RELATIONSHIPS BETWEEN VERTICAL ZONATION, ZONE-HOMING BEHAVIOR AND HEMOLYMPH ACID-BASE-BALANCE IN MONODONTA-LINEATA (GASTROPOD, TROCHID)

Monodonta lineata (Gastropod. Trochid) is present only at high levels of the intertidal zone. When experimentally placed at low levels, it c rawls up towards its characteristic position on the seashore. Our data about the pH, P-CO2 and [HCO3-] values of the haemolymph lead to the conclusion that these parameters could be used as internal signals by M. lineata in order to choose its optimal level on the seashore. Indee d, (1) relative extracellular acidosis and alkalosis are induced by em ersion and submersion respectively and (2) the range of these induced disturbances is influenced by the percentage of exposure to the air (P EA) imposed on the animals: a high PEA (typical of high shore levels) induced lower disturbances than a low PEA (typical of low levels, neve r occupied by M. lineata in the field). We propose the following hypot hesis: the haemolymph hypocapnia (P-CO2 around 1 torr) occurring in M. lineata during submersion, characteristic of low levels of the shore, induces a negative geotactism and/or a positive phototactism which re sults in the observed crawling up towards the higher levels of the int ertidal zone.