Digestive physiology of minke whales

9 1995ElsevierScienceB.V. All fights reserved Whales, seals, fish and man A.S. Blix, L. WallCeand ~. Ulltang,editors

351

Digestive physiology of minke whales S.D. Mathiesen, T.H. Aagnes, W. S~rmo, E.S. Nordc~y, A.S. Blix a n d M.A. Olsen Department of Arctic Biology and Institute of Medical Biology, University of Tromsr Tromsr Norway Abstract. The anatomy and principal function of the gastro-intestinal tract of minke whales were investigated. The stomach consists of four compartments, including an initial non-glandular forestomach followed by a glandular fundic chamber, a connecting chamber and a pyloric chamber. The length of the small intestine of minke whales is short, only four times body length, and the colon and caecum are poorly developed. The forestomach is small, containing between 5 and 801 of contents, with as much as 24.8% dry matter (DM). High population densities of anaerobic bacteria were found in the forestomach fluid, and adherent to the food particles, pH in the forestomach fluid ranged between 5.36 and 7.43, and the concentration of volatile fatty acids (VFAs) ranged between 49 and 486 mM. Based on these results we conclude that minke whales primarily utilize the prey they eat by microbial digestion. The contribution from VFAs to the daily energy requirements of minke whales seems to be of less importance than in ruminants. The multi-chambered stomach probably is an adaptation which increases passage time and, hence, microbial and enzymatic digestion. We suggest that the relatively small size of the stomach of minke whales, compared with that of ruminants, reflects their carnivorous diet, but does not necessarily indicate any reduced importance of the forestomach microbial digestion. Key words: baleen whales, gastro-intestinal tract, anatomy, function, stomach compartments

Introduction

The principal function of the gastro-intestinal tract of animals is to provide for digestion and absorption of nutrients. In the terrestrial ecosystem a variety of different adaptations for assimilation of food have been developed. Carnivores have a relatively simple stomach. In such animals the stomach is essentially a pouch-like structure which contains glands which secrete HC1 and pepsinogen. The small intestine, caecum and large intestine are short and uncomplicated. In terrestrial ecosystems, herbivorous animals have been very successful. They have developed stomachs and intestinal modifications which enable them to utilize plant cell wall polysaccharides such as cellulose and hemicellulose. The cellulose (1--4)-fl linked glucoside unit is one of the most abundant organic compounds available to terrestrial animals. For unknown reasons vertebrates have not developed the capacity to produce enzymes capable of hydrolysing plant cell wall polysaccharides, such as cellulose. The basis for the ability to utilize these dietary components is the symbiotic relationship between the host animal and the microbial population in the gastro-intestinal tract. In ruminants, the stomach has evolved into four chambers where the dominant organ, the rumen, provides for extensive pregastric microbial

Address for correspondence: S.D. Mathiesen, Department of Arctic Biology, N-9037 Tromsr Norway. Tel: +47 7 76 44871; Fax: +47 7 76 45770.

352 fermentation of structural polysaccharides [1,2]. The development of a forestomach system allows retention of food particles and growth of anaerobic bacteria. In the marine ecosystem the cellulose analogy is chitin, a major structural component of algae, plankton and crustaceans. Chitin is the (1--4)-r-linked unbranched homopolymer of N-acetyl-D-glucosamine. Gooday [3] estimated both its annual production and standing crop to be in the order of 10-100 billion tons. The energy available from the chitin biomass in the ocean supports most marine ecosystems. However, in fish like rainbow trout (Salmo Gairneri), chitin is poorly digested, even though chitinolytic enzymes have been identified [4]. The only explanation for this finding is that the simple gastro-intestinal system found in most fish, is too simple and does not allow enough retention of food particles for microbial fermentation. According to Gaskin [5], a great increase in the number of mammals took place during the palaeocene and ecocene. The local pressure on terrestrial feeding areas probably was intense, with one population after another reaching a state of overabundance and subsequent collapse. Interactions within riverside communities were undoubtedly very dynamic and always in a state of flux. Exertion of such pressures on ancestral populations of whales may well have stimulated them to go to sea to exploit new vacant niches of food. This implies that ancestral whales were herbivores in contrast to seals, which have developed from camivorous creodonts [5]. Molecular evidence for the inclusion of cetaceans within the order of Artiodactyla has recently been found [6]. Few investigations have focused on the digestive functions of marine mammals, but it appears that the anatomy and function of the gastro-intestinal tract of modem whales and seals reflect their evolutionary origin [715]. Large baleen whales have a multi-chambered stomach system [9,14,15] which resembles that of ruminants [ 1]. The functional organization of the baleen whale stomach, however, is somewhat different from that of ruminants. The first three chambers of the ruminant stomach are nonglandular, followed by a gastric chamber. In whales only the initial chamber is nonglandular, and the multi-chambered arrangement primarily involves compartmentalization of the glandular stomach. The north Atlantic minke whale (Balaenoptera acutorostrata) and the harp seal (Phoca groenlandica) seasonally share the same geographical area where they feed on crustaceans, and fish, such as herring (Clupea harengus), capelin (Mallotus villosus) and cod (Gadus morhua) [ 16-23]. In the Antarctic, however, the minke whale is known to feed exclusively on pelagic crustaceans [24]. Information on the structural and functional features of the digestive systems of these animals obviously is of major importance for the understanding of their ability to utilize the food they eat.

