Ready to harvest? Spine colour predicts gonad index and gonad colour rating of a commercially important sea urchin

An important problem limiting productivity and profitability of sea urchin aquaculture is the inability of producers to predetermine if sea urchins are ready for harvest. One way to overcome the 10–100% of lost production due to inopportune harvest times is to use external morphological characteristics to determine harvest readiness. To determine whether spine colour may be a useful indicator of harvest readiness, we tested whether there are relationships between spine colour and the size and colour of gonads of the commercially important sea urchin Tripneustes gratilla. Adult T. gratilla were grown in all combinations of three densities and three seawater exchange rates, simulating a range of environmental conditions occurring in culture systems. After six weeks, we measured the size and colour of gonads using standard protocols, quantified spine colour using colour rating and RGB (red, green, blue) intensity, and tested for relationships among these variables using linear models. The models identified significant positive relationships between spine colour predictors and gonad index and gonad colour rating. Our models indicated higher gonad indices were associated with brighter coloured spines, whilst gonads with the highest colour ratings were associated with stronger orange or red coloured spines. Patterns in spine colour identified by our models may reflect variation in colour producing pigments, carotenoids and naphthoquinones, in the epithelium. It is not clear why colour pigments in spines may depend on the colour and size of gonads, but it may be that healthy sea urchins that can grow large gonads also have sufficient energy to obtain and store colour pigments. Alternatively, colour pigments may provide health benefits that facilitate greater gonad size. Overall, our findings suggest that spine colour may be a suitable proxy for determining the harvest readiness of T. gratilla and possibly other sea urchins, but improvement in the reliability of our models and full automation of colour assessments are required before this novel technique can be implemented at commercial scales