This document describes the gross morphology and topography of the intestinal tract of the Nile tilapia (Oreochromis niloticus). The intestinal tract follows a complex path involving multiple loops and coils. Five main regions were identified: 1) the hepatic loop, 2) the proximal major coil, 3) the gastric loop, 4) the distal major coil, and 5) the terminal segment. The proximal and distal major coils form a spiral, cone-shaped mass nested inside each other. Though the topography allowed identification of regions, the external features were similar, making excised segments difficult to identify by region. The nested spiral pattern of the tilapia intestine is novel compared to other fish.

1. Original Paper
Cells Tissues Organs 2000;166:294–303
Gross Morphology and Topography of
the Adult Intestinal Tract of the
Tilapian Fish, Oreochromis niloticus L.
Bonnie J. Smitha Stephen A. Smitha Bundit Tengjaroenkula
Terry A. Lawrenceb
aDepartment of Biomedical Sciences and Pathobiology and bBiomedical Media Unit, Virginia-Maryland Regional
College of Veterinary Medicine, Virginia Polytechnic Institute and State University, Blacksburg, Va., USA
Accepted after revision: April 27, 1999
Bonnie J. Smith
Department of Biomedical Sciences and Pathobiology
Virginia-Maryland Regional College of Veterinary Medicine
Blacksburg, VA 24061-0442 (USA)
Tel. +1 540 231 9024, Fax +1 540 231 7367, E-Mail bjsmith@vt.edu
ABC
Fax + 41 61 306 12 34
E-Mail karger@karger.ch
www.karger.com
© 2000 S. Karger AG, Basel
1422–6405/00/1663–0294$17.50/0
Accessible online at:
www.karger.com/journals/cto
Key Words
Intestine W Morphology, gross W Tilapia W Fish W
Oreochromis niloticus
Abstract
The intestinal tract of the Nile tilapia, Oreochromis niloti-
cus L., follows a complex course involving multiple loops
and coils arranged in a previously undescribed form.
From cranial to caudal, five principal regions were identi-
fied and designated as the hepatic loop (HL), proximal
major coil (PMC), gastric loop (GL), distal major coil
(DMC), and terminal segment (TS). The first four of these
regions each possessed a reversal flexure and thus could
be divided into proximal and distal limbs. Only the termi-
nal segment was straight and undivided. The PMC and
DMC were disposed in a spiral, cone-shaped mass (spiral
intestine) – their proximal and distal limbs are thus desig-
nated as centripetal and centrifugal limbs. These spiral
limbs were arranged with each successive limb nested
internal to the previous one. Beginning from the stom-
ach, the complete course of the gut including designa-
tions of the subdivisions of the major regions was as fol-
lows: proximal limb of the HL, distal limb of the HL, cen-
tripetal limb of the PMC, centrifugal limb of the PMC,
proximal limb of the GL, distal limb of the GL, centripetal
limb of the DMC, centrifugal limb of the DMC, and the TS.
Though the topographical relations of the various gut
loops permitted ready identification of each, external
surface features were so similar among the segments
that extirpated segments of gut could not be identified as
to region of origin. The nesting of successive intestinal
loops of the spiral intestine in this fish is novel among
patterns previously described, and also among the more
intricate of those that have been described.
Copyright © 2000 S. Karger AG, Basel
Abbreviations used in this paper
DCfL distal centrifugal limb PCfL proximal centrifugal limb
DCpL distal centripetal limb PCpL proximal centripetal limb
DMC distal major coil PMC proximal major coil
FL fish length TIL total intestinal length
GL gastric loop TS terminal segment
HL hepatic loop

2. Gross Morphology of Tilapian Intestine Cells Tissues Organs 2000;166:294–303 295
Introduction
Fish are a remarkably diverse taxonomic group, con-
sisting of approximately 20,000 living species [Lagler et
al., 1975]. Utilization of a wide spectrum of niches pro-
viding a tremendous variety of food items is reflected in
extensive variation in the complexity and disposition of
the fishes’ alimentary canal. Indeed, the structural diver-
sity of the intestine among fish has been described as
greater than that in any other vertebrate group [Stevens,
1988]. Earlier reviews addressing these features include
Kapoor et al. [1975], Stroband and Debts [1978], Sis et al.
