Definition of a new unbiased gonad index for aquatic invertebrates and fish: its application to the sea urchin Paracentrotus lividus

The gonad index is a tool widely used for studying the reproductive cycle of a large range of species, although its validity has been questioned in various scientific studies. One of the main criticisms is the assumption of an isometric relationship between gonad size and body size, a premise that has not been verified in most of the cases in which it has been applied. In this study we define the standardized gonad index (SGI), an indicator of the reproductive cycle based on the differences between the observed and expected weights of the gonads for an individual of a given size. Unlike other gonad indices, the SGI takes into account the possible allometric gonadal growth and is suitable for comparative analyses with samples composed of individuals of different sizes as well as samples that have been collected in different areas or at different times of the year. In order to show its advantages over other gonad indices, we applied this new index to 2 sea urchin Paracentrotus lividus populations and compared the results with those of the gonad index most frequently used for these organisms.