Investigating the effect of molting on gastric mill structure in Norway lobster (Nephrops norvegicus) and its potential as a direct ageing tool

• Gastric mill ossicles of Nephrops norvegicus are lost in the stomach at molting.
• Ossicles are subsequently digested, resorbed, and recalcified postmolt.
• Similar ossicle loss was also observed in molted Austropotamobius pallipes.
• Unlikely that apparent ossicle growth increments are indicative of age.
• Chemical tags retained through molting due to resorption of lost ossicles postmolt.

Recent research has indicated through chemical tagging of some species of decapod crustacean that the mesocardiac, zygocardiac, and pterocardiac ossicles of the gastric mill are retained through molting and contain apparent growth increments potentially indicative of age. Though other recent studies have suggested that in some decapod species, these ossicles are either lost in the stomach exuvia at molting, or are partially resorbed prior to molting. This study investigated the fate of the gastric mill ossicles of Nephrops norvegicus throughout the molt cycle, in order to ascertain their utility for age determination. Wild-caught individuals were immersed in a calcein solution to stain the ossicles, reared in an aquaculture facility, and sacrificed at different stages of the molt cycle. In all individuals sacrificed less than 24 h after molting, the mesocardiac, zygocardiac, and pterocardiac ossicles of the gastric mill were found loose within the stomach of the individual, no longer attached to the stomach wall, among the disintegrated gastroliths. New ossicles are subsequently calcified postmolt to support the gastric mill and stomach wall. The same phenomenon of ossicle loss was also observed in two Austropotamobius pallipes individuals sacrificed 1 h postmolt. In some N. norvegicus individuals sacrificed at varying periods of time postmolt, the calcein stain was reincorporated into the newly calcified ossicles. It is suggested that this is because once the calcifying gastric mill is strong enough to cope with food, the old ossicles and gastroliths are digested, and their calcium, to which the calcein is bound, is resorbed. The results of this study indicate that the growth increments previously observed within the gastric mill ossicles of N. norvegicus are unlikely to be of annual periodicity, but are a result of the postmolt calcification process. It therefore seems unlikely that N. norvegicus can be directly aged using the gastric mill ossicles. This may be the case for a number of decapod crustacean species.