Anatomy The stomach of minke whales consists of four compartments, including an initial nonglandular forestomach followed by a glandular fundic chamber, a connecting

353

Har

I

-

.,

Minke whale

Fig. 1. Gastrointestinal tracts of a harp seal (120 kg) and a minke whale (7000 kg). The minke whale intestine is 4 x body length, while the harp seal intestine is 14 x body length. Bar = 40 cm.

chamber and a pyloric chamber [25] (Fig. 1). The forestomach is lined by a white mucosa, composed of a keratinized stratified squamous epithelium. The epithelial lining resembles that of the rumen of ruminants [26] although it lacks papillary projections. Organization of the stomach in minke whales is therefore similar to that reported for larger baleen whales [9,13,27-29]. The mean tissue wet weight of the forestomach in minke whales contributes 10% of tissue wet weight of the total gastro-intestinal tract, compared to as much as 60% in ruminants, like the reindeer (Rangifer t. tarandus) [25] (Mathiesen, unpublished) (Table 1). The in situ contents of the forestomach of minke whales (body mass 2000-7000 kg) range between 5 and 80 1. The in situ wet weight of the rumen contents in ruminants, such as reindeer (75 kg) eating a grass diet in summer varied between 12 and 18% of body mass (Mathiesen, unpublished). The tissue wet weight of the second stomach compartment, the fundic chamber, is approximately 14% of the total weight of the gastointestinal system (Table 1). The entire inner surface of the fundic chamber is lined with columnar mucous cells. Gastric pits lined with columnar mucous cells provide an entrance into the glands throughout the fundic chamber. The mucosal lining of the main fundic chamber of minke whales possesses gastric glands with parietal cells and chief cells [25]. The orifice connecting the forestomach with the fundic chamber is rather large (average diameter 28 cm), compared to the reticulo-omasal orifice in ruminants where only plant fibers less than 2 mm are allowed to pass [1]. Thus, it seems likely that large volumes of digesta including fish bones of considerable size, may easily enter the

354 Table 1. Mean tissue wet weight of the different compartments of the gastro-intestinal system in percent of the total gastrointestinal tract a Compartment

Reindeer (n = 10)

Minke whale (n = 3)

Harp seal (n = 12)

Forestomach(s) Glandular stomach(s) Small intestine Caecum Large intestine

60.1 5.7 19.8 1.5 10.6

10.4 16.8 63.3 0.8 8.7

27.0 67.6 0.2 5.3

a Data from Mathiesen et al. (unpublished); Olsen et al. [25,30].

