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
distributions, especially if they are skewed. The change in
median particle size between 0 and 500 grinds is indicative of
the overall effects of grinding for each plant species. These
values (=‘breakability index’) were compared.
Results
Size of mouthparts
The occlusal surface area of cheek teeth ranged from 24 mm 2
in a dugong aged o0.5 years to 485 mm 2 in an adult dugong
aged 21 years, and was significantly regressed against skull
length and age (F=32.12; d.f.=1, 51; Po0.0001; adj
R 2 =0.36; n=52; Fig. 2). However, the coefficient of variation
(CV) in occlusal surface area was high (CV=35.05),
even when only adults were compared.
The surface area of the lower horny pad ranged from
901 mm 2 in dugongs aged 0.5 years to 6280 mm 2 in the
largest adults, with a mean of 4933 109 for adult dugongs.
Horny pad surface area was highly significantly regressed
against body size (F=433.28; d.f.=1, 53; Po0.0001; adj
R 2 =0.91; n=54) and followed an exponential growth
curve against age (Fig. 2). The CV for the horny pad surface
area of adult dugongs (CV=12.73) was considerably lower
than that for the dental occlusal surface area of adults. The
absolute surface area of the horny pads was up to nine times
greater than the occlusal surface area of the cheek teeth.
Digesta analysis
Diet composition
Twenty-seven of the 29 dugongs had fed on more than one
seagrass genus and some on up to four or five. Seagrass
genera recorded most frequently included Halophila, Halodule,
Cymodocea, Thalassia and Zostera. One dugong had
ingested some Enhalus. The most frequently ingested genera
Surface area (mm )
7000
6000
5000
4000
3000
2000
1000
0
SA teeth
Sa horny pad
Satotal
Log. (SA teeth)
Log. (Sa horny pad)
Log. (Satotal)
y = 1102.9 Ln(x) + 1793.1
R = 0.9309
y = 1044.7 Ln(x) + 1674
R = 0.9182
y = 58.382 Ln(x) + 118.46
R = 0.3648
a
0 5 10 15 20 25 30 35 40 45 50
Age (years)
Figure 2 Surface area (mm 2 ) of (a) functional cheek teeth, (b) lower
horny pad and (c) total functional mouthparts (cheek teeth plus lower
horny pad; all log-transformed) plotted against age (years). Lines of
best fit included.
c
b
recorded were Halophila and Halodule. Two dugongs had
fed on only one seagrass genus, Halodule. Ten per cent of
dugongs had identifiable algal fragments in their digesta.
However, because digesta could only be identified from the
largest particle size classes, seagrasses that were more amenable
to breakdown during mastication (see below) would
have been underestimated. Consequently, composition of
diet could not be examined as a variable affecting particle
size breakdown.
Stomach contents
In general, the stomach contents were well macerated.
However, there was considerable variation in particle size
distribution curves for the stomach contents of different
dugongs. Figure 3 shows some representative stomach
particle size distributions of dugongs of different ages.
Median particle size ranged between 302.8 and 3454.3 mm,
with a mean of 1241.8 141.98 mm (n=29; Table 1). The
CV in median particle size (for stomachs pooled) was high
at 62.6.
The multiple regression model, with median particle size
as the dependent variable, and total surface areas of the
cheek teeth and the horny pad as independent variables,
accounted for only 1% of the variation, R 2 =0.01, with
none of the factors significantly regressed (P40.05). This
suggests that particle size distributions in the stomachs of
dugongs are not simply a function of the masticatory surface
areas.
Whole guts
Particle size frequency histograms for each of five gut
regions of three dugongs (for which the contents of each
gut region were available) are shown in Fig. 4. Table 2
compares the range, mean SE and CV for median particle
size of each of five gut regions. Median particle size
decreased progressively down the gut with a reduction
in particle size distribution occurring at each gut region
(Fig. 5), indicating significant post-oral particle size reduction.
There was variation between specimens, both in
particle size distributions within each gut region and differences
in distributions between regions (Fig. 4).
There was consistent particle reduction down each gut
despite the fact that diet composition probably altered along
each gut. Species composition of digesta was based on
stomach digesta analysis only, because it was usually impossible
to identify seagrass to generic level past the
stomach, due to further particle reduction. However in the
case of two of the three dugongs, Z. capricorni was identifiable
in the hindgut.
The amount of particle size reduction that was attributable
to mastication by the mouthparts could not be quantified.
However, based on the degree of mastication of digesta
in the cardiac region of the stomach (i.e. all particles were
less than 8 mm in maximum dimension), this was generally
considerable. By examining the changes in median particle
280
Journal of Zoology 270 (2006) 277–289 c 2006 The Authors. Journal compilation c 2006 The Zoological Society of London