JOINTS AND MUSCLES OF THE DORSAL FIN OF TILAPIA NILOTICA L. (FAM. CICHLIDAE).

by

P. J. GEERLINK AND J. J. VIDELER (ZoologicalLaboratory, Universityof Groningen,The Netherlands)

SUMMARY

The dorsal fin of the teleost fish Tilapia nilotica is described, emphasizing the joints of the spines and soft rays. Both kinds of joints are composed of the same basic elements. In the spine joint one axis of movement is present; in the soft ray joint there are two axes perpendicular to each other. An account is given of the musculature of the dorsal fin. Separate muscles are distinguished, situated between the inclinator muscles of the spines, for which the name interinclinatorial muscles is introduced. Their function is not clear.

INTRODUCTION

There are many publications describing the comparative anatomy of the dorsal fins of teleost fishes. A number of questions, however, concerning the immediate relation between structure and movements of the fin remain unclarified. In order to elucidate this kind of problem attention should be focussed on the fin structure and movement of one single species. Only a few authors have done this (e.g. HOOGLAND, 1951, who investigated the fixing mechanism of the dorsal spines of Gasterosteus aculeatus). The aim of this paper is to describe and compare some anatomical details of the two parts of the dorsal fin of Tilapia nilotica. Many Per- ciform fishes have dorsal fins consisting of two different parts or even two different fins occur. Tilapia nilotica has one single dorsal fin; the rostral part containing spiny rays and having restricted movability; the caudal part having flexible, segmented, soft rays, which can move in different directions. The joints between the rays and their supporting elements will receive special attention, as their structure is an impor- tant factor in limiting the potential of movement.

MATERIAL AND METHODS S

For dissections, fishes preserved in formalin, as well as frozen animals were used. Their size varied from 15 to 30 cm. Median and transverse sections were prepared from frozen fishes of about 10 to 15 cm in length, after decalcification in 5% nitric acid, fixation in Bouin solu- 280 tion and embedding in paraffine. The sections were cut at a thickness of 7 [1.m and stained according to the azan method. From these schema- tical drawings were made (Figs 5, 6, 7, 9, 10). To elucidate the possible movements of some structures simple three dimensional diagrams were drawn in which the relevant anatomical details were outlined.

' RESULTS S The skeleton a. General features, nomenclature The formula of the dorsal fin of Tilapia nilotica is D (XV)XVI-XVII (XVIII)/11-15 (STERBA, 1968). The rays are supported by a row of pterygiophores (Fig. 1), situated between the vertebral column and the fin base. These rays articulate in a joint at the distal end of the pterygiophores. Each pterygiophore has developed from three cartilagi- nous elements (only two in Cichlidae, according to FRANÇOIS, 1959) : the proximal, mesial and distal radial (EATEN, 1945). During the ontog-

TABLE I Names and descriptions of ray support skeleton, commonly found in literature whole ray support proximalpart distal part of ray support of ray support

Fig. 1. Skeleton of dorsal fin.