Biology Reference
In-Depth Information
Second, grazers can be separated by size, with a main separation between meso-
and macrograzers (Table
8.1
); virtually nothing is known about the impact of the
smallest group, micrograzers (
0.1 cm), on seaweeds. Mesograzers are small
(mostly 0.1-2.5 cm), and can include juveniles of some larger species, with
typically small spatial feeding ranges (Hay et al.
1987
; Arrontes
1999
).
Mesograzers can occur in very high densities on their hosts and their grazing effects
are often specific to the algal host rather than the entire seaweed community.
Conversely, macrograzers are larger (
<
2.5 cm) with intermediate to large feeding
ranges. The voracious feeding activity of some macrograzers can denude large
seaweed stands and consequently influence overall community structure.
A third way of classifying grazers is by their diet specificity, distinguishing
specialists feeding on one or few seaweed species from generalists feeding on a
large variety of seaweed species and types (Table
8.1
). Most marine herbivores tend
to be generalists, and many also incidentally ingest some animal material. Feeding
selectivity can be driven by a number of factors, among others predation pressure
on the herbivore. For example, small, sedentary amphipod mesograzers can spe-
cialize to live and feed on chemically defended seaweeds to gain protection against
predators (Hay et al.
1989a
,
b
). Food selection can also be driven by the nutritional
content of the seaweed. Some crustaceans have higher reproductive fitness when
feeding on mixed algal species or on higher quality species (Hemmi and
Jormalainen
2002
). Specialization of herbivores on a single or few select algal
species can also be due to the structural habitat an alga provides the herbivore. For
example, the isopod
Idotea baltica
prefers feeding on phlorotannin-rich seaweeds
although this causes reduced growth rates, likely because the stable habitat structure
of these algae affords protection from predators (Jormalainen et al.
2001a
).
>
8.2 Digestive Adaptations in Grazers
One of the “problems” of seaweed food is its low nutritional quality and the
restricted digestibility and assimilation (Montgomery and Gerking
1980
).
Seaweeds have a high structural carbohydrate content with limited enzymatic
digestibility by herbivores, low nitrogen content compared to animal diet, tough
tissues, and sometimes metabolites that reduce digestion (e.g., phlorotannins;
Targett and Arnold
2001
; Amsler and Fairhead
2006
). Hence, while seaweeds are
often relatively easy to acquire as food because of their abundance and stationary
distribution, processing large amounts of algal food to satisfy metabolic needs of
the grazer poses many challenges. Digestion and assimilation of seaweeds are best,
albeit incompletely, known for herbivorous fishes (for reviews, see Horn
1989
;
Clements et al.
2009
). Adaptations in fishes often include a gizzard or other
grinding mechanism and/or acidic stomach environment to mechanically break
up and/or chemically lyse the food. They also typically possess a long alimentary
tract to optimize assimilatory surface area and gut retention time, and hindgut
fermentation of complex carbohydrates through microorganisms. Some of these
