Biology Reference
In-Depth Information
Figure 3.1. Anterior ends of five species of polyopisthocotylean Monogenea from the
gills of Carangoides emburyi (a,b) and Scomberomorus commerson (c to e) on the Great
Barrier Reef, Australia. All species are blood feeders and live in identical or largely
overlapping microhabitats on the gills of their respective fish hosts, i.e., they exploit
the same resource. Nevertheless, there are marked differences in the sizes of pharynx
and oral suckers. Scales 0.1mm. From Rohde (
1979b
). Reprinted by permission of
Blackwell Science Ltd.
displacement in parasites was discussed by Butterworth andHolmes (
1984
):
two species of trematodes of the racoon, Pharyngostomoides procyonis and P.
adenocephala, show character displacement (size differences) when they co-
occur in the same geographic area, interpreted as meaning that they use
different food particles.
However, importantly, character displacement can have causes other
than competition. A particularly important aspect of character displacement
is the difference in size of feeding organs in sympatric species, supposedly to
avoid competition for the same resource (see discussions in Christiansen
and Fenchel
1977
; Schoener
1986b
; Eadie
1987
). However, which null
model does apply? Is it possible that differences are fortuitous? Andrewartha
and Birch (
1984
) cite a paper by Pulliam and Enders (
1971
)whofound
three to five species of finches in the same habitat in the southeastern
United States in the summer. The species differed in the size of their
beaks but there was almost complete overlap in the size of seeds eaten.
Also, seed-eating heteromyid rodents seem to eat seeds of identical size, and
seed husking efficiency seemed identical (Rosenzweig and Sterner
1970
).
Rohde (
1979b
,
1991
) has demonstrated that the feeding organs (pharynx,
oral suckers) of monogeneans infecting the gills of the same host species and
using the same food, blood, differ in size and shape (Figure
3.1
). Hence, he
