Cockroache; Ecology, behavior & history - W.J. Bell

Fig. 5.2 Unidentified nymph feeding on bird excrement, Ecuador.
Photo courtesy of Edward S. Ross.
Fig. 5.3 Detritivore-microbial interactions during coprophagy.
When a cockroach feeds on a refractory food item (A), any
starches, sugars, lipids present are digested, and endogenous
cellulases permit at least some structural polysaccharides to be
degraded as well. Much of the masticated litter, however, may
be excreted relatively unchanged (B), and serve as substrate for
microbial growth (C). Ingested microbes, whether from the
substrate (D) or from the fecal pellets of conspecifics (C), may
be digested, passed in the feces, or selectively retained as mutualists.
Microbes on the food item, on the feces, and in the
hindgut are sources of metabolites and exoenzymes of possible
benefit to the insect (E). Metabolites of the insect and of the
gut fauna excreted with the feces (F) may be used by microbes
colonizing the pellets or reingested by the host during coprophagy.
Various authors shift the balance among these components,
depending on the arthropod, its diet, its environment,
and its age. From Nalepa et al. (2001a), with the permission of
Birkhäuser Verlag.
Coprophagy as a Source of Microbial Protein
and Metabolites
As food, the feces of detritivores are not fundamentally
different from rotting organic matter; the feces of many
differ very little from the parent plant tissue (Webb, 1976;
Stevenson and Dindal, 1987; Labandeira et al., 1997). The
differences that do occur, however, are important ones:
feces are higher in pH, have a greater capacity to retain
moisture, have increased surface to volume ratios, and
generally occur in a form more suitable for microbial
growth (McBrayer, 1973). Fecal pellets are colonized by a
succession of microbes immediately after gut transit, with
microflora increasing up to 100-fold (Lodha, 1974; Anderson
and Bignell, 1980; Bignell, 1989). Fragmentation
of litter is particularly important for bacterial growth, for
unlike fungi, whose hyphae can penetrate tissues, bacterial
growth is largely confined to surfaces (Dix and
Webster, 1995; Reddy, 1995). The process is similar to gardeners
creating a compost pile: microbially mediated decomposition
occurs best when plant litter is moist and
routinely turned. Coprophagy exploits the microbial
consortia concentrated on these recycled cellulose-based
foodstuffs (Fig. 5.3); the microorganisms serve not only
as a source of nutrients and gut mutualists, but they also
“predigest” recalcitrant substrates. Microbial dominance
is so pronounced that fecal pellets may be considered living
organisms. They consist largely of living cells, they
consume and release nutrients and organic matter, and
they serve as food for animals higher on the food chain
(Johannes and Satomi, 1966).
Coprophagy as a Mechanism for Passing
Hindgut Mutualists
All developmental stages feed on feces, but coprophagy is
most prevalent in the early instars of gregarious domestic
cockroaches (B. germanica, P. americana, P. fuliginosa)
(Shimamura et al., 1994; Wang et al., 1995; Kopanic et al.,
2001). Feces contain protozoan cysts, bacterial cells, and
spores, and are the primary source of inoculative microbes
(Hoyte, 1961a; Cruden and Markovetz, 1984).
Very young cockroaches, with a hindgut volume of 1 l,
already show significant bacterial activity (Cazemier et
al., 1997a). Repeated ingestion of feces is no doubt required,
however, because a successional colonization of
the various gut niches by microbes is the norm (Savage,
1977). Obligate anaerobes have to be preceded by facultative
anaerobes, and a complex bacterial community has
to precede protozoan populations (Atlas and Bartha,
1998). Because cockroach aggregations are generally species
specific, horizontal transmission of microbial mutualists
from contemporary conspecifics may be considered
typical. Mixed-species aggregations are occasionally re-
MICROBES: THE UNSEEN INFLUENCE 79