Abstract
Macrostylis marionae n. sp. is described by means of integrative taxonomy. It is the second isopod species (Crustacea) described from the hadal Puerto Rico Trench. It differs from other Macrostylidae (Janiroidea) predominantly by the lack of the tergal borders between the fossosome segments. Adult males of M. marionae n. sp. differed from females and juveniles of both sexes in a higher number of peritrich ciliate epibionts. In total, 64.3% of adult males and only 10.5% of adult females were infested. Because macrostylids are generally considered infaunal, with a digging lifestyle, while peritrichs require aquatic medium for filter feeding, this observed pattern led to the hypothesis of a behavioural sexual dimorphism in this species of Macrostylidae. Accordingly, the epibionts may prefer adult male macrostylids as basibionts because of a rather epifaunal and roaming lifestyle. In contrast they may avoid females due to a stationary and infaunal behaviour. Additionally, the stage progression of the M. marionae male ontogeny was reconstructed.
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