Impacts of Endogenous Factors and Ontogenetic Stages on the Metabolic Rate of the Mediterranean Spider Crab <i>Maja squinado</i> (Herbst, 1788)

In the Mediterranean, populations of the spider crab <i>Maja squinado</i> are drastically declining. As a critical step toward restoration efforts, this study investigates ontogenetic metabolic changes from larvae to adults, accounting for size, molt stage, and sex. Routine metabolic rates were measured in reared larvae and juveniles, and wild-caught adults. Zoea 1, the first planktonic stage, showed higher metabolic rates than zoea 2, likely due to a greater proportion of metabolically inactive tissue and differing energy sources (egg reserves vs. exogenous feeding). From megalopa to benthic juvenile stages, metabolic rates increased exponentially, probably reflecting increased organ complexity and activity. However, rates declined significantly from 7-month-old juveniles to adults, potentially due to reduced growth, longer intermolt periods, and behavioral adaptations. Among adults, males exhibited metabolic rates twice as high as females, likely linked to greater territorial and reproductive activity. In 7-month-old juveniles at 14°C, mass-specific metabolic rate is inversely correlated with body size: individuals <20 g consumed oxygen at twice the mass-specific rate of those >80 g. The log₁₀ of oxygen consumption positively correlated with log₁₀ wet mass, with a "b" factor of 0.83. Molting also drastically influences metabolic activity, with lower rates observed in postmolt individuals than in individuals in premolt stages. The successful rearing of <i>M. squinado</i> and the significant physiological insights gained into the different developmental stages enhance our understanding of the species' biological processes, and pave the way for further analyses before the implementation of restoration trials.