Gastric mill ossicles record chronological age in the Caribbean spiny lobster ( Panulirus argus )

Information on individual age and size relationships is crucial for sustainable management of fished populations. Previous research has indicated it may be possible to directly age crustaceans by counting annular bands deposited within gastric mill ossicles. The method has been applied to multiple crustacean species; however, its applicability has been controversial because studies have shown gastric mill ossicles are reworked and/or replaced during ecdysis. Over the last 8 years, we have refined and improved upon methods of processing gastric mill ossicles and investigated the validity of this method for ageing Caribbean spiny lobster ( Panulirus argus ) using captive-raised animals of known age from the Florida Keys (USA). Although the biological process that creates annular bands within gastric mill ossicles remains unclear, validation of the method for P. argus based on known-age lobsters from 10 months -5.5 years old ( n = 33) confirms that the deposition of bands correlates with chronological age. We compared precision and accuracy of age estimates for ossicles processed using two imaging techniques (transmitted vs. reflected light) and found that transmitted light provided the best images for ageing P. argus . The mean coefficient of variation (CV) of age estimates between two readers was 20.2 %. The consensus age estimates provided by the readers were, on average, within 0.5 years of the known age and overall, 94 % of consensus age estimates from readers were within 1 year of the known age. We also compared lobsters of the same size but different age as well as those of the same age but different size to address whether the number of endocuticular bands in the ossicle is a measure of true chronological age or simply related to the size of the animal. Our results suggest that the gastric mill ossicles record chronological age in P. argus , providing a reliable ageing method with which to inform age-based management of the Caribbean spiny lobster population. Future research is needed to understand the mechanism that controls the deposition of endocuticular bands within the crustacean gastric mill ossicles and to determine the applicability of the method for other crustaceans.