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The Mode of Terrestrial Locomotion in
Alf G Johnels
A number of African as well as Indian catfish species belonging to the family Clariidae have more or less amphibious habits. These fishes are able to move over land from a desiccating pond to another body of water. They are also known to be able to withstand periods of drought by burrowing in the mud of dried-up pools. Shoals of Clarias lazera may leave water temporarily for a walk over dry land without being forced to do so by drought (Welman, 1948). Already in 1895 it was reported that specimens of Clarias lazera burrow in the ground of dried-up pools and that they may temporarily leave their burrows by night in search of food (Vaillant, 1895). Thus it seems that these fishes are able to move actively on dry land outside their normal medium. Welman (1948) observed that the fishes "wriggled through the grass with snake-like movements." Locomotion on uncovered ground (Clarias anguillaris) has been described by Boulenger (1907: 281): "When placed on the ground the fish uses the spines of its pectoral fin for progression, moving first one spine and then the other alternatively." In 1950 the present author observed and filmed a specimen of Clarias in the act of progressing on dry land. The species was probably C. senegalensis but as afterwards C. lazera was also recorded in the collections from the Gambia River (Johnels, 1954) it cannot be excluded that the specimen belonged to the latter species. The observed specimen was about 40 cm long and was brought to our quarters by a native fisherman. Placed on the ground the Clarias specimen suddenly started to move along the ground and progressed for a considerable distance (about 10 m); the last few "steps" were recorded on an eight mm colour film.
Fig. 1. The locomotion on dry land of a Clarias specimen. The pictures (and Fig. 2) are enlarged from an 8 mm Kodachrome film. The film was exposed at the rate of about 16 pictures/s, and with an approximate exposure of 1/30 s. The time interval between the pictures: I-II 1.0 s, II-III 0.2 s., III-IV 0.3 s, IV-V 0.25 s, V-VI 0.4 s, VI-VII 0.4 s. (192 K MPEG MOVIE!)
Fig. 2 a. Locomotion of Clarias. The body is raised from the ground (note the shadow) and pectoral and pelvic fins are rigidly dilated. b. The speed of movernent in Kombination with the long exposure (1/30 sec.) caused a fogged picture of the body. The sharp focussing of the left pectoral fin (note the spine in front) indicates the position of the fixed point of the momentum of the locomotion.
Analysis of the individual frames of this film disclosed the mode of terrestrial locomotion used by the Clarias specimen (Figs. 1, 2). The spines of the pectoral fin play an important role but the progression is not caused by the alternate movement of the spines in relation to the body. On the contrary they are fixed in the erect position during locomotion. The paired fins, held at right angles to the body, serve to maintain equilibrium on land. It is likely that the pectorals help to prevent backward movement. As in many other catfishes the spines of Clarias can be locked by a special mechanism in the erect position. At the end of a step the spine may be momentarily bent caudally but this seems to be due to the effect of the sliding motion at the end of a step. It is typical that not only the pectorals but also the pelvic fins are rigidly expanded during the steps, although the latter fins can hardly be of importance for locomotion.
Fig. 3. Sketch illustrating the mode of locomotion of Clarias. The direction of movement is indicated by an arrow. The length of movement of the pectoral spine equals twice the length of a "step".
The progression of the body is effected by the alternate bending of the body from side to side caused by the contractions of the lateral muscles. Simultaneously the body is rolled laterally so that the body is alternately inclined downwards at the side of contraction. This rolling movement brings the tips of the pectoral spines alternately in firm contact with the ground. The spine serves as a lever by means of which the body is heaved from the ground. In the middle of a step the inclination is very pronounced and most of the body weight rests on the pectoral spine directed down towards the ground. During locomotion the body is alternately curved laterally. Consequently the head and the anterior part of the body as well as the tail are set in an oblique position in relation to the actual direction of the locomotion at the end of each step. The pectoral spines are situated just behind the head and therefore take part in the shift of position in relation to the direction of locomotion. The length of one step equals half of the distance between two consecutive points of contact with the earth of one and the same pectoral spine. The tip of the spine represents the fixed point during locomotion which transmits a niomentum to the ground. On a less firm ground, as in muddy pools, the surface of the pectoral fin may be of importance for progression. The rate of the steps demonstrated by the film is five in slightly more than five seconds. There is often a short pause between the end of one step and the onset of another, and consequently the bending of the body is quicker than is indicated by the rate of one step/second. The speed of the movement may be estimated at 4-5 m/min. in the case of the filmed specimen. However, it may be suspected that the specimen was more or less exhausted by the conditions during its capture in a seine and subsequent handling.
Clarias specimens swim with a movement of the anguilliform type. This means that the body has more than one lateral curve due to the contractions of the lateral musculature. The contractions which cause the locomotion on land are different as they alternately affect the whole of the lateral musculature of the body halves. Further, the contractions are coordinated in a regular way with a rolling movement causing the body to incline laterally and ventrally to the side of active contraction. This rolling is of fundamental importance for the progression on land but is altogether lacking during normal swimming activity. As a consequence it seems that the muscular activity on land is different from that in water. This implies that control of the movement on land must be based on an intrinsic neural process different from that in the normal medium, water. Thus, the ability of Clarias specimens to travel by land seems to be a very exceptional attribute which will most probably have evolved in response to particular environmental conditions. In this connection the faculty to move actively on land, typical of several of the species of the Clarias heterobranchoides group, is of particular interest. In fact this faculty must be interpreted as an additional adaptation to the same environment as the accessory breathing organs. The shallow waters inhabited by these fishes are often more or less temporary in conjunction with a change between dry and wet seasons
BOULENGER, G.A. 1907. The fishes of the Nile. In Anderson's "Zoology of the Egypt". London. JOHNELS, A.G. 1954. Notes on fishes from the Gambia River. Arkiv för Zoologi 6: 327-411. JOHNELS, A:G. 1957. The mode of terrestrial locomotion in Clarias. Oikos 8: 122-129 VAILLANT, L. 1895. Sur les habitudes terrricoles d'un Siluroide africain Clarias lazera, Cuvier et Valenciennes. Bull. Mus. Nat. Hist. 1, 271. WELMAN, J. B. 1948. Preliminary survey of the freshwater fisheries of Nigeria. Lagos.