[1979], Clark and Whitcomb [1980], and Ezeasor and Sto-
koe [1980, 1981]. Much of the variation in alimentary
tract morphology is expressed in the intestine, which
ranges from a simple tube with little or no coiling in spe-
cies such as salmon and trout, through moderate to com-
plex degrees of looping and/or coiling in species such as
carp and ocean sunfish [Suyehiro, 1941; Harder, 1975].
Tilapian fish are members of the large and diverse
family Cichlidae. Though gross intestinal morphology has
been examined in several members of this family [Ya-
maoka, 1985, Reinthal, 1989], tilapia are a large group of
cichlid fish in which intestinal morphology has not been
entirely described. Tilapia are currently utilized in several
manners which raise interest in their anatomy beyond the
self-evident academic interest: tilapian fish figure promi-
nently worldwide in aquaculture [Ackefors et al., 1994],
and are also proving notable as a laboratory animal [Hart
et al., 1997; Augusto et al., 1996]. Thus, a detailed charac-
terization of the normal gross morphological features of
the intestinal tract of tilapia will be of interest to a wide
segment of the scientific community. This study begins
such characterization by describing the complex gross
anatomy of the intestinal tract in mature individuals of
the Nile tilapia, Oreochromis niloticus.
Methods
Determination of in situ Relations
Two 23-cm (both male) and eight 12- to 13-cm fish (4 males and
4 females) were used to determine the in situ emplacement of the
intestinal tract. Following a 12-hour fast, each fish was anesthetized
using tricane methanesulfonate (MS-222, Sigma Chemical Co., St.
Louis, MO, USA) and killed by cervical separation. The right and left
body walls covering the body cavity were removed and the organs
examined in situ from each side (fig. 1).
Identification of Intestinal Regions
Fourteen adult fish (8 male and 6 female) ranging in size from
12.5 to 18.5 cm were examined. The digestive visceral mass and asso-
ciated organs (esophagus, stomach, spleen, liver, gall bladder and
intestine) were removed en masse by cutting the esophagus as far
cranially as possible and cutting the intestine caudally where it joined
the body wall. The tight mesenteric attachments within the visceral
mass permitted its easy removal from the body cavity without dis-
turbing topographical relations among its various portions. The
entire visceral mass was immediately immersed in 10% neutral buff-
Fig. 1. In situ position of the visceral mass
within the body cavity, left view. From this
aspect, the liver (L), the proximal (PH) and
distal (DH) limbs of the intestinal HL, the
PCpL of the PMC (Cp 1), and the intestinal
TS (T) are visible. Note the fat lying between
the intestinal coils. The gall bladder (GB)
and gonads (G) are also visible. White ar-
rows indicate direction of ingesta flowing
toward the center of the first major coil. Bar
= 2 cm.

3. 12.5
296 Cells Tissues Organs 2000;166:294–303 Smith/Smith/Tengjaroenkul/Lawrence
ered formalin. To promote fixation of the delicate intestinal wall ade-
quate to permit extensive manipulation with minimal breakage, the
mass was fixed at room temperature for a minimum of 3 days. The
fixative was changed on the 2nd day.
After fixation, topographical relations were first documented
with the digestive visceral mass and associated organs intact. Subse-
quently, each digestive tract was dissected to determine features and
relations of the inner coils. Beginning at the stomach, the intestine
was progressively dissected free from its mesenteric attachments to
other surrounding structures. As regions of the tract were identified,
length measurements of each were made.
