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
This paper investigates the degree of breakdown of
seagrass digesta along the gut of dugongs and relates this to
the functional surface area of the mouthparts and nature of
the diet. We examine how dugongs cope with a fibrous plant
diet in the absence of an effective hard dentition.
Methods
Digesta analysis
Digesta samples were obtained from 41 dugongs. These
samples included stomach samples in 10% neutral buffered
formalin from 29 dugongs that had accidentally drowned in
set nets in north Queensland, and now form part of the
Museum of Tropical Queensland specimen collection. Skull
and mouthparts measurements were possible for 28 specimens.
Faecal samples were available from two captive and
five wild animals.
In addition, five fresh whole digestive tracts (four adults
and one calf) were obtained from dugongs that had been
accidentally drowned or hunted in northern Australia during
the course of this study. These tracts were frozen soon
after collection. Digesta samples (up to 250 g) were obtained
from five regions along each complete adult digestive tract
when available: (1) the anterior region of the stomach,
(2) the anterior small intestine (duodenum), (3) the posterior
small intestine, (4) the caecum and (5) the posterior colon
(=faeces). In the calf, digesta samples were available from
the stomach and colon only. Extracted gut contents were
fixed in 10% neutral buffered formalin.
Dietary composition
The dietary composition of each digesta sample was determined
to genus using a micro-stereological technique
(Channells & Morrissey, 1981; Lanyon, 1986). In most
cases, digesta could only be identified from leaf material of
the largest particle size classes (41.6 mm) so that the bulk of
each stomach sample could not be identified. Apparent
dietary composition was expressed in terms of percentage
volume of each identifiable genus of seagrass present.
Particle size distribution
Digesta samples were wet sieved into eight particle size
classes using Endecott sieves with mesh sizes of 4 mm,
1.6 mm, 1 mm, 500 mm, 250 mm, 125 mm and 75 mm. To
alleviate the problem of particles being deformed and forced
through the sieve, water pressure was minimized. Particles
were separated on the basis of maximum dimension. The
particulate content of each sieve was oven dried at 55 1C
overnight to constant weight. Particulate matter from the
washings of the finest sieve was collected through centrifuging
and filtering. It was considered that the amount of
matter filtered through paper with pore size 6 mm was
negligible because the filtrate was clear. All particles from
the largest sieve size, that is 44 mm, had maximum dimensions
of o8 mm.
Mouthparts
The results of the investigation into the morphology and
functional occlusion of the dentition of a total of 57 dugong
specimens have been presented elsewhere (Lanyon & Sanson,
2006). Crown surface areas of cheek teeth from these
dead stranded dugongs from tropical north Queensland
were measured. For 29 of these specimens, digesta contents
were also available (see above). The surface area of each
erupted cheek tooth was digitized in planar view from
tracings made using a Nikon V12 profile projector at 5
magnification under reflected light. These measurements
were considered representative because dugong teeth wear
relatively flat with age. Occlusal or functional surface area
was estimated by subtracting non-functional tooth areas
[i.e. those coated with calculus, partially erupted or nonoccluding
teeth (Sanson, 1980)], from the total tooth surface
area (Lanyon & Sanson, 2006).
For each of 57 specimens, the surface area of the lower
horny pad was measured through approximation to the
surface area of the underlying bony attachment area on the
symphysial part of the mandible (Lanyon & Sanson, 2006).
The anterior edge of the horny pad was defined as running
along the edge of the most anterior incisor socket, and the
posterior edge along the dorsal margin of the deflected
portion of the dentary, that is the posterior end of the lower
incisor socket row (Fig. 1). The surface area of each traced
lower pad was measured digitally on an image analyser.
Because the upper and lower pads work in opposition, the
lower pad was considered to be representative of the functional
pad area. The surface area of mouthparts (dentition
and horny pads) was expressed in terms of age across
specimens from a wide age range (o1 to 46 years; animals
of age o1 year were given an age of 0.5 years) for which
intact skulls were available. The age of each dugong specimen
had previously been estimated by counting the dentinal
growth layer groups in the erupted and non-erupted tusks
(Marsh, 1980).
In vitro breakdown of seagrass
Leaf samples of five seagrasses that form part of the
dugong’s diet [Halophila ovalis, Halodule (narrow-leaf
morph), Halodule uninervis (broad-leaf morph), Cymodocea
serrulata and Zostera capricorni] and three common Australian
terrestrial pasture grasses [Triticum aestivum
(19-day-old wheat), Koeleria setacea and Erharta erecta]
were subjected to in vitro mechanical damage to compare
their susceptibility to fracture. Similar-sized samples of each
of the fresh grasses were placed in 40 mL of 0.1 M phosphate
buffer (pH 6.8) within individual lubricated, thin-walled,
dot-ribbed, supersensitive latex balloons. Each sample was
subjected to 60 min of grinding under constant pressure in a
peristaltic pump fitted with metal rollers. Resultant plant
breakdown was assessed visually and qualitatively. Further,
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Journal of Zoology 270 (2006) 277–289 c 2006 The Authors. Journal compilation c 2006 The Zoological Society of London