An unexpected gills roommate from museum collections: description of a new species of Microcotyle Van Beneden & Hesse, 1863 (Mazocraeidea: Microcotylidae) from the ballan wrasse Labrus bergylta Ascanius (Teleostei, Labridae) off Sweden with novel

Abstract Natural history museums serve as vital repositories of biodiversity, offering extensive parasitological datasets crucial for taxonomical and ecological studies, supporting the work of taxonomists for conservation efforