Adaptive design of locomotion and foot form in prosobranch gastropods

Abstract

A comparison of the locomotor types, speed, tenacity, and foot form of nearly 300 species in 52 families of marine prosobranchs has revealed that foot size and shape and even subtle variations of locomotion affect the speed and strength of adhesion to the substratum. Gastropods inhabiting soft substrata move primarily by pedal cilia or by discontinuous locomotion in which shell and foot move alternately. Both types of movement are accompanied by low tenacity. A specialized type of discontinuous locomotion, namely, leaping, surpasses all other methods of movement in speed. Species with ciliary locomotion have a very large foot while those with discontinuous movement have an exceedingly small foot relative to shell size. The majority of prosobranchs inhabit hard substrata, move by continuous pedal muscular gliding, and have moderately high tenacity during movement. Arhythmic pedal locomotion yields lower maximum speeds and tenacities than do rhythmic pedal waves. Foot size and shape relative to shell length in species with arhythmic locomotion vary from very short and broad to long and narrow. Studies of transects at several temperate and tropical marine littoral stations showed that these species are confined to low littoral or sublittoral habitats that are sheltered from heavy wave action. High speed and tenacity are simultaneously attained only by species with rhythmic pedal waves. Speed and tenacity do not represent competing selective pressures on the size and shape of the foot. Speed increases among species as the foot approaches or exceeds shell length and is highest if the foot is also broad; the greatest tenacities are attained by species with a long, broad foot whose dimensions do not exceed that of the shell. The optimal shape for both high tenacity and speed is a broad foot somewhat shorter than the shell; neither speed nor tenacity are much compromised by this form. In general, only species with rhythmic pedal waves whose foot size and shape approximate the optimal form for high tenacity and speed are found in habitats exposed to much wave action. Long rhythmic waves, moving a large proportion of foot area at once, are in theory energetically more economical than small, very rapid waves resulting in the same overall speed, but experiments showed that tenacity is significantly reduced in gastropods which increase speed by enlarging the waves. The optimal wave pattern of a species should be a balance between the demand for speed with the least expenditure of energy, favored by a pattern of many large waves at once, and the demand for tenacity, favored by a pattern of few and small waves. Retrograde ditaxic waves of elongation are the most common pattern encountered among prosobranchs, and are associated with a large range of foot sizes and shapes. Such waves are at least one third as long as the foot, while direct waves and other waves of compression are frequently much smaller. The range of foot forms of species with waves of compression is restricted, tending to be optimal for high tenacity or to be long and narrow. Waves of compression appear to be a specialization with the potential for maintaining high tenacity even at high speeds since the waves can be very small, and for giving superior speed since they can travel very rapidly.