fundic chamber to be further digested by acids and enzymes. The fundic chamber, the connecting chamber, and the pyloric chamber correspond to the abomasum in ruminants and to the stomach of monogastric animals, such as the harp seal. In minke whales the glandular stomachs comprise 17% of the tissue wet weight of the GI-tract compared to 6% in ruminants, such as the reindeer, and 27% in the harp seal. The tissue wet weight of the total stomach system relative to body mass in minke whales and seals is similar [30] (Table 1). The narrow orifice of the connecting channel (Fig. 2) probably prevents the passage of large components, such as fish bones from the preceding chamber, until they are acted upon by gastric juices and broken down. The proximal portion of the duodenum consists of a duodenal ampulla, which is a dilated sac with less capacity than the fundic chamber. Histological analysis of the tissue has revealed that the mucosa is similar to that of the pyloric chamber. The duodenal ampullae leads directly into the duodenum proper. The length of the small intestine ranges from 16 to 36 m being on average four times the body length [25]. In harp seals, the length of the small intestine ranges between 20 and 25 m, being on average 14 times the body length [30] (Fig. 2). The lengths of the small and the large intestines measured in minke whales in percent of total length of the intestines were 89 and 11%, respectively, while in harp seals the corresponding values were 97% and 3%, respectively. In ruminants, such as the reindeer, with a forestomach fermen-

r

ps

Fig. 2. Illustration of the minke whale stomach and cranial duodenum, showing the forestomach (FS), the fundic chamber (FU), the connecting channel (cc), the pyloric chamber (PY), the pyloric sphincter (ps) and the duodenal ampulla (DA).

355 tation of plant polysaccharides, the small intestine is 24 m which is approximately 12 times body length (Mathiesen, unpublished). In reindeer the contribution of the small and the large intestine to the total length of the intestines is 71 and 29%, respectively (Mathiesen, unpublished). Although the relative length of the intestines in the minke whale is short compared to that of harp seals [25,30], the mean tissue wet weight in percent of total gastrointestinal tract wet weight is not very different, being 63 and 68%, respectively. The absolute length and tissue wet weight of the caecum in both the minke whale and the harp seal were small, the length being on average only 24 cm and 35 cm, respectively [25,30]. Based on these observations, we conclude that the hindgut of minke whales and harp seals is of minor importance.

Function

The role of the cetacean forestomach in digestion has been disputed. The forestomach may function as a temporary storage chamber for larger quantities of ingested food, and an extensive muscularis externa indicates that it also grinds or chums the contents mechanically [9]. Keratinization may protect the forestomach wall against mechanical damage by the prey, which in mysticetes is filtered from the sea with the baleen plates and swallowed intact. The composition of the forestomach contents of minke whales vary from undigested to extensively dissolved food. This indicates that digestion is initiated in the forestomach, even though digestive glands are absent [25]. The dry matter content in the forestomach varies between 14 and 25% [25]. The chemical composition of the forestomach contents of minke whales is much different from that of herbivorous ruminants. In herring-eating minke whales, the protein and lipid contents as percent of the dry matter contents were as high as 40% and 59%, respectively, compared to 73% protein and 40% lipid in the krill-eating minke whales [31]. High concentrations of anaerobic bacteria in the forestomach fluid of minke whales were found [31,32] (Mathiesen, unpublished). The number of bacteria growing in an anaerobic habitat simulating medium [31] ranged between (7145) • 10s bacterial cells per ml forestomach fluid in herring-eating whales, between (1-12) x 10s bacterial cells per ml in krill eating whales, and 3 x 10s bacterial cells per ml forestomach fluid in one capelin-eating whale. These numbers are comparable to the total number of anaerobic bacteria found in the rumen contents of ruminants [1,33]. Transmission electron microscopic analysis of the forestomach fluid revealed high numbers of bacteria with different morphology (Fig. 3). By use of scanning electron microscopical analysis, Olsen et al. [31 ] were able to show that bacteria with different morphology were attached to food particles obtained from the forestomach of herring-eating whales, which indicates that the bacteria actually attack and digest the prey. In herring-eating whales, bacterial species such as Lactobacillus spp., Streptococcus spp. and Ruminococcus spp. were the most common strains. All bacterial strains isolated from the prey using similar microbiological

