Monstrilla mahahualensis sp. nov., SEM-prepared adult male from Mahahual, Mexico A right antennule fifth segment showing terminal setal elements, ventral view B distal part of urosome showing genital complex, fifth legs with long setae (L5s) C integumental structure 1 (is), on third antennulary segment, semi-lateral view D integumental structure 2 (is) on fourth antennulary segment, ventral view E genital complex showing genital opening and paired genital lappets(gl) F detail of anal somite, ventral view. Setal elements on segments 1–4 labeled following Grygier and Ohtsuka’s (1995) nomenclature; armature of fifth segment labeled following Huys et al. (2007); ae2 on distal segment follows Huys et al. (2007) sequence notation of antennulary aesthetascs. Arrows indicate proximal rounded protuberance on fifth antennulary segment. Scale bars: 75 μm (A, B).