Pixelar index: a new quantitative approach for the assessment of reproductive condition in the purple sea urchin, Paracentrotus lividus, by image analysis

Abstract

The assessment of the gametogenic cycle and gonadal stage in commercial species relevant for shellfisheries and aquaculture is essential for the optimal management of the resource. However, currently this assessment frequently relies on the subjective classification of the specimens into discrete stages of development, with a strong dependence on the observer. The present study developed a quantitative, continuous, and observer-independent method for the analysis of the gonadal stage of the edible sea urchin, Paracentrotus lividus. This new approach takes advantage of image analysis to assess in histological preparations of the gonads the cellular growth of germ cells and calculate a Pixelar index (PI). The PI can be calculated taking into account just the proportion of the area covered by germinal vs somatic cells in the gonad follicle (PI1) or computing also the empty space created in the follicle lumen after spawning (PI2), which reduces the variability of the index. In both cases, PI showed a bell-shaped dependence on the traditional gametogenic stage that could be fitted to a Gaussian model, providing a tool that can be used for the quantitative evaluation of effects on sea urchin gonads of any kind of environmental factors, from aquaculture rearing conditions to ecotoxicological effects of pollutants on wild stocks. Further assessment of intraindividual—both among gonads and within gonads—variability in this index will be necessary for refinement of this quantitative tool.