Table 1. Sequential organization of the five
main regions and their subdivisions of the
intestinal tract of O. niloticus
Hepatic loop
Proximal limb
Distal limb
Spiral intestine
Proximal major coil
Proximal centripetal loop
Proximal centrifugal loop
Gastric loop
Proximal limb
Distal limb
Distal major coil
Distal centripetal loop
Distal centrifugal loop
Terminal segment
Statistical Analysis
The means, standard deviations, and ranges of the fish length and
total intestinal length (TIL) were determined. The lengths of the indi-
vidual intestinal segments as well as the percentage of the TIL con-
tributed to by each segment were also determined. Linear regressions
of the TIL, individual segment length, and percentage of segments on
fish length were calculated. Intestinal segment lengths were evaluated
for gender differences by analysis of variance with inclusion of the
continuous variable fish length as a covariate in the model. Evalua-
tions were performed using the SAS (Sas System for Windows,
Release 6.12; SAS Institute, Cary, NC) for all analyses. Specifically,
the UNIVARIATE, REG, CORR (Pearson and Kendall’s tau-b) and
GLM procedures of this system were used in the evaluations.
Results
Table 1 presents the sequential organization of the five
main intestinal regions and their subdivisions in O. niloti-
cus. Table 2 shows the number, sex, and size of individual
fish examined, as well as the lengths of individual intesti-
nal segments and the percentage of TIL for each segment.
Table 3 displays the mean, standard deviation, and mini-
mum/maximum of fish length and individual intestinal
segment lengths, as well as those values for percentage of
TIL provided by individual intestinal segments.
The general form of the intestine was an unadorned
tube, with pyloric ceca entirely absent. Surface features of
the various gut regions were essentially similar among all
regions identified. Though a faint tendency was present
Table 2. Intestinal tract in O. niloticus:
length of individual segments, total length,
and individual segments’ percentage of
the TIL
Fish
length
cm
Fish
sex
HL
cm
PMC
cm
GL
cm
DMC
cm
TS
cm
TIL
cm
m 65 (21) 109 (35) 26 (8) 87 (28) 21 (7) 308
12.5 f 72 (22) 115 (35) 25 (8) 88 (27) 28 (9) 328
13:0 f 52 (17) 131 (41) 18 (6) 87 (28) 27 (9) 315
13.0 m 51 (17) 109 (36) 21 (7) 95 (32) 25 (8) 301
13.0 f 52 (18) 121 (41) 24 (8) 80 (27) 19 (6) 296
13.2 f 59 (18) 125 (37) 25 (7) 99 (30) 26 (8) 334
14.8 f 74 (78) 129 (12) 58 (14) 141 (34) 20 (5) 402
15.0 f 66 (22) 103 (33) 35 (12) 77 (26) 12 (4) 314
15.5 m 49 (16) 107 (35) 50 (16) 96 (33) 12 (4) 294
15.7 m 57 (17) 124 (37) 41 (11) 96 (29) 14 (4) 332
16.0 m 65 (20) 98 (31) 45 (14) 89 (28) 21 (7) 318
16.0 m 74 (16) 154 (34) 52 (12) 139 (31) 32 (7) 451
16.0 m 70 (17) 135 (33) 54 (13) 125 (31) 24 (6) 408
18.5 m 69 (17) 138 (34) 70 (17) 106 (26) 25 (6) 408
Numbers in parentheses report the percentage of a given intestinal segment of the TIL in
individual fish.

4. Fish length, cm
Gross Morphology of Tilapian Intestine Cells Tissues Organs 2000;166:294–303 297
Fig. 2. In situ position of the visceral mass
within the body cavity, right view. From this
aspect, the centripetal limb of the PMC (Cp
1), two centrifugal limbs of the PMC (Cf 1),
the medial surface of the HL curving around
the caudal border of the liver, and the intesti-
nal TS (T) are visible. The cranial extremity
of the liver (L) is also visible. Note the fat
lying between the intestinal coils. White ar-
rows indicate direction of ingesta flowing
toward the center of a coil, while black
arrows indicate flow away from a center. Bar
= 2 cm.
for more distal regions of the gut to be slightly smaller in
diameter than more cranial regions, this feature was both
inconsistent and also greatly affected by the presence or
absence of fixed peristaltic waves along the gut wall. Short
segments excised from any one region of the gut were
essentially indistinguishable from any other. Thus, the
sole reliable means of precise identification of a particular
gut region depended upon the ability to visualize its topo-
graphical relations to other gut regions and/or other
organs.
