ecology

Ecology of Cephalopods

POPULATION SIZE STRUCTURE

Cephalopods are structure by body size and seasonally with one and two cohorts. In Chilean waters jumbo squid Dosidicus gigas have seasonal variations in population structure, in winter specimens of large (71 to 98 cm MDL) and small size (< 44 cm MDL) in both sexes are present; while in spring medium size squids are abundant (26 to 60 cm MDL) (Chong et al. 2005). Size frequency distribution analysis in the both seasons shows two cohorts of D. gigas in the southern Pacific Ocean (Fig. 1) (Chong et al. 2005, Ibáñez and Cubillos 2007). Recently, Ibáñez and Cubillos (2007), they found in the austral winter, medium-sized, maturing individuals were found in oceanic waters while large-sized mature males and maturing females were present in the coastal zone. In the austral spring, maturing medium-sized and large-sized individuals were observed in the oceanic zone. In contrast, two groups of individuals were observed in the coastal zone: one consisting of small and immature individuals, and one consisting of medium-sized maturing specimens. In the austral summer, only the oceanic zone was sampled, and samples contained small immature and maturing medium-sized individuals.

Fig. 1 Length frequency distributions of Dosidicus gigas from Chile (from Chong et al. 2005, Ibáñez and Cubillos 2007).

TROPHIC ECOLOGY

Cephalopods play an important role in the trophic structure of world marine ecosystems because they are voracious predators due to their high metabolic rates and they are prey of sevral marine predators (Fig. 2) (Amaratunga 1983; Rodhouse and Nigmatullin 1996). In cephalopods, the digestion and evacuation rates vary between species and are temperature dependent (Boucaud-Camou and Boucher-Rodoni 1983). Also, the digestion and evacuation rates are faster and more efficient in cephalopods than in fish (Boucher-Rodoni 1973). According to Amaratunga (1983) and O’Dor and Wells (1987), the energetic and metabolic requirements decrease during cephalopods ontogeny (Amaratunga 1983; O’Dor and Wells 1987) because food incorporation in a juvenile squid is greater with respect to its body weight than for an adult. Thus, the food consumption vary inter and intra-annually as well as during the ontogeny (Amaratunga 1983).

Fig. 3 Diet of Dosidicus gigas from Chilean waters (Chong et al. 2005).

Incirrate octopods of the family Octopodidae are typically benthic cephalopods that live in crevices and caves where they hide from predators, including conspecifics (Roper et al., 1984; Aronson, 1986). Common prey items for most species include clams, snails, fish and crustaceans. The diet of a single octopus species may include up to 55 different species as reported for Octopus bimaculatus Verrill 1883 (Ambrose, 1984). Thus, octopuses have been considered generalist predators of the third or fourth trophic level in marine communities (Guerra, 1978; Summers, 1983; Ambrose and Nelson, 1983; Rodhouse and Nigmatullin, 1996). Octopuses move through a broad range of trophic positions during their short life cycle (Rodhouse and Nigmatullin, 1996). Some octopus species show ontogenetic changes in their diets. For example, juveniles of Octopus bimaculatus feed mainly on small crustaceans, whereas adults feed on a wide variety of benthic invertebrates (Ambrose, 1997). Smale and Buchan (1981) reported similar ontogenetic variations in the diet of Octopus vulgaris from the coasts of South Africa. There, juveniles feed mainly on the bivalve Perna perna (Linnaeus, 1758), whereas larger individuals feed on crustaceans and other mollusks. In these cases the diversity of prey increases in relation to body size of octopuses. In Southern Chile the diet of Enteroctopus megalocyathus primarily consisted of brachyuran and anomuran crustaceans, fish and conspecifics. The diet differed in frequency of prey between fishing zones (Fig. 4) (Ibáñez and Chong, 2008).

Fig. 4 Diet of Enteroctopus megalocyathus from southern Chile (Ibáñez and Chong, 2008).

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REFERENCES

1. Amaratunga T (1983) The role of cephalopods in the marine ecosystem. In: Caddy JF (ed) Advances in Assessment of World Cephalopods Resources. FAO Fish Tech Papers 231: 379-415
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3. Aronson RB (1986) Life history and den ecology of Octopus briareus Robson in a marine lake. J. Exp. Mar. Biol. Ecol. 95: 37 – 56.
4. Boucher-Rodoni R (1973) Vitesse de digestion d’ Octopus cyanea (Cephalopoda: Octopoda). Mar Biol 18: 237-242
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