The digestive tract of Nautilus pompilius (Cephalopoda, Tetrabranchiata): an X-ray analytical and computational tomography study on the living animal
1 Institut für Allgemeine und Spezielle Zoologie, Bereich
Entwicklungsbiologie, Stephanstrasse 24, D-35390 Giessen, Germany
2 Zentrum für diagnostische Radiologie der Chirurgie,
Justus-Liebig-Universität Giessen, Klinikstrasse 29, 35392 Giessen,
Germany
3 Institut für Geflügelkrankheiten der
Justus-Liebig-Universität Giessen, Frankfurter Strasse 91, 35392 Giessen,
Germany
* e-mail: Bettina.Westermann{at}chemie.bio.uni-giessen.de
Accepted 25 March 2002
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Summary |
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Key words: digestive organ, morphology, histology, three-dimensional reproduction, X-ray examination, computational tomography, Nautilus pompilius, Cephalopoda
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Introduction |
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In contrast to the predatory coleoids, nautiloids are largely scavengers,
using their strongly developed calcified jaw to grind food. Analyses of the
contents of the digestive tract of the benthic-living Nautilus
macromphalus showed, however, that hermit crabs and brachyuran crabs are
their most important food sources (Ward
and Wicksten, 1980).
Previous morphological studies showed that the digestive tract of
Nautilus pompilius is divided into the buccal complex with the
anterior salivary glands, the radula, jaw, crop, oesophagus, stomach,
vestibulum, caecum, midgut gland, midgut and rectum
(Fig. 1). The digestive tract
of the Nautiloidea differs from that of the Coleoidea. It has no posterior
salivary glands and the pancreatic appendages are missing. Within the proximal
part of the midgut, two longitudinal folds, the so-called typhlosolis major
and minor, separate this organ into an inferior and superior part
(Griffin, 1900;
Naef, 1913
;
Bidder, 1966
;
Westermann and Schipp, 1998b
).
Cytological and enzyme-histochemical investigations and tracer experiments
indicate that the midgut gland and the caecum are involved in the absorption
of nutrients (Westerman and Schipp,
1998a
,
1999
;
Westermann et al., 2000
).
However, there is no information about the duration and sequence of the
different phases of digestion. In addition, an analysis of the precise
topography of the digestive tract in dead animals is insufficient because the
soft parts do not retain their exact positions after removal of the shell. To
determine the topography of this organ complex and the duration and dynamics
of a digestive cycle, Nautilus pompilius was investigated using
histological methods and in vivo using X-ray analytical studies and
computational tomography. This approach makes experimental observations
possible without killing the animal.
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Materials and methods |
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X-ray analyses
For the X-ray analytical studies, three juvenile Nautilus
pompilius L. (shell diameter 10-12 cm) from Philippine coastal waters
were used. The animals were kept without food in a closed seawater system for
5 days to stimulate their appetite. The contrast medium barium sulphate was
used to visualize the digestive tract. The medium has a particle size of 1-3
µm and is not water-soluble (Elke et
al., 1982). Each animal was fed with four medium-sized
barium-sulphate-labelled shrimps (Crangon crangon) together
containing 1 ml of concentrated contrast medium. For these investigations, a
special acrylic aquarium (40 cmx25 cmx20 cm,
lengthxwidthxheight) was designed and constructed. At 5, 6, 7, 8
and 9 cm along the aquarium length, a partition could be fixed to constrain
the animal to the front of the aquarium. A plastic stick was placed in the
aquarium for the animal to grip. As the nautiloids usually remained motionless
during the day, adhering to the wall of the aquarium, no anaesthesia was
necessary. During the X-ray analyses, the sea water was maintained at 18-19
°C.
For the X-ray pictures, the animals were restricted to the front of the aquarium using the partition. For lateral-view photographs, the X-ray film (Ektascan DNB for mammography from Kodak) was placed in the aquarium 80 cm from the X-ray unit (Siemens, series number 01120 S 02) behind the partition with a plastic film cover. For top-view photographs, the film was placed at the same distance from the X-ray unit below the aquarium, and the sea water was removed until the animal was just covered. The lateral-view X-ray exposures were taken at 63 kV and 12.6x10-3 C, those from above at 70 kV and 12.6x10-3 C approximately 5 min later. X-ray pictures were taken 20 min after food intake, and at 40 min and 60 min. Thereafter, photographs were taken at hourly intervals until the digestive cycle was finished (12 h).
