Important changes described in the marine littoral zones throughout the world are attributed to short term phenomena such as marine pollution. Recently, a few authors have emphasized medium to long-term modifications of the ocean conditions (Mearns, 1988). Increasing concentrations of carbon dioxide and other infrared absorbing gases are widely proposed as a mechanism of global surface warming over the next several decades (Ausubel, 1983). Over the past 20-30 years, a significant increase in the average temperature of the waters of the western Mediterranean basin has been observed: between 1973 and 1987, an increase of 0.4°C at 80 m depth (Pascual, pers. comm.), and between 1959 and 1989, an increase of 0.12°C under 400 m (Bethoux et al., 1990). Similar observations have been made in the other ocean basins (Bindoff & Church, 1992). In view of our present-day knowledge of the shallow waters of the Mediterranean, we can only put forward a model-based assumption of temperature increase (Bethoux et al., 1990). Winds, currents and albedo can strongly induce variations of the surface temperature (lvanoff, 1972) but physical measurements cannot demonstrate a significant mean temperature increase at present, even if it exists. However, the possibility of an increase is suggested by the marine flora and fauna which integrate medium-term changes in ambient conditions (Soule & Keppel, 1988). Unusual occurrences in marine life may also be used as indicators of changing ocean conditions (Mearns, 1988).