ON THE LOCOMOTION OF THE MUD‐SKIPPER PERIOPHTHALMUS KOELREUTERI (PALLAS): (GOBIIDAE)

SUMMARY 1The following structural modifications of Periophthalmus koelreuteri (Gobiidae) are of particular important in locomotion on land. (a) A strenghtening of the pectoral girdle by increased ossificationm, and an enlargement of the areas for muscle attachment. The postaxial rays of the fin are stiffened considerably, and are of the semi-spinous type. No fusion of the radial elements occures in this species. (b) The relative extension and depression of the pectoral fin axis is greater for P. koelreuteri than other species. This enables the postaxial rays to be swung in almost a vertical plane. The pectoral fin rays are not spread in this species when moving on land, but are bundled together to form a strut. (c) In addition to adducting the basal lobe, the M. coraco-radialis takes on the new role of maintaining the stability of the shoulder joint, and supporting the body weight. (d) The pelvic bones articulate with the cleithrum, and act as compression structs transmitting the body weight to the pelvic fins. The M. protractor pelvis and the anterior hypaxial musculature play an important part in supporting the body, and in providing the vertical component of the forces used in skipping. The pelvic fin rays are short and stiff. The first is spinous and the remainder semi-spinous. The pelvic fins are modified for support, and have lost their suctorial ability. (e) The caudal fin is truncated ventrally. The shor stiff ventral rays are used to anchor the tail during the skip, and to support the hind end of the body when moving on land. 2Six modes of locomotion are defined and analysed. (a) Fast swimming is effected by the tail without the assistance of the paired fins. (b) Paddling is a slow mode of swimming in which the pectoral fins are used as paddles. Air in the bucco-pharynx causes the fish to swim with the head partly out of the water. (c) Crutching is a slow halting motion on land in which the fish is hitched forward on the pectoral fins. The trunk and tail remain stiff, and do not make any sinuous movements. The caudal fin acts as a skid to support the hind end fo the body off the ground. A bilateral synchronization of the paired fins is necessitated by the instability of a three point supporting system. Motion is discontinuous, i.e., it comes to a momentary halt at the end of each locomotory cycle. Analysis of motion films has given conclusive evidence that the pelvic fins contribute to the propulsive thrust, but their contribution is very small. A speed of about 10 cm/sec. is achieved by adult fish (c. 14 cm. body lenght). (d) In skipping the fish is projected through the air by a propulsive force developed in the axial muscles of the tail. A vertical component is producted by the M. protractor pelvis and the hypaxial muscles. The caudal fin acts as a fulcrum for the skip. (e) Climbing. The ability of P. koelreuteri to climb is limited, due to the fact that the pelvic fins have been modified for a locomotory, rather than a suctorial, function. (f) Skimming is an activity in which the fish makes a series of bounds over the water. The movements of the tail. A speed of about 2.5 metres/sec. is achieved. Skimming is an escape reaction. 3The adaptations shown by Periophthalmus and tetrapods to terrestrial progression are constrasted, and its bearing on the evolution of the two groups from aquatic ancestors is discussed.