Computational tomography
For computational tomography, the acrylic aquarium was made smaller (20
cmx25 cmx20 cm, lengthxwidthxheight) to reduce the
volume of sea water causing random noise and beam-hardening effects. The
Nautilus pompilius used in these studies were the same as in the
X-ray analyses. To stimulate their appetite, the animals were kept in a closed
seawater aquarium without food for 5 days. For these experiments, each animal
was also fed with four medium-sized barium-sulphatelabelled shrimps
(Crangon crangon) containing in total 1 ml of concentrated contrast
medium. These investigations were carried out at a water temperature of 18-19
°C. Twelve hours after feeding, scans (slice thickness 1 mm,
94x10-3 A, 140 kV per kernel Ab 82, slice spacing contiguous,
reconstruction interval 1 mm) in frontal and sagittal orientation were
performed using a helical CT scanner (Somatom plus 4; Siemens, Erlangen). The
animal's hard body regions such as the shell, siphuncle channel and jaw show
up as high density. The density of the soft tissue is comparable with
(although slightly higher than) that of the surrounding sea water. The
digestive tract was easily discernible because of the orally administered
contrast material. The image of the shell has to be removed digitally to
isolate the digestive tract.
Using a post-processing tool (Magic view VA 31; Siemens, Erlangen), a region of interest was drawn manually that included the digestive tract and excluded the shell (program edited for three dimensions). Using this data set, a shaded surface display was generated using a surface threshold calculation (lower limit 100 Hounsfield units; upper limit 2500 Hounsfield units; see Fig. 4A-C). To transform the two-dimensional CT scans into the final three-dimensional presentation of the complete animal, for comparison with the X-ray photographs, a special post-processing software was used (Vitrea 2 2, version 2.2 for Windows NT; Vital images, Plymouth, MN, USA). A three-dimensional volume-rendered view in grayscale was obtained that enables the user to see through one structure to visualise another (program, bone CT; three-dimensional, see-through; contrast, see-through bone shading, 0.7; transparency, CT bone surface 50; colour, monochrome; slice thickness, 1 mm; Fig. 4D,E)
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Results |
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X-ray analyses
Using X-ray photographs, the calcified organs of Nautilus
pompilius, i.e. the shell, siphuncle channel, renal appendages and parts
of the contrasted digestive tract, could be demonstrated in a living animal
(Fig.
3A-F,3G-I).
The X-ray photographs show the contrasted crop, stomach, midgut and rectum;
the caecum and the midgut gland were not clearly identified by this method.
The photographs show that the food has entered the stomach 20 min after food
intake and has been reduced to small pieces
(Fig. 3A). Most of the food is
stored in the crop localised below the umbilical region and enlarged to
approximately four times its original size. The chyme reaches the midgut,
including the caecum and the midgut gland, 3 h later
(Fig. 3D). Four hours after
feeding, indigestible food enters the ascending branch of the rectum
(Fig. 3E) and 1 h later the
rectal loop. After 8h, the excrement can be observed at the end of the
descending branch of the rectum (Fig.
3G), and excretion into the mantle cavity occurs after 12 h
(Fig. 3I). The excrement of
these animals is in the form of redbrown threads, 2 cm in length and with no
solid components, so that it disintegrates approximately 36 h after excretion.
The X-ray pictures also show that the branches of the rectum are coplanar and
adjoin the last septum (Fig.
3G-I). The crop and the stomach still contain food 12 h after food
intake. In addition, the X-ray photographs reveal an enormously dynamic
digestive tract. The stomach produces strong contractions and dilations, and
undergoes changes in position during the digestive process depending on its
volume (Fig.
3B,H).
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Computational tomography
Using computational tomography, the exact position of the digestive tract
of Nautilus pompilius could be reconstructed from a living animal
(Fig. 4). This method enables a
threedimensional image of this organ complex to be produced. The shell with
the siphuncle was removed digitally and is therefore not visible in the
three-dimensional reproduction (Fig.
4A-C). As in the X-ray photographs, the tomograms show the jaw,
the renal appendages and parts of the contrasted digestive tract; the caecum
and the midgut gland and other soft parts could not be depicted. The
computational tomography results confirm the results of the X-ray study in
which the crop and the stomach are still filled with food after 12 h
(Fig. 4).
The results of the morphological topography show that the digestive tract of Nautilus pompilius consists of the buccal cavity, the foregut, which is widened to a crop, the stomach, the vestibulum, the caecum, the midgut gland, the midgut and the rectum (Fig. 5). The caecum connects to the midgut gland via the paired ductus hepatopancreas. The foregut narrows before entering the stomach. The view from the dorsal side of the animal shows that the stomach is situated on the left and the caecum on the right side, connected via the vestibulum. Before entering the mantle cavity, the rectum forms a loop that lies dorsal to the caecum and adjoins the last septum (Fig. 5).
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Discussion |
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Descriptions of the digestive tract by previous authors based only on
morphological studies of dissected material contain some errors. Griffin
(1900) describes the position
of the vestibulum as between the oesophagus and the entrance to the stomach.