356 techniques had phenotypic patterns different from those of the strains isolated from the bacterial population in the forestomach, indicating that the microbiota is indigenous to the forestomach of the whales [31 ]. In krill-eating whales, bacterial strains such as Lactobacillus spp., dominated in one whale, while strains of Bacteroides spp., Clostridium spp. and Streptococcus spp. dominated in another (Mathiesen, unpublished). Of the isolated bacterial strains from the forestomach fluid of one whale, 47% were able to hydrolyze chitobiose, while 5 of 37 strains were chitinolytic. Some of the chitinolytic bacteria were able to produce lactate. By use of a selective medium for proteolytic, lipolytic and N-acetyl glucosamine-using bacteria, bacterial populations as high as 3 x 10 9, 1 x 10 9 and 18 x 10 9 bacterial cells per ml forestomach fluid were isolated from krill-eating minke whales (Mathiesen, unpublished). The pH in the forestomach contents of minke whales as measured immediately after death, varied between 5.95 and 6.69 (n = 18) [25], which is comparable to that found in the fermentation chamber of herbivorous mammals [1]. In herring-eating minke whales, the pH of the forestomach was 5.36-6.87, compared to 6.17-7.34 in krill-eating animals [31] (Mathiesen, unpublished). High concentrations of volatile fatty acids (VFAs), such as acetate, butyrate and propionate, have been found in the forestomach of large baleen whales [10,11]. In minke whales, the concentration ranged between 49 and 486 mM in the forestomach fluid (Table 2). However, Olsen

Fig. 3. Transmission electron micrograph of strained forestomach contents from a herring-eating minke whale, showing bacteria with different morphology, some surrounded by a glycocalyx (arrows). Bar = 1/~m.

357 Table 2. Concentration of volatile fatty acids (VFAs) (n = 8) and anaerobic bacteria (n = 4) in the forestomach fluid of minke whales

VFAs (mM)a

Median Range

Viable bacterial cells (109/ml)b

Total

Acetate

Propionate Butyrate

94 49-486

65 28-332

12

5-66

21

12-89

3.7 0.7-14.5

a Data from Olsen and Mathiesen (unpublished) and Mathiesen et al. (unpublished). bData from Olsen [31].

and Mathiesen (unpublished) were able to show that bacterial fermentation of prey in the forestomach varied depending on the volume and quality of the digesta. In ruminants much of the VFAs diffuse across the rumen wall [34] and some 70% of the ingested metabolizable energy passes through the ruminal VFA pool [35]. The contribution of VFAs to the daily energy requirements in minke whales seems to be of less importance than in ruminants. NordCy et al. [36] developed a three stage in vitro digestibility technique to simulate digestion in minke whales, and found that as much as 70% of the initial dry matter of the substrate disappeared into solution by bacterial degradation in the forestomach. The forestomach microbial digestion therefore seems to be of prime importance for the digestion of the prey, while bacterial fermentation products, such as VFAs, contribute less to the daily energy needs of minke whales (Olsen and Mathiesen, unpublished). This is also reflected in the relatively small size of the forestomach, compared to that of ruminants [25]. The stomach system of minke whales is followed by a short intestine, and the compartmentalization of the stomach is therefore thought to aid in retention of food, and hence to increase the passage time through the gastrointestinal tract. With the different structural and functional approaches to digestion in harp seals and minke whales, their ability to utilize food is different. The % digestible energy of fish (herring and capelin) seems to be equally high in both harp seals and minke whales, while the utilization of crustaceans is significantly lower in harp seals (Table 3) [3638]. It could be argued that the forestomach fermentation of food particles like chitin of the crustacean exoskeleton by indigenous bacteria contributes to an increased utilization of crustaceans in whales compared to seals. We suggest that the multi-

Table 3. Digestible energy (%) of different prey species in minke whale and harp seal

Herring (Clupea harengus) Capelin (Mallotus villosus) Krill (Thysanoessa sp.) a Nordr et al. [36]. bKeiver et al. [38]. CM~rtensson et al. [39]. dMhrtensson et al. [37].

Minke whale

Harp seal

92 (n = 16)a 95 (n = 5)c 93 (n = 5)c

95 (n = 4)b 94 (n = 4)d 82 (n = 4)d

358 chambered stomach of minke whales is an adaptation to increase passage time and consequently to increase the time available for both microbial and enzymatic digestion of food particles. The development of a relatively small forestomach in minke whales, compared to ruminants, may reflect an adaptation to a carnivorous diet and does not necessarily indicate a reduced importance of microbial digestion in these animals.

Acknowledgement This study was supported by the Norwegian Research Council, grant no. 4001408.007.

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