Biology Reference
In-Depth Information
herbivore, or some combination thereof. Defenses synthesized only when under
attack or in greater amounts when under attack are referred to as
induced defenses
.
Alternatively, some defenses, known as
activated defenses
, are produced constitu-
tively but in an inactive or less toxic form and are rapidly changed into the bioactive
form upon herbivore attack. There are a number of theories based on terrestrial
plants (Stamp
2003
; Agrawal
2007
) that have been used for predictions of what
kinds of defensive compounds seaweeds should produce as well as when and where
they should make them. These ideas were recently reviewed by Amsler and
Fairhead (
2006
) and Pavia and Toth (
2008
).
The red alga
Delisea pulchra
is a well-studied example of constitutive chemical
defenses against predation (and against biofoulers, see also Chap.
8
by Iken,
metabolites known as
furanones
(de Nys et al.
1993
) which deter grazing by many,
but not all, herbivores (Wright et al.
2004
). Interspecific variations in furanone
levels are heritable (Wright et al.
2004
) and have a cost to the seaweeds in terms of
reduced growth and fecundity under some circumstances (Dworjanyn et al.
2006a
).
That defensive levels vary within a species, are heritable, and come at a cost to the
producing organism are assumptions underlying chemical defense theories (Stamp
2003
; Agrawal
2007
). However, they are rarely documented, particularly in
seaweeds, making
D. pulchra
a particularly important model.
Constitutive (and other) defenses are also well studied in a number of brown
seaweeds, particularly in members of the Orders Fucales and Dictyotales (Amsler
and Fairhead
2006
). In the Fucales, one important model species has been
Fucus
vesiculosus
, particularly from northern Europe where the defensive compounds
appear to be phlorotannins, a class of large, phenolic compounds that are widely
distributed in brown algae (Amsler and Fairhead
2006
; Jormalainen and Honkanen
2008
). Phlorotannin levels in these seaweeds have a heritable component and also
vary geographically, over time, and with a variety of environmental parameters,
which has facilitated tests of a diverse range of ideas in defensive chemical ecology
(Jormalainen and Honkanen
2008
). Seaweeds in the Dictyotales utilize a variety of
smaller compounds for defense with the best studied examples being terpenoids,
particularly in the genus
Dictyota
(Pereira and da Gama
2008
). These seaweeds
have also been used in a wide variety of studies of how defensive chemicals mediate
seaweed-herbivore interactions. For example, they have been used to demonstrate
that chemical defenses interact with the nutritional quality of an alga to influence
not only how much of the alga is consumed by a herbivore, but also the ultimate
impact on herbivore fitness (Cruz-Rivera and Hay
2003
). They have also been
particularly useful in studies of how some herbivores are able to tolerate algal
chemical defenses (Hay
2009
; Sotka et al.
2009
).
Activated chemical defenses are best understood in several green seaweeds
including two siphonous genera:
Halimeda
and
Caulerpa
.
Halimeda
spp. constitu-
tively produce the diterpenoid halimedatetraacetate which itself deters fish grazing,
but upon wounding the seaweeds rapidly convert it to an even more potent grazing
deterrent, halimedatrial (Paul and Van Alstyne
1992
).
Caulerpa
spp. produce the
sesquiterpene caulerpenyne constitutively but upon wounding rapidly convert it to