Furthermore, he describes the ductus hepatopancreas as having three openings.
In reports by Naef (1913
), the
digestive tract of Nautilus pompilius is represented as a short tube
without the complex rectum. The caecum and the midgut gland were not visible
in the X-ray photographs or the computational tomograms because the contrast
medium, barium sulphate, cannot be absorbed and therefore did not penetrate
the absorptive organs because it is not water-soluble
(Elke et al., 1982
). These
results confirm previous investigations that identified the caecum and the
midgut gland as the organs of nutrient absorption
(Westermann and Schipp, 1999
;
Westermann et al., 2000
).
The advantages of X-ray analysis and computational tomography compared with
morphological and histological methods alone are that the digestive tract is
visible in its unaltered position within the living animal. Furthermore, the
different phases of digestion can be determined. Previous studies on the
digestive mechanisms of coleoid cephalopods were carried out on animals killed
for this purpose at defined intervals after food intake
(Bidder, 1950;
Boucher-Rodoni, 1973
;
Boucher-Rodoni and Mangold,
1977
). Using X-ray analyses and computational tomography, the
exact topographic position of the renal appendages of Nautilus
pompilius were also identified because of their high content of calcium
compounds (Crick et al.,
1985
). These organs insert ventrally on the base of the four
afferent branchial vessels (Kefersein,
1866
; Vigelius,
1880
; Griffin,
1900
; Naef, 1913
)
and are described as a mineral-storage tissue found only in cephalopod species
that have shells composed of calcium compounds
(Schipp and Martin, 1981
).
Duration of digestion
The present investigations show that the cycle of digestion in Nautilus
pompilius takes 12 h at a water temperature of 18-19°C, which is
approximately the time reported for the necto-benthic sepioids and benthic
octopods (Table 1). In
Octopus vulgaris, digestion takes 12 h at the same temperature
(Boucher-Rodoni and Mangold,
1977) and in Sepia officinalis, a necto-benthic species,
the duration of digestion is 15 h at a water temperature of 20°C
(Boucaud-Camou, 1973
). However,
in the actively swimming predator Loligo vulgaris, the digestion
cycle takes only 4-6 h at 18°C and is thus faster than in sepioids,
octopods and nautiloids (Bidder,
1950
). These results suggest that the benthic mode of life of
nautiloids influences their rate of digestion. Within a species, the duration
of digestion is dependent upon temperature; e.g. in Eledone cirrhosa,
the duration is 15 h at 20°C, 20 h at 15°C and 30 h at 10°C
(Boucher-Rodoni, 1973
;
Wells, 1978
). Investigations
on the digestive tract of coleoid cephalopods indicate that sex and stage of
maturation can also influence the duration of digestion: the passage of chyme
is faster in immature than in mature males and is faster in males than in
females (Boucher-Rodoni and Mangold,
1977
).
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The present investigation provides no information of the efficiency of the
digestive process. To obtain this information, the rate of digestion must be
calculated as ingested food (as a percentage of body mass) divided by
digestion duration (in h) (Wells,
1978). Wells showed that the rate of digestion in coleoid
cephalopods was at least as efficient as in their fish competitors.
Course of digestion in nautiloids
Nautilus pompilius grinds its food into small pieces using its
strongly developed jaw; the food is coated with mucus in the buccal cavity and
then rapidly moved into the stomach by peristaltic movements of the foregut.
In loliginids and octopods, the passage of food into the stomach is also very
fast and slows down progressively further along the gut
(Bidder, 1950;
Boucher-Rodoni and Mangold,
1977
). Nautiloids can store food in the crop while the first part
of the meal is digested, as can octopods. In benthic species, storage of food
in the digestive tract and the ability to fast for several weeks represent
adaptations to extended periods of starvation, whereas nectonic species will
migrate in search of food (Boucaud-Camou,
1987
). The X-ray photographs and tomograms in the present study
show that 12 h after food intake the crop and the stomach still contain food,
indicating that the food moves along the gut in portions. In the stomach, the
food is degraded by enzymes and by the mechanical activity of the organ
itself. The chyme is then transported to the midgut gland, passing through the
caecum. These organs were identified as the sites of nutrient absorption by
previous tracer studies (Westermann and
Schipp, 1999
; Westermann et
al., 2000
). Indigestible debris is transported directly from the
stomach to the rectum via the division of the vestibulum and proximal
midgut. The mechanism by which chyme and indigestible debris are separated in
the vestibulum in Nautilus pompilius is still unclear. In coleoid
cephalopods, sphincters enable the substances present in the stomach to enter
the caecum or the intestine, but again the mechanism is still unclear
(Boucaud-Camou and Boucher-Rodoni,
1983
).
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Acknowledgments |
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