MORPHOLOGY AND MORPHOMETRY OF CAECA IN THE BLACK SCOTER MELANITTA NIGRA

Ewa Działa-Szczepańczyk, Katarzyna Betlejewska

ABSTRACT

The subject of the research was caeca of 52 individuals of the Black Scoter Melanitta nigra – 32 males (5 young and 27 adult ones) and 20 females (11 young and 9 adult ones). The analysis comprised the relations among the values of four measurements of caeca (length – CLL and weight – CLW of left caecum, length – CRL and weight – CRW of right caecum) and the values of three body parameters (body length BL and weight BW – and sternum length) taking into consideration the birds’ ages and sexes. Ontogenetic differences in caeca sizes were revealed – young individuals of both sexes had longer caeca than adult individuals. Moreover, young females had heavier caeca than adult individuals of this sex. However, no significant dimorphic differences in the analysed parameters of caeca were noticed. Definite majority of black scoters had longer (47 individuals) and heavier (29 individuals) left caeca than the right caeca. The fluctuating asymmetry coefficient of caeca for the studied group of b

Key words: caeca, morphology, Black Scoter Melanitta nigra..

INTRODUCTION

The characteristic feature of alimentary systems of a determined majority of bird species is the presence of two caeca. They are located in the transition zone of a small intestine into a large intestine and are situated opposite each other along an ileum [20,22]. In the representatives of Anseriformes all three types of caeca, specified in birds using the criterion of those organs’ sizes, occur [21].

The Black Scoter Melanitta nigra is a member of the order of Anseriformes, suborder Anseres, family Anatidae, subfamily Anatinae, tribe Mergini [12]. In Poland it is a migrating species, numerously wintering on the Baltic [42]. It spends the summer in tundra and at that time feeds on food of animal origin with a small share of plant material (seeds, roots, and tubers). In winter it is exclusively a predatory bird and its food mainly consists of bivalves and, to a lesser extent, crustaceans, polychaetes, and fish [10,11,12,19,35]. This species is rated among the group of birds with moderate sizes of caeca [2,21].

The purpose of this study is a morphological and morphometric characteristic of caeca in the Black Scoter Melanitta nigra taking into consideration the sizes, sexes, and ages of the birds. The study also analyses the observed asymmetry of caeca lengths and weights.

MATERIALS AND METHODS

The subject of the study was caeca of 52 individuals of the Black Scoter Melanitta nigra – 32 males (5 immarure and 27 adult ones) and 20 females (11 immature and 9 adult ones). The birds were obtained in autumn and winter seasons in the years 1993-2001 from the sea fishing bases of West Pomerania Province (in Dziwnów, Międzyzdroje, Międzywodzie, Wisełka, Świnoujście). The birds’ ages were determined by the look of tertiary flight feathers, wing covers, and the size of bursa Fabricii [9,18]. Three measurements describing the birds’ body sizes were made: body length (BL) – measured with a flexible tape from the tip of a beak to the end of a rump (exact to 0.5cm), sternum length (SL) – measured with a slide calliper exact to 0.5cm, and body weight (BW) – determined exact to 50g. The linear body measurements were made according to the suggestion of Dziubin and Cooch [6]. The morphometry of caeca included: length (CLL) and weight (CLW) of left caecum (caecum sinister), and length (CRL) and weight (CRW) of right caecum (caecum dexter). The lengths of caeca were measured with a slide calliper exact to 0.1mm with the Leopold’s method [17], whereas the weighs were determined exact to 0.01g after cleaning the intestines off chyme and drying them with filter paper.

The obtained results were worked out statistically. Interrelations occurring between the parameters describing the birds’ body sizes and caeca parameters were characterised with appropriate values of Pearson’s linear correlation coefficient (r). In order to determine the significance of differences in parameters between a left and right caecum t-Student test was used. The quantity of asymmetry was characterised with the fluctuating asymmetry coefficient (FAC) proposed by Van Valen (1962): FAC = 1 – r², where r is a correlation coefficient.

RESULTS

All studied Black Scoters had well developed both caeca. The organs had a shape of small outgrowths situated opposite to each other, of equal diameter on their whole length (fig. 1). The average caeca length of the studied birds was: 21.7mm – for the left caecum, and 18.36 mm – for the right caecum. The average weight of those organs was as follows: 0.12g – left caecum, and 0.11g – right caecum (table 1). No interrelation between the length and weight of both caeca and the parameters describing the birds’ body sizes were revealed.