Five principal gross regions of the intestinal tract were
identified. Progressing caudally from the stomach, these
were designated as: (1) hepatic loop (HL); (2) proximal
major coil (PMC); (3) gastric loop (GL); (4) distal major
coil (DMC), and (5) terminal segment (TS) (fig. 1–6). The
first four of these regions each possessed a reversal flexure
and thus could be divided into proximal and distal limbs.
Only the TS was straight and undivided.
The intestine exited the cranial region of the stomach
at an acute angle, adjacent to the esophagus and to the left
of the midline (fig. 4, 6). The intestine immediately
turned dorsally and entered the HL. The proximal limb of
the HL coursed caudally closely following the dorsal
hepatic border, reflected cranially around the caudal edge
of the liver, and continued cranially as the distal limb of
the HL (fig. 1, 3, 4, 6). At a level approximately even with
the stomach, the distal limb of the HL turned medially
and entered the spiral region of the intestine (fig. 1, 3,
Table 3. Fish length, intestinal segment length, and percentage of
TIL contributed to be individual intestinal segments: means, stan-
dard deviations, and minima/maxima
Variable Mean SD Minimum Maximum
14.6 1.8 12.5 18.5
HL
cm 62.5 9.0 49.0 74.0
% of total 18.3 2.0 16.4 22.0
PMC
cm 121.3 15.5 98.0 154.0
% of total 35.5 3.1 30.8 41.6
GL
cm 38.9 16.3 18.0 70.0
% of total 11.1 3.8 5.7 17.2
DMC
cm 100.4 20.5 77.0 141.0
% of total 29.1 2.8 24.5 35.1
TS
cm 21.9 6.1 12.0 32.0
% of total 6.4 1.6 3.8 8.6
TIL, cm 343.5 51.0 294.0 451.0

5. 298 Cells Tissues Organs 2000;166:294–303 Smith/Smith/Tengjaroenkul/Lawrence
Fig. 3. Visceral mass removed from the
body cavity, superficial surface, left view.
The proximal (PH) and distal (DH) hepatic
loops are visible in their course around the
liver (L). The PCpL (Cp 1) and PCPL (Cf 1)
of the PMC of the spiral intestinal region are
plain. Note the fat between the intestinal
loops. The TS (T) of the intestine and the gall
bladder (GB) are visible. White arrows indi-
cate direction of ingesta flowing toward the
center of a coil, while black arrows indicate
flow away from a center. Bar = 5 cm.
Fig. 4. Visceral mass removed from the
body cavity, superficial surface, right view.
The dorsal exit at an acute angle of the proxi-
mal HL (PH) from the stomach (S), the GL,
three PCpL of the PMC (Cp 1), and a por-
tion of the intestinal TS (T) are visible. The
liver (L) and spleen (Sp) are also demon-
strated. Note how the proximal HL follows
the dorsal border of the liver. Also note that
the GL emerges from the left side, ap-
proaches the stomach, and then reverses di-
rection to regain the left side and reenter the
spiral intestinal region. The esophagus (E)
and gall bladder (GB) are also visible. White
arrows indicate direction of ingesta flowing
toward the center of a coil, while black
arrows indicate flow away from a center. Bar
= 4 cm.
table 1). The spiral intestinal region was disposed as a
truncated cone lying partially to the left of the midline and
oriented dorsoventrally, with its base dorsally and apex
ventrally (fig. 1–4). The spiral intestine comprised the
PMC and DMC, each with its own centripetal and centri-
fugal limbs (table 1, fig. 4, 6).
The PMC was positioned superficially (fig. 1–4). This
coil began with the proximal centripetal limb (PCpL) that
descended on the outside of the cone, requiring three to
four turns to gain its apex (fig. 1–4). The PCpL then
passed through a U-turn and, now as the proximal centri-
fugal limb (PCfL), ascended the cone deep (internal) to
the previous segment (fig. 2–4). Glimpses of this (deeper)
PCfL were occasionally visible in the intact mass between
loops of the proximal centripetal coil (fig. 2). The PCfL
also required three to four turns to traverse the cone.
