ROLE AS CONSUMERS

Cephalopods are voracious, versatile predators. They generally have a short life span and a single spawning event followed by death. Populat ions are subject to dramatic fluctuations and their impact on prey pop ulations is equally variable. The prehensile arms and tentacles of cep halopods, coupled with a highly evolved sensory system, allow them to occupy a broad trophic niche and migrations enable populations to expl oit the temporal and spatial variability of production systems and pop ulations of prey. Shoaling is a common behavioural feature of many spe cies which facilitates prey capture and contributes to the impact of c ephalopods on prey populations. Research on cephalopod stomach content s is hampered because the beak is used to bite the prey into small pie ces so hard parts, which are usually needed for identification of prey species, are often rejected causing potential bias in estimation of d iet. Cephalopods may also feed unnaturally in the presence of sampling gear. Despite these problems there is a growing body of data on cepha lopod predation collected using direct observations, conventional visu al analysis of stomach contents and serological methods. Most species feed on small crustaceans as juveniles and shift the diet to larger fi sh and other cephalopods during growth. This shift is accompanied by o ntogenetic changes in the allometry of the brachial crown. There is in creasing evidence that myctophid fishes are an important food resource for oceanic squid. The diet and stock size of some commercially explo ited squid populations is sufficiently well known to quantify the impa ct of a single generation on the prey community. Where there is predat ion on commercial stocks of fish and crustaceans, the effect of cephal opod feeding on recruitment may be significant. Cephalopods are trophi c opportunists in marine food webs from polar to equatorial seas.