Table 1. Differences in absolute body and caeca parameters between males and females of the Black Scoter Melanitta nigra, M - males, F - females, - arithmetic average, SD - standard deviation, CV – coefficient of variation, t – value of t-Student’s statistics, NS – statistically insignificant differences, p – level of significance, BW – body weight (g), BL – body length (mm), SL – sternum length (mm), CLL – left caecum length (mm), CRL – right caecum length (mm), CLW – left caecum weight (g), CRW – right caecum weight (g)

In the studied group of Black Scoters no statistically significant differences in the length and weight of caeca between males and females were recorded, although such differences were noticed in the parameters describing their body sizes (table 1). But the comparison of young individuals with adult ones within each sex in the scope of the discussed features revealed an opposite situation – despite the lack of significant differences in body sizes, significant differences in caeca parameters between age categories of the studied birds were recorded. Young drakes had significantly longer and heavier caeca than adult individuals of this sex. In the group of females significant differences were only determined for caeca weights – young ducks had heavier caeca than adult individuals of this sex (table 2).

Table 2. Differences in absolute body and caeca parameters between young and adult individuals of the Black Scoter Melanitta nigra (M - males, F - females, ad – adult individuals, im – immature individuals, - arithmetic average, SD – standard deviation, CV – coefficient of variation, t – values of t-Student’s statistics, NS – differences statistically insignificant, p – level of significance). Measurements symbols – see table 1

The analysis of the differences in length and weight between left and right caeca showed distinct asymmetry of those organs, particularly with reference to the former of the mentioned parameters. Applying t-Student tests for the whole group of Black Scoters the significance of differences between the average lengths the left caecum and the average length of the right caecum was recorded. No such differences with reference to weight were revealed. Moreover, it was found out that 94.4% (47 individuals) of the studied Black Scoters had a longer left caecum compared to the right one. The direction of asymmetry in the weight of those organs was significantly less visible – only 55.8% (29 individuals) were characterised by a heavier left caecum than the right one (table 3). The fluctuating asymmetry coefficient FAC [44] of caeca of the studied group of black scoters reached for their lengths the value of 0.368, and the value of 0.502 for the weight.

DISCUSSION

Relatively small sizes of caeca of the Black Scoter may be related to the diet of this species. Many researchers [3,7,8,14,24,31] point at a great dependence of birds’ caeca lengths on a kind of food they feed on. Particularly with reference to representatives of the order Galliformes a distinct positive correlation of the length of those organs with the number of raw fibre in their diet was revealed [17,25,26,31,32]. This interrelation is a result of already well-known digestion function of caeca in this group of birds. The function of caeca in water birds, however, has not been fully explained [3]. Miller [25] suggests that caeca lengths of those birds are also related to the content of fibre in a diet. Miller [25] presumes that, similarly like in the birds of the order Galliformes, indigested food remains may be directed to caeca where microbial fermentation of cellulose and other constituent of the diet takes place. In consequence the caeca increase in volume. This phenom ena refers in particular to duck species that feed on vegetable or mixed vegetable and animal food. Miller [23] even suggests that the morphology of caeca of the representatives of the genus Anas undergoes too rapid changes to be used as a good indicator of food preferences of those birds. Bernes and Thomson [1], studying 18 duck species divided into three trophic group – carnivorous, herbivorous and feeding mixed food, drew attention to the fact that predatory species, with no indigestible carbohydrates in their diet, were characterised with poor caeca development. The researchers explain that with the fact that the main place of food absorption in ducks is a small intestine, so the presence of developed caeca in predatory species would cause unnecessary, in their opinion, increase of maintenance costs of the alimentary systems in those animals. Also Kehoe and Ankney’s [13] research carried out on five duck species of the genus Aythya prove a distinct relation of the caeca length with the diet of those birds. The authors stated that the observed interspecific differences in caeca lengths and weights were not only results of body weight diversity between the studied duck species, but they were also conditioned by the content of raw fibre in their diet. The Black Scoter is a predatory species feeding on highly concentrated animal food – little varied and consisting mainly of bivalves, frequently dominated by the Edible Mussel Mytilus edulis [11,35]. In this connection, the function played by caeca in the digestion process in this species may not be large.

