Biology Reference
In-Depth Information
coralline green algae,
Rhipocephalus
and
Udotea
(Hay et al.
1994
). Calcification in
seaweeds can also be paired with additional non-coexistence (avoidance) strategies.
One example is the tropical green coralline alga
Halimeda
, which produces new,
non-calcified segments only at night when herbivorous reef fishes are inactive (Hay
et al.
1988
). The price for herbivore resistance of calcareous seaweeds is very low
growth rates, which prevent coralline algae from easily replacing tissue that is lost
through grazing. A different example of structural defense is referred to as bet
hedging and can occur in heteromorphic seaweeds. Here, a fleshy, upright form has
high photosynthetic and growth rates but is susceptible to grazers, while a crustose
form is more grazer resistant at the expense of reduced growth rates (e.g., Slocum
1980
). With this strategy, the crustose form prevails in situations of high grazer
abundance while the upright form occurs when herbivores are fewer or absent.
Another type of structural (mechanical) defense is where water motion creates a
whiplash effect of some kelps, which will prevent urchins from entering kelp beds
to graze (Konar
2000
).
Coexistence strategies using chemical defenses are probably the best-studied area
of seaweed defenses against herbivores, and a recent topic provides an excellent
review of the topic (Amsler
2008
). These defenses also are covered from a mecha-
nistic perspective in Chap.
9
by Amsler, while they are viewed in this chapter from a
strategic and community perspective. Marine seaweeds from all latitudinal ranges
and ecosystem types produce a wide variety of secondary metabolites that function
as herbivore deterrents (Maschek and Baker
2008
; see Table
8.3
for examples from
different latitudinal systems). These chemical defenses mostly reduce the palatabil-
ity or nutritional quality of the seaweed, or can also reduce the fitness and survivor-
ship of the herbivore through toxic effects. The functions and mechanisms of the
multitude of defensive chemical metabolites have evoked a number of predictive
models. Many of these models are based on the assumption that the production,
storage, or degradation of defensive metabolites is costly, and that implementation
of defenses is thus under regulation to optimize energy use in seaweeds (Pavia and
Toth
2008
). Models of seaweed chemical defenses that are mostly based on resource
availability are reviewed elsewhere in detail (Cronin
2001
; Amsler and Fairhead
2006
; Pavia and Toth
2008
). One model, the Optimal Defense Theory (ODT;
Rhoades
1979
), is specifically based on herbivory as the selective driver of defensive
strategies. The ODT predicts that allocation of defensive compounds to specific
parts within the alga is in direct proportion to the value of the thallus part for the
seaweed's fitness as well as to the risk of attack, and in inverse relation to the cost of
defense production. Within the framework of the ODT, induction of these defenses
should occur during times when the risk of herbivore attack is greatest and unpre-
dictably variable. The preferential allocation of defensive chemicals and induction
of defenses after initial damage in seaweeds has been abundantly tested and reported
in the literature and much, although not unanimous, support has been found (see
Toth and Pavia
2007
for review). In part the debate about differential defense
allocation to the most valuable thallus parts may be hampered by the definition of
“valuable.” One particularly well-studied system of the ODT is phlorotannin allo-
cation in brown algae, even though the defensive role of phlorotannins has been
