A practical identification guide to the zoeae of the invasive European green crab, <i>Carcinus maenas</i> (Linnaeus, 1758) (Decapoda: Brachyura: Carcinidae), and to the zoeae of the families of brachyuran crabs in Washington state, USA

Abstract Although the brachyuran crab Carcinus maenas (Linnaeus, 1758) is a globally invasive species, early-life history information for its larvae, such as hatch timing, stage duration, dispersal, and behavior, is limited, particularly in newly invaded locations. This is, in part, due to the need for region-specific guides on larval taxonomy that incorporate C. maenas. We addressed this issue for Washington state marine waters by developing a dichotomous key for the zoeae of C. maenas and a matrix that compares the morphological characters of C. maenas zoeae to those of brachyuran crab families reported from the state. To assess whether morphological characters of C. maenas from this region reflect taxonomic descriptions from the species’ native range, we reared zoeae from females collected in Washington state. We found that zoeae from each crab family reported from Washington state have at least two characters that distinguish them from C. maenas zoeae: the presence of lateral carapace spines in most of these families, and for three families lacking lateral carapace spines (Epialtidae, Grapsidae, Pinnotheridae) antennae that are nearly equal to the length of the rostral spine, the presence of posterolateral extensions on any of the somites, or a laterally expanded last somite, respectively. Rearing of larvae showed that the rostral spines of the Washington zoeae I of C. maenas were ~ 0.06 mm longer than reported in the literature. We also found that late-stage zoeae may retain more furcal spines than previously reported. The region-specific taxonomic tools we developed allow easier and quicker identification of C. maenas zoeae in Washington. These descriptions should facilitate early detection in newly invaded bays, a more rapid understanding of C. maenas larval timing and behavior, and better evaluation of larval dispersal that can be used in risk-assessment tools.