phlorotannins, common
metabolites in brown algae, have the potential to bind to digestive enzymes as
well as with plant nutrients and thus limit assimilation efficiencies in the grazer
(Targett and Arnold 2001 ). For example, the assimilation efficiency in fishes was
greatly reduced when feeding on a phlorotannin-rich diet (Boettcher and Targett
1993 ), and egg production in periwinkles was reduced by phlorotannins (Toth et al.
2005 ). Similarly, the terpenoid metabolites produced by the brown alga Dictyota
significantly reduced growth, survival, and reproduction in some grazing
amphipods (Cruz-Rivera and Hay 2003 ). These fitness effects can depend on
metabolite concentration and on the specific herbivore species, as there is consid-
erable variation in the effects of the same algal metabolite on even closely related
herbivores. Conversely, some mesograzer populations can ultimately benefit from
seaweed defenses by gaining protection from predators when living associated with
a chemically defended host, even if this comes with reduced fitness as a trade-off
(Hay et al. 1988 , 1989a , b ).
Relatively little understanding exists on the structuring role of seaweed chemical
defenses on overall communities beyond the specific interactions between a partic-
ular seaweed and herbivore (Hay 2009 ). Chemically well-defended seaweeds can
be very abundant and even dominate communities, independent of latitude or algal
group; this attests to the competitive advantage chemical defenses may provide. On
tropical reefs in Brazil with high grazing pressure, members of the tropical green
algae Bryopsis and Caulerpa, the red alga Laurencia, and the brown alga Dictyota
chemically deter herbivore grazing and also are highly abundant (Marques et al.
2006 ; Villa¸a et al. 2010 ). Fucus vesiculosus is the dominant habitat-providing
seaweed along rocky shores in the Baltic Sea, and phlorotannins act as effective
defenses in the adult seaweed against most grazers (Jormalainen and Honkanen
2008 ). Antarctic shallow waters are dominated by large brown algae of the order
Desmarestiales, which are unpalatable to most sympatric grazers. In some cases,