None of the analysed measurements of caeca of the studied black scoters was significantly related to the birds’ body parameters. The lack of the relation of the right and left caecum length to the body weight and length was also recorded in the Velvet Scoter Malanita fusca [36] the Long-Tailed Duck Clangula hyemalis [37]. A similar lack of the relation was found between the total caeca length of the Mallard Anas platyrhynchos, and the length and weight of body and sternum length of this species [4]. This phenomenon may probably be explained with a large anatomical and morphological plasticity of those organs that rapidly react to changes in a kind and amount of food, which was confirmed in research on several bird species (Coot Fulica atra - Sitna [34], Red Grouse Lagopus lagopus scoticus - Moss [26], Mallard Anas platyrhynchos – Miller [24], Kehoe et al. [14], Japanese Quail Coturnix coturnix japonica – Fenna et al. [7], Savory et al. [3 3], Spruce Grouse Cnachites canadensis – Fenna et al. [7]. The values of parameters characterising the birds body sizes are more stable and do not change so rapidly in time.

In the studied group of black scoters no distinct dimorphic differences with reference to caeca lengths and weights were noticed. No such differences were either observed in the Greater White-Fronted Goose Anser albifrons [38] the Bean Goose Anser fabalis [41]. No differences between ducks and drakes with reference to the total caeca lengths were found in the Long-tailed Duck Clangula hyemalis [39]. However, ducks and drakes of the Mallard Anas platyrhynchos differed significantly only in the right caecum length [5].

The revealed ontogenetic differences in the lengths and weights of caeca in the studied black scoters may be conditioned by the diversity of the body sizes of birds of different ages. Young birds – smaller than adults – must take more food in winter to satisfy increased demand of their bodies for energy resulting from faster metabolism and less advantageous relation of body area to its size. Taking more food may be connected to the increase of birds’ intestines. Such a hypothesis was formulated by Pulliainen [31] studying wintering willow grouses Lagopus lagopus. The researcher stated that lighter individuals of this species (females and young individuals) had relatively longer small intestine and caeca than heavier birds (males and adult individual). No such regularity was found in the Mallard Anas platyrhynchos – the differences in the lengths and weights of caeca between young individuals and adult ones of this species were statistically insignificant [5].

The recorded phenomenon of asymmetry in the structure of the caeca of the studied group of black scoters was already revealed in earlier morphometric research on the alimentary system in this species [37] – it was noticed in 20 birds out of 23 that the left caecum was longer than the right one. Also in other species caeca size asymmetry in favour of the left caecum was observed. Sitna [34], on the material of 16 individuals of the Coot Fulica atra, revealed that 13 birds were characterised by the presence of longer left caeca. Similarly, in the Velvet Scoter Melanitta fusca – out of 34 studied individuals, 30 had longer left caeca [36]; similarly – in the Long-tailed Duck Clangula hyemalis: 23 out of 26 [39], in the Mallard Anas platyrhynchos: 93 out of 99 [5]. All 15 studied individuals of the Red-Necked Grebe Podiceps grisegena had longer left caeca [40]. In two goose species, however, a reverse situation was noticed – the majority of individuals had lon ger right caeca – this feature occurred in 26 out 42 studied individuals of the White-Fronted Goose Anser albifrons [38] and 38 out of 64 studied individuals of the Bean Goose Anser fabalis [41]. The obtained values of fluctuating asymmetry coefficient for caeca lengths and weights of the studied group of black scoters are comparable to the ones obtained for the Long-Tailed Duck Clangula hyemalis [39] and the Mallard Anas platyrhynchos [5]. The fluctuating asymmetry coefficient for caeca lengths for the former of the mentioned species was 0.36, and for the latter species – 0.259 for the lengths of these organs and 0.476 for their weights. Some authors claim that fluctuating asymmetry is a kind of variability and a measure of deviation from a genetically conditioned bilateral symmetry [16,28,30]. It may reflect a state of an organism homeostasis and its ability to defend itself against unfavourable environmental factors [15,27,29,43]. According to the above, the degree of asymmetry of this type in animal organs, particularly in wild species, may become valuable indirect information on changes going on in the environment.

CONCLUSIONS

The obtained results and their analysis allow us to state the following. No relation whatsoever was found between the lengths and weights of both caeca and the parameters describing the birds’ body sizes. Ontogenetic differences in caeca sizes were revealed – young individuals of both sexes had longer caeca than adult individuals. Moreover, young males had heavier caeca than adult individuals of this sex. No significant dimorphic differences in the analysed parameters of caeca were recorded. An asymmetry in caeca lengths and weights were revealed. It had a distinct direction – 90.4% of the individuals had longer left caecum, and 55.8% had heavier left caecum. Fluctuating asymmetry coefficient of caeca reached the value of 0.368 for their length, and the value of 0.502 for the weight.

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