The buoyancy and orientation of the upside-down catfishes of the genus Synodontis (Pisces: Siluroidei)

Authors

  • N. G. Willoughby

    1. Kainji Research Project, P.M.B. 666, New Bussa, Kwara State, Nigeria, West Africa
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    • *Fisheries Research Unit, Makhanga, Chiromo, Malawi.


Abstract

The forces involved in the buoyancy of the genus Synodontis are unusually large owing to the well-developed cephalo-nuchal shield possessed by all members of the genus, and the large swim bladder and high levels of body fat needed to counteract this. A new method of determining the quantity of crude lipid in the tissues was developed and tested against a recognized method. Negative correlations were found between the total quantities of fat present and the volumes of the swim bladder for several species. Two species which maintained neutral buoyancy but occupied different ecological niches were found to achieve neutrality by different means, one possessing an abnormally large swim bladder and the other very large quantities of fat. The amount of bone present was found to be considerably greater than in species from other genera.

Several species of this genus regularly invert and swim on their backs. Three of the species known to do this were found amongst the 16 species present in the man-made Lake Kainji on the River Niger in Nigeria. The orientations of many of the species were studied both under natural conditions and in the laboratory. Several possible causes of inversion were investigated, including both physical and behavioural factors. The position of the food source was found to be the most important factor controlling the orientation of the species. By manipulating this it was possible to persuade normally orientating species to invert, and inverting species to return to normal orientation. Three theories to explain the inverting habit are advanced and their merits discussed.

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