Hemolymph-Based Molt Prediction in the European Green Crab, Carcinus maenas: Potential Tools for a Soft-Shell Industry

As invasive European green crabs (Carcinus maenas Linnaeus) continue to impact Gulf of Maine ecosystems and fisheries, harvesters and coastal resource managers are exploring strategies to mitigate the effects of this species. One option is to establish and promote a soft-shell crab fishery for this species; however, it can be difficult to accurately and consistently identify premolt crabs for harvest using external morphological cues alone. This study evaluated the application of several hemolymph indices as diagnostic tools for determining the molt stage in C. maenas. Captured crabs were tagged, measured, and held in individual compartments in a floating enclosure for up to 51 days. Hemolymph samples were collected twice per week from individual crabs (n = 126) and evaluated for hemolymph color, total protein (refractive index, RI), and calcium concentration. The resulting data suggest a positive relationship between these metrics: darker hemolymph samples had higher protein and calcium concentrations; crabs with darker hemolymph and higher initial RI were more likely to molt within 3 wk; and initial RI ≥9.5 resulted in a >50% chance of molting within 3 wk. In addition to improving our understanding of the biological underpinnings of molting in brachyuran crabs, these results provide practical information for creating an accessible molt assay tool (i.e., hemolymph color) that can be used by harvesters to further facilitate the development of a soft-shell crab market.