The striking intertidal gradient from marine to terrestrial physical conditions is correlated with an equally obvious spatial partitioning of the habitat by groups of animals. This has repeatedly given rise to the assumption that terrestrial physical factors, acting during tidal emersion, determine the upper range limits of intertidal populations. This assumption is elevated to the level of a hypothesis in this study and tested with scientific rigor. If conditions in the physical environment serve as "limiting factors," determining the partitioning of the intertidal, it should be demonstrable that (a) interspecific differences in physiological tolerances exist and permit differential exploitation of the habitat, and (b) conditions in the microenvironment of the animals exceed the physiological tolerances at the fringes of ranges and, by causing mortality, prevent range extension.

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Late stage juveniles and adults of Callinectes sapidus in Chesapeake Bay, USA, and Maja squinado off the Ria de Arousa, Spain, were compared for ontogenetic changes in movement patterns (speed, distance, orientation) and habitat selection (depth, substrate) using ultrasonic telemetry and published information. After settling in submerged grass beds in the lower Bay, 20-mm juvenile C. sapidus disperse long distances into low salinity sub-estuaries to feed and grow to maturity in two years. Within the Rhode River sub-estuary, juvenile C. sapidus moved with a mean speed of 12 m h 1 in nearshore shallows (1-1 m); whereas adults averaged 24 m h" 1 in the deeper (2-9 m) channel areas and moved freely in and out of the main estuary. Individuals of both life stages exhibited a pattern of slow meandering (juveniles, 2 m h 1 , adults 10 m h" 1 ) within a limited area, alternating with faster, directionally-oriented movement (both stages >50 m h' 1 ) between meandering sites. Juvenile and adult males over winter in deeper water nearby, while inseminated females migrate long distances into high salinity areas to incubate the eggs. Callinectes sapidus completes the migration cycle only once per 2-5-y generation. Maja squinado settles on rocks in shallow kelp forests in the coastal zone, where they grow to maturity in 2 y. Juveniles moved slowly (0-5 m h" 1 ) while meandering without directional orientation on shallow (4 m) small patch reefs during summer. After the pubertal moult in summer, adults also meandered slowly (1 m h" 1 ) mostly on rocks at slightly greater depth (7 m). In late summer and autumn, newly mature and older adults moved with directional orientation into deeper (10-40 m) water for the winter, until migrating back to the shallows for the summer; whereas juveniles remained inshore on rocks for the winter. Adult M. squinado live several years after puberty and complete the seasonal migratory cycle several times during their lives.Despite marked differences between the two species in life histories and habitats, their similarities in behaviour and shifts in habitat utilization during ontogeny reflect adaptation to similar selective pressures. For both species, juvenile movement and habitat selection primarily indicates adaptation to intense predation pressure and growth optimization; whereas adult behaviour and migration indicates relaxed predation pressure but optimization of energy needs and site of larval release.

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