Mechanical disruption of seagrass in the digestive tract of the dugong

Mechanical disruption of seagrass by the dugong
J. M. Lanyon and G. D. Sanson
2000
(a) mm 206
100
Median particle size
(μm) (mean ± )
1000
% of total dry weight
80
60
40
20
0
Stomach Duodenum Small
intestine
Caecum
Colon
Figure 5 Change in median (M) particle size (mm) along the digestive
tract of three dugongs.
Table 3 Food particle breakdown (as a percentage of total breakdown
occurring between the anterior stomach and posterior colon) within
each of the principal gut regions
Dugong Stomach Small intestine Caecum Large intestine
mm 311 42.7 33.3 0.1 23.9
MB1 70.8 15.1 0.00 14.1
MB2 28.5 26.6 28.8 16.1
Mean 47% 25% 10% 18%
Mean, mean breakdown occurring in each gut region with the three
specimens pooled. Dominant seagrass genera: mm 311, Halodule;
MB1, Halophila/Zostera; MB2, Zostera.
the small intestine, 10% in the caecum and 18% in the large
intestine.
There was considerable variation in the amounts of
breakdown per gut region between specimens (Table 3). It
is possible that these differences in particle breakdown may
be attributable to composition of the diet or other factors.
The diet of dugong MB2 was quite distinct from the other
two specimens with a high proportion of Z. capricorni. The
degree of digesta breakdown between the anterior stomach
and duodenum for this dugong was less than for the others.
Conversely, particle reduction in the caecum of this animal
was greater than for the other two specimens. Despite the
high variation in median particle size in the stomachs,
duodena, small intestines and caeca (Table 2), the CV for
the colonic samples (faeces) was relatively low (16.05).
A comparison was made of the particle distributions of all
sieved stomach (n=35)andfaecal(n=11) samples (Table 1).
Representative particle distributions of stomachs and faeces
are shown in Fig. 6. In all dugongs, except the calf mm 312
(Fig. 6c), the median particle size of the faeces was significantly
lower than that of the stomachs (e.g. Fig. 6a, b). The
CV was high (60.98) for the stomach samples, and also quite
high when the faecal samples were considered collectively.
However, the faecal samples could be divided into two
groups based on seagrass species composition (Table 1). In
those cases where large particles of leaf and greater proportions
of these were present (n=3), these were always
identified as Z. capricorni; the mean particle size was
138.6 8.4 mm (CV=10.49). Large pieces of Z. capricorni
pass through the gut in a recognizable state. The other faecal
0
(b) mm 201
100
% of total dry weight
80
60
40
20
0
(c) mm 312
100
% of total dry weight
80
60
40
20
0
samples (n=8) had no recognizable leaf material and the
mean particle size was lower at 62.24 2.92 mm (CV=
13.26). This does not necessarily mean that Z. capricorni
was absent, but that it was not apparent. Thus, despite the
wide variation in stomach particle size distributions, variation
among the faecal samples was low. In most of the faecal
samples, particles caught on the 500 mm sieve consisted of fine
fibres only. The dugong calf mm 312 (Fig. 6c) had markedly
different particle size distributions because its stomach
contained mainly milk, in addition to some fragments of
Halophila ovalis and algae, and its faeces contained an
undigested alga.
In vitro breakdown of seagrass
Fresh seagrass
Stomach
Stomach
Faeces
Faeces
Stomach
Faeces
Gut region
> 4.0 mm
> 1.6 mm
> 1.0 mm
> 500 μm
> 250 μm
> 125 μm
> 75 μm
< 75 μm
Figure 6 Comparison of the particle size distributions (% total weight)
in the stomach contents and faeces of three dugongs.
Each of the five seagrasses ruptured more easily than the
terrestrial pasture grasses. Of the leaf fractions compared,
the thin oval leaves of Halophila ovalis ruptured most easily.
Next were C. serrulata, Halodule uninervis (n) and Halodule
uninervis (b) with similar rupturing qualities. All of these
282
Journal of Zoology 270 (2006) 277–289 c 2006 The Authors. Journal compilation c 2006 The Zoological Society of London