On gaining the dorsal border of the cone, the PCfL
entered the GL (fig. 4, 6). The proximal limb of the GL
3
4

6. Gross Morphology of Tilapian Intestine Cells Tissues Organs 2000;166:294–303 299
Fig. 5. Visceral mass removed from the
body cavity, deep structures, left view. The
HL and PMC (PCpL and PCfL) have been
removed. The liver has been slightly dis-
placed dorsally to demonstrate the GL. The
entrance from the proximal centrifugal coil
(Cf 1) into the GL has also been displaced to
simplify its demonstration. Note again how
the gastric loop approaches the stomach (S)
and then reverses direction. In this view, the
continuation of the distal limb of the GL
into the initial portion of the DCpL (Cp 2, of
the DMC) is evident. Similarly, the origina-
tion of the DCfL of the DMC (Cf 2) from the
reversal flexure, and its continuation up the
spiral intestinal region is also demonstrated.
The gall bladder (GB) and liver (L) are visi-
ble. White arrows indicate direction of inges-
ta flowing toward the center of a coil, while
black arrows indicate flow away from a cen-
ter. Bar = 4 cm.
Fig. 6. Visceral mass removed from the
body cavity, deep structures, right view. The
HL and PMC (PCpL and PCfL) have been
removed. The exit of the proximal HL (PH)
from the stomach (S) at an acute angle is visi-
ble, as is the initial course HL along the dor-
sal border of the liver (L). The GL is shown
overlying the structures from the opposite
side. The disposition of coils in this area
makes following direction of the intestine
difficult. The distal end of the PCfL of the
PMC (Cf 1) is shown entering the GL
(ghosted in on the opposite side of the stom-
ach). The DCpL of the DMC (Cp 2) contin-
ues from the GL back to the left side, to enter
the spiral intestinal region. The DCfL (Cf 2)
of the DMC are also visible, as is the intesti-
nal TS (T). E = Esophagus; GB = gall blad-
der. White arrows indicate direction of in-
gesta flowing toward the center of a coil,
while black arrows indicate flow away from a
center; open arrows indicate flow in intesti-
nal segments lying deep to the stomach and
GL in this view. Bar = 4 cm.
5
6
turned cranially and progressed until it nearly touched the
stomach. At this point it passed through a U-turn and par-
alleled its own proximal limb to reenter the dorsal portion
of the spiral region by becoming continuous with the
DMC.
Both limbs of the DMC lay entirely internal to the limbs
of the PMC. The DMC began with the distal centripetal
limb (DCpL) (fig. 5, 6), which descended just deep to the
PCfL. Being more internal, the turns of the DCpL were
tighter than those of the preceding limbs. The DCpL typi-
cally required three turns to traverse the cone. The DCpL
passed through a U-turn at the cone’s apex, and continued
as the distal centrifugal limb (DCfL) (fig. 5, 6). This rever-
sal flexure of the DMC was intimately nested with the
reversal flexure of the PMC. From this reversal flexure, the
DCfL ascended to the dorsal surface of the cone by passing
through three turns internal to the DCpL. Being the most
internal of the four spiral limbs, the turns of the DCfL were

7. 300 Cells Tissues Organs 2000;166:294–303 Smith/Smith/Tengjaroenkul/Lawrence
the tightest of all regions of the spiral intestine. From here,
the intestine turned caudally, straightened, and gradually
extended ventrally en route to the anus as the TS (fig. 1–6).
The cranial region of the TS paralleled and lay in direct
contact with the dorsal border of the hepatic limb. A fold of
mesentery attached the TS to the liver.
Statistical analysis showed that male fish in this study
(n = 8, mean length = 15.4 cm, SD = 1.9 cm) tended to be
longer than female fish (n = 6, mean length = 13.6 cm,
SD = 1.0 cm, p = 0.06). However, after adjustment for fish
length, gender did not affect either length of intestinal seg-
ments or percentage of individual segments of TIL. No
effect of fish length on the length of most intestinal seg-
ments (HL, PMC, DMC and TS) was observed. However,
TIL [r2 = 0.36, p = 0.02; TIL = 95.2 + 17.0 ! fish length
(FL)] and length of the GL (r2 = 0.83, p ! 0.01; GL = 82.1
+ 8.3 FL) increased with fish length. The percentage of
TIL contributed to by the PMC (r2 = 0.28, p = 0.052) had
a tendency to decrease, and the TS (r 2 = 0.76, p ! 0.01)
absolutely increased.
