EXPERIMENTALLY PRODUCED ABNORMAL COILING AND ANALYSIS OF GROWTH IN GASTROPODS - ROLE OF DEVELOPMENTAL CONSTRAINT IN EVOLUTION -

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Samples of Turbo cornutus were used to investigate how snail reacts to environmental disturbance like attachment of Oyster which blocks the normal course of the shell growth. In this project, silicon rubber balls were used as an obstacle. After one or two years, most of individuals succeeded in keeping their growth and the following patterns were induced as reaction to the attachment.1. With the rubber ball attached in upper part (on abapical side) of aperture overlap zone on the previous whorl, the shell growth changes downward and then returns back to upward after passing by the obstacle.2. With the rubber ball attached in lower part (on adapical side) of aperture overlap zone, the forward growth of the shell stops for a while the aperture expands outwards.These two patterns of the reaction to the obstacle suggest a certain role of the head-foot contact on the aperture margin. In order to test the role of the contact on the shell morphogenesis, a computer simulation has been performed for the shell morphogenetic model involving the head-foot contact on the aperture margin. Results of computer simulation of this model are as follows.1. Most popular model of coiling, regular and tight coiling, can be produced only by the head-foot contact on the aperture margin even if there is no intrinsic nature of coiled shell.2. The head-foot/aperture contact mechanism can simulate the reaction of the natural snail to the-obstacle attached on the previous whorl.All the experiments and computer simulations indicates that the head-foot contact mechanisms imposes the developmental constraints on the evolution of the shell coiling.