Monstrilla mahahualensis sp. nov., SEM-prepared adult male from Mahahual, Mexico, female holotype A detail of right genital lappet showing cuticle ornamentation, ventral view B same, left genital lappet, semi-lateral view C apical surface of genital complex showing genital opening and paired opercular flaps (ofl), ventral view D swimming les 2–4 showing setation pattern and long exopodal spines (arrowheads) E integumental structure on antennule segment 4, ventral view F urosome, genital complex, and caudal rami showing setation pattern (setae II–V).