Meloriastacus ctenidis gen. et sp. nov.: a primitive interstitial copepod (Harpacticoida, Leptastacidae) from Tuscany

A new interstitial copepod, Meloriastacus ctenidis gen. et sp. nov. (Harpacticoida, Leptastacidae), is described from the shallow Meloria Shoals, Tuscany (Italy). It represents the most primitive leptastacid known to date, displaying several unique plesiomorphic character states previously unknown in the family. A sistergroup relationship with Archileptastacus Huys is suggested on the basis of the detailed structure of the caudal ramus, the fifth legs, the distai endopodal armature of P2-P4 and the presence of spinule rows on either sfide of the anal opening. The ancestral leptastacid condition for the P5 setation is re-assessed and the homology of the caudal ramus setae discusseti. The structure of the mouthparts in Meloriastacus suggests that mucus-trap feeding was adopted only secondarily in the evolution of the family.