Effects of size dominance on the survival, growth and physiological activities of juvenile pharaoh cuttlefish (Sepia pharaonis)

Abstract

The impact of individual size differences on social interactions and social groups are being increasingly recognized by researchers in different disciplines, but studies on cephalopods are still scanty. A two-stage trial was conducted to evaluate the effect of size ratios on growth and physiology of juvenile pharaoh cuttlefish (Sepia pharaonis). Juvenile cuttlefish were divided into two size groups (large (L): weight 1.0 ± 0.08 g, mantle length 1.5 ± 0.1 cm; small (S): weight 0.3 ± 0.05 g, mantle length 0.8 ± 0.1 cm), and groups with the following size-based ratios (L:S) were established: 0:30, 3:27, 6:24, 9:21, 12:18, 15:15, and 30:0. During the first phase of the 4-week trial, the growth parameters indicated that small cuttlefish experienced growth depression at the size ratio of large cuttlefish >9:21, which seemed to be irreversible. When large cuttlefish were removed from the 9:21 group, compensatory growth occurred, and define specific growth rate was significantly higher than that in other groups (P < .05). The occurrence of low levels of glycogen in tissue and of stress-induced physical damage indicated size dominance affects feeding, that small individuals were suppressed by larger individuals, and that small cuttlefish suffer from malnutrition, especially at the size ratios of 12:18 and 15:15. Expression of arginine kinase (AK) and heat shock protein 90 (HSP90) implied that the size ratio of 9:21 was a turning point; exceeding this ratio, resulted in significantly decreased growth performance and physiological activity. In summary, the differences in size seem to be a direct consequence of social dominance where the larger cuttlefish are aggressive against smaller individuals, resulting in suppressed growth, and were closely related to cannibalism and size-dependent mortality. This study provides evidence of size dominance in cuttlefish with implications for ethical and welfare considerations when using cephalopods as experimental animals and aquaculture practice.