Discussion
Tilapia are similar to most other species of fish in lack-
ing gross surface distinction among various intestinal
regions. Only the tight mesenteric attachments and subse-
quent constant topographical relation of one area to
another permitted immediate identification of the var-
ious regions in the intact or partially dissected intestinal
mass.
The absence of intestinal ceca in the Nile tilapia may
relate to their herbivorous/omnivorous diet. Though in-
testinal ceca are present among various species of fish
including both carnivores and herbivores, ceca tend to be
better developed in carnivorous than herbivorous fish,
and best developed in carnivorous fish with short intesti-
nal length [deGroot 1971]. The role of intestinal ceca has
long been debated, with numerous functions having been
suggested including absorption, fermentation, storage,
and breeding sites for gut microbiota [Saddler and Ashley,
1960; Reifel and Travill 1978, 1979]. However, cecal
function has subsequently been more clearly demon-
strated as absorptive and similar to that of the cranial por-
tion of the intestine. Buddington and Diamond [1986]
provided convincing evidence that the ceca serve an
absorptive function similar to that of the cranial intestine
by demonstrating an essentially similar uptake of various
nutrients in the ceca as in the cranial intestine. More qual-
itatively, Hossain and Dutta supported these findings by
first [1988] showing that the ceca develop from mucosal
folds of the intestine rather than the stomach (indicating
their derivation from an intestinal region dealing with
absorption), and later [1992] by demonstrating that the
expansive capacity of intestinal ceca is minimal (which
rules out a prominent role in food storage). Given that
intestinal ceca likely function mainly to increase the
absorptive area of the gut, the possession of a long intes-
tine by O. niloticus would obviate the need for ceca.
Indeed, when Buddington and Diamond [1986] pointed
out gut absorptive area of fish increases with either length-
ening the gut or adding ceca, they noted ‘tilapia’ (without
mentioning genus name) as an example of the former.
Apparently the length of the continuous tubular portion of
the Nile tilapia’s gut accommodated by the intricate coil-
ing of the intestine provides sufficient surface area to
enable the fish to derive adequate nutrition from its herbi-
vorous/omnivorous diet.
The percentage of total length of a given intestinal seg-
ment displayed fair consistency among adult O. niloticus
of varying size (table 2). The GL was the only segment
that departed notably from this generalization. In smaller
fish (12–13 cm), the GL averaged about 7% of the TIL,
while in larger fish (15–23 cm) the GL was significantly
longer, averaging 13.5% of the TIL. In addition to propor-
tionate change, statistical evaluation showed the increase
in absolute length to be significant. The longer GL in larg-
er fish contributed to the other length change observed,
which was significantly longer TIL in larger fish. Though
other features of length demonstrated no significant dif-
ferences, a tendency was observed for the proportionate
length of the PMC and TS to decrease. The increase in the
absolute and percentage length of the GL likely contrib-
uted to this effect.
Among fish (as also among most other vertebrate spe-
cies), greater intestinal length and more complex disposi-
tion of the gut tube is generally characteristic of herbi-
vores and onmivores, while shorter intestinal length and
simpler disposition usually typifies carnivorous species
[Al-Hussani, 1949; deGroot, 1971; Harder, 1975; Reifel
and Travill, 1979; Zihler, 1982; Geevarghese, 1983; Rib-
ble and Smith, 1983; Korovina et al., 1991; Menin and
Mimura, 1992; Kramer and Bryant, 1995]. Considerable
overlap in TIL exists among these trophic groups. Harder
[1975], summarizing the work of Jacobshagen [1915],
reported that intestinal length relative to body length in
carnivorous fish generally ranges between 0.2 and 2.5
times body length, in omnivorous fish, between 0.6 and
8.0 times body length, and in herbivorous fish, between
0.8 and 15.0 times body length. The Nile tilapia thus falls

8. Gross Morphology of Tilapian Intestine Cells Tissues Organs 2000;166:294–303 301
within the range characteristic of herbivorous fish, albeit
nearer the shorter end. The relatively short length of the
tilapian intestine as compared to other herbivorous fish
likely relates to the tilapian’s well-known ability to readily
adapt to different foodstuffs: though adult tilapia are
largely herbivorous, they are quite opportunistic and free-
ly make insects and crustaceans a significant part of
their diet [Philippart and Ruwet, 1982; Trewavas, 1983;
Wheeler, 1985].
