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INTRODUCTION
The colossal squid (Mesonychoteuthis hamiltoni) is the world's largest invertebrate with a maximum record of approximately 500 kg. Its large size and some unique morphological characters have fuelled speculation that it is an aggressive top predator in the circum-Antarctic Southern Ocean. It has the largest eyes and beak of any cephalopod and its position near the top of the food chain is confirmed by stable isotope analysis (Cherel & Hobson, 2005) that reveals large fish, most notably the Antarctic toothfish, are its primary prey. The colossal squid also constitutes an important component in the diets of apex predators, especially sleeper sharks and sperm whales (Clarke, 1980; Xavier et al., 2002; Cherel & Duhamel, 2004).
However, like other cranchid squids, M. hamiltoni is thought to live in the meso- and bathypelagic zones (Filippova & Yukhov, 1979), although the juveniles may be found near the surface (Voss et al., 1992), and attains neutral buoyancy by storing ammonium fluid in the coelomic chamber (Filippova, 1991). Furthermore, its mantle wall is soft and semigelatinous (Roper et al., 1984). Great depth, neutral buoyancy, and gelatinous musculature are usually associated with relaxed requirements for strong locomotory abilities and a passive mode of life (Seibel et al., 1997; Nixon & Young, 2003; Boyle & Rodhouse, 2005).
Here, we estimate the mass-specific routine metabolic rates of M. hamiltoni based on extrapolation from scaling relationships within the family, Cranchiidae. We project the daily energy consumption (kcal day-1) along a depth gradient, and estimate the prey requirements (per day) of this cold-water deep-sea giant in the Southern Ocean.
MATERIALS AND METHODS
In the absence of direct measurements for M. hamiltoni, we used a dataset of routine metabolic rates (RMR) for cranchid species (Seibel et al., 1997; Seibel, 2007), across four orders of magnitude size-range, to estimate daily energy consumption and prey biomass requirements for the colossal squid. The scaling relationship (MO2 = aMb) for cranchiid squids was solved for 500 kg. The scaling coefficient (b) was -0.19 for cranchiids (Seibel, 2007) and a temperature coefficient (Q10) of 2 (O'Dor & Wells, 1987; Rosa & Seibel, in press) was assumed to normalize...