STUDIES ON SOME ECOLOGICAL AND BIOLOGICAL ASPECTS OF CUTTLEFISHES (CEPHALOPODA) IN SUEZ.CANAL

Abstract

The duration of life in molluscs varies between two months and hundred of years. The most long-lived molluscs are bivalves, but there are short-lived species in both marine and fresh water and in terrestrial habitats. Molluscs are considered short-lived if their lives are less than two years, or if they live more than two years but have only one reproductive season (Heller, 1990). All living cephalopods except Nautilus belong to this category.

The Cephalopoda is an ancient group, pre-dating the arrival of fish in the oceans by many millions of years. Originally protected by heavy external shells, cephalopods have evolved into progressively more fish-like creatures, paralleling the activities of fish in their habitats and life-styles. In many cases they are in direct competition with fish, close to the tops of marine food chains. They are important in the affairs of men for the same reasons as fish, as food and as competing users offood. Clarke (1996) has given figures which suggest that the cephalopod species of the continental shelves comprise only about 15% of all cephalopod genera; they live in water of less than 300 m depth, which covers only 6% of the Earth's surface. They do not represent the majority of cephalopod species or much of their total biomass; 85% of genera are spread in the upper 2000 m and across the bottom of the deep oceans, which occupy 66% of the Earth's surface. Over 40% of these genera are neutrally buoyant by oil or chemical means and may have very different lifestyles from the forms we know from shallow water.

Although studies on anatomy and physiology show that cephalopods have evolved many mechanisms in parallel with fish (Packard, 1972), the muscular cephalopods are different from other nekton, including fish, in many details of their biology and ecology (Boyle and Boletzky, 1996; Rodbouse and Nigmatullin, 1996; Wells and Clarke, 1996). Their populations show wide fluctuations of abundance (Boyle and Boletzky, 1996), largely because they usually grow rapidly to maturity, in one or two years, they carry few food reserves (Rodhouse and Nigrnatullin, 1996), have little overlap of generations, and their migratory patterns make them particularly susceptible to changes in oceanographic conditions. On the other hand, they show great resilience to fluctuations in conditions by their capacity to vary their growth rates, extend their breeding seasons, vary the depth of their spawning grounds and maintain complex