Patterns of intestinal looping are constant within, but
vary tremendously among, the thousands of fish species
[Mohsin, 1962; Harder, 1975; Kapoor et al., 1975; Reifel
and Travill, 1979; Zihler, 1982; Reinthal, 1989]. Coiling
of the intestinal tube in most fish is relatively simple, typi-
cally exemplified as a simple sigmoid curve, a screw-type
spiral, or a flat disc with all coils occupying the same plane
[Suyehiro, 1941; Harder, 1975]. Even in fish character-
ized by a spiral colon, such as the bitterling (Rhodeus seri-
ceus amarus), the spiral typically consists of a single cen-
tripetal limb passing into and a single centripetal limb
extending out of the spiral, with no loops that leave and
then return to the spiral [Harder, 1975]. Certain other
species such as the mullet (Mugil cephalus) and the gold-
fish (Carassius auratus) [Harder, 1975] and the algal feed-
ing cichlids Cyathopharynx lucifer and Ophthalmotilapia
nasuta [Yamaoka, 1985] show a more complex looping
pattern, but such instances are not as common.
Zihler [1982] described the gross morphology and con-
figuration of the digestive tracts of 71 species of cichlid
fish (not including Oreochromis). Allowing for variation
related to general body form and feed preferences, he con-
sidered the general morphological characteristics of the
digestive tract of the family Cichlidae as synapomorphic.
In many respects, O. niloticus conforms with the pattern
described by Zihler as characteristic of cichlid fish. Such
features include the extensible blind pouch of the stom-
ach, the exit of the intestine from the left side of the stom-
ach, and the first intestinal loop lying on the left side of
the body. Zihler [1982] described several forms of intesti-
nal looping among cichlids, ranging from short and simple
to lengthy and complex, again related to diet. Compared
to these patterns, the looping pattern of O. niloticus is
most similar to the coiling pattern identified by Zihler as
type H and exemplified by the African cichlid, Sarothero-
don mossambicus. Both O. niloticus and S. mossambicus
possess a cone-shaped mass of intestines and a relatively
short gastric loop (referred to by Zihler as the ‘flap-back
loop’). However, S. mossambicus does not demonstrate
the doubled form of the spiral colon (i.e., does not possess
both the proximal and distal major coils).
Among the species described by Zihler [1982], cichlid
fish were characterized as possessing a form of intestinal
loop arrangement he defined as convoluting. In the con-
voluting form, the intestine leads once into a spiral and
once out. The intestinal morphology of the Nile tilapia [as
well as certain other cichlids; Reinthal, 1989], however,
departs from this pattern and instead more closely follows
a pattern described by Zihler [1982] as coiling. Zihler
[1982] described the coiling form of intestinal disposition
as leading twice into a spiral region and twice out, with the
reversal flexures of the two sets of loops being closely
nested into each other.
Complex patterns of intestinal coiling have indeed
been previously characterized in cichlid fish. In addition
to characterizing some of these patterns, Yamaoka [1985]
also demonstrated that their ontogeny can be used to
determine phylogenetic relationships among some spe-
cies. Reinthal [1989] described the intestinal morphology
of six genera and 16 species of cichlid fish (not including
Oreochromis). His study confirmed that longer intestinal
length is associated with a diet high in plant material (as
well as in species feeding mainly on detritus), but also
described an intestinal coiling pattern similar to that of
O. niloticus. Most species he described possessed an intes-
tine that passed into, then out of a spiral, through a loop
related to the stomach (termed by him the ‘haplochrom-
ine’ loop, i.e. the ‘flap-back’ loop of Zihler and the gastric
loop of this study), then back into and out of the spiral
region again. This pattern of coiling was largely similar
among most species he examined, with the main varia-
tions he observed lying in the number of loops in each
coil. Thus, O. niloticus fits the general pattern described
by Reinthal as characteristic of cichlids. However, O. nilo-
ticus is unique in the manner of disposition of the spiral
intestine’s coils. The quadruple-looped spiral intestine of
the Nile tilapia, with each successive loop lying internal to
the preceding loop, was not included in Reinthal’s de-
scriptions. Thus, though adhering to certain generalized
features of the cichlid family, O. niloticus also nonetheless
presents certain striking unique features as well.
