ABSTRACT
The gill area to body weight relationship of two species of loach, Noemacheilus barbatulus (L.) and Cobitis taenia (L.) was measured. It was found that the secondary lamellar area and the total length of the gill filaments were independently related to the lengths of the fish in each of the species. Both these measurements were similar in the two species. The number of secondary lamellae per mm of gill filament was independent of fish length, and was greater in C. taenia than in N. barbatulus (45·5 and 36·4 respectively), being the main factor for the far greater total gill area/g calculated for C. taenia than for N. barbatulus (507·9 and 316 mm2/g respectively). These gill area measurements are, in general, higher than those of other freshwater fish. It is suggested that this is related to the sedentary burrowing habits of the two species.
INTRODUCTION
The measurement of gill areas of different fish species has been undertaken by several previous workers. >Gray (1954) and Hughes (1966) considered mainly marine species. Dubale (1951) made a comparative study of tropical freshwater air breathing species, drawing some evolutionary and ecological conclusions. The topic was thoroughly reviewed by Hughes & Morgan (1973), who also discussed the ecological significance of some of the morphological adaptations.
MATERIALS AND METHODS
Specimens of Cobitis taenia and Noemacheilus barbatulus were collected from the same site on the River Great Ouse at Newport Pagnell in Buckinghamshire as described by Robotham (1977).
All specimens were examined fresh in order to avoid the effects of fixatives or preservatives on the gills. All measurements were carried out using a binocular microscope with micrometer eyepiece. Secondary lamellae area measurement was undertaken using a camera lucida. The problems and errors incurred by not ‘weighting’ the secondary lamellae selection are outlined by Hughes (1973). In this case, owing to large quantities of mucus present and the very small size of the specimens, the recommended procedure of selecting secondary lamellae from certain areas on the filaments could not be carried out. A compromise was reached by taking a large number of lamellae to measure (60–100) and attempting to remove them from different areas of the filaments. This rather random method, adopted to obtain a mean lamellar area, is possibly the main source of error in the measurements, and should be borne in mind when comparing these figures with those taken by a more accurate ‘weighting’ system.
RESULTS
The mean value of gill area/g body weight was considerably greater in C. taenia than in N. barbatulus (507·9 mm2/g and 316 mm2/g respectively), especially as the values for L and bl were broadly similar between species and closely related to the total length of the fish for N. barbatulus (length of fish, L; correlation coefficient r = 0·9406; length of fish, bl; correlation coefficient r = 0·9423) and the sample fish of N. barbatulus were 20% larger than the specimens of C. taenia. The relationship between length of fish and L and bl was less striking for C. taenia (correlation coefficient r = 0·6551 and 0·7703 respectively) although a positive correlation was present.
The component which caused the most interspecific variation was that of the number of secondary lamellae per mm, giving a figure of 36·4/mm for N. barbatulus and 45·5/mm for C. taenia. This factor was found to be the same for all individuals of the species (cf. Hughes, 1966) down to the level of accuracy attainable, and is therefore independent of fish length.
DISCUSSION
The mean surface area/g body weight of these two species are similar to those of Tinca tinca (L.) (383) and Salmo trutta (L.) (339) recorded by Hughes (1966) but higher than Carassius auratus (Bloch) (116) and Cyprinus carpio (L.) (139) (Hughes & Morgan, 1973). However, both the loaches have gill areas approximately ten times those of the air breathing fish Anabas testudineus (Bloch) (64 mm2/g) (Hughes, Dube & Munshi, 1973) and Heteropneustes fossilis (Bloch) (74 mm2/g) (Hughes et al. 1974). Munshi (1976) has shown a logarithmic relationship between fish size and gill area, with the weight specific gill area decreasing with increased body weight. As the two species studied here are very small (maximum length 87 mm) this may partly explain their greater gill area to body weight. Owing to the lack of other data on fish of such a small size, it is not possible to make any further comparisons.
The greater gill surface areas found in these two species may result in more efficient gaseous transfer and may be a necessary aid to a mode of life in which the majority of time is spent stationary either burrowed (Cobitis taenia) or under stones (Noemacheilus barbatulus) in areas of low water flow. The efficiency of gaseous exchange is also affected by the thickness of the water-blood barrier; no measurements regarding this were taken however.
Neither species is an obligate air breather, although air breathing does occur at low oxygen tensions, beginning at approximately 30% (Robotham, 1976). This ability to withstand lowered oxygen tensions may in part be the result of greater gill areas, which is supplemented at lower oxygen tensions by air breathing.
This research was carried out for the degree of Ph.D. of the University of London under the tenure of a Westfield College Studentship and the author wishes to acknowledge Professor J. Green for his supervision of the project.