The coiling of the intestinal tract in the Nile tilapia
plainly achieves the self-evident advantage in accommo-
dating into the body cavity an intestinal tract many times
longer than the cavity itself. However, an intricate pattern
of looping and coiling is unnecessary for this goal, which
can be achieved as simply and uncomplicatedly as suspen-
sion of unfixed lengthened intestinal loops from a single
long mesentery, as in the jejunum of many other verte-
brates. Thus, the complex nature of the Nile tilapia’s
intestinal coiling pattern suggests that some goal other

9. 302 Cells Tissues Organs 2000;166:294–303 Smith/Smith/Tengjaroenkul/Lawrence
than this most apparent feature may also be served. One
possible basis could be that, beyond simply adding length
to the gut, the topographical disposition of the loops in
some manner favors absorption of nutrients from the gut.
Characteristics that would slow passage of ingesta through
the intestine and/or increase the efficiency of nutrient
absorption from the intestinal wall would contribute to
this effect. The Nile tilapian intestine passes through four
reversal flexures. Three of these flexures (between the two
parts of the PMC, GL and DMC) are extremely acute,
180° turns. Flow rate would necessarily be slowed in order
to complete these turns. Also, the course followed by the
ingesta through the three acute reversal flexures as well as
through the immediately succeeding segment of gut re-
quires ascending against the pull of gravity, which could
also tend to slow the rate of ingesta passage. As an addi-
tional if less likely possibility, the exact paralleling of the
proximal and distal limbs of the PMC and DMC (as well
as of the GL) together with the opposite direction of inges-
ta flow within each of these respective limbs suggest some
chance of some form of counter- or cross-current mecha-
nism in these regions. Unpublished results of studies
involving distribution of various digestive and absorptive
enzymes through the length of the Nile tilapian gut show
that digestive and absorptive enzymes are indeed plenti-
ful in the PMC and GL, and also present in the DMC,
though they are lacking in the TS. Investigations as to the
nature of the blood supply to these regions may provide
insight as to whether such mechanisms are indeed at work
in these areas.
In summary, the intestinal tract of O. niloticus was
characterized by a series of loops set in a constant and
intricate pattern that is both unique among species de-
scribed to date, and also one of the more complex patterns
reported in fish. The intestine departed the stomach, fol-
lowed the elongate borders of the liver, and entered the
spiral region of the intestine. The spiral intestine con-
sisted of two paired major coils (proximal and distal),
each of which comprised a centripetal and centrifugal
loop. The short gastric loop (‘flap-back’ or haplochromine
loop) was interposed between the two major coils. The ter-
minal segment of the intestine departed the spiral region
and followed a straight course to the anus.
Possession of an intestine of a length greatly exceeding
that of the body cavity as well as the disposition of that
elongated gut into loops or coils of some sort, as possessed
by O. niloticus, are characteristic of adult herbivorous fish
in general. With a TIL approximating 2.5 times the total
body length, the Nile tilapia falls nearer the shorter end of
the range of 0.8–15 times body length characterizing most
herbivorous fish. This characteristic may reflect the fish’s
adaptability in diet, which can be readily modified from
its typical diet of phytoplankton to include animal food in
the form of crustaceans.
Acknowledgments
The authors would like to thank the Commercial Fish and Shell-
fish Technology (CFAST) program of Virginia Polytechnic Institute
and State University for providing the funding for this project, and
Ms. Sandy Brown for maintenance of the fish in the Aquatic Medi-
cine Laboratory of the Virginia-Maryland Regional College of Veteri-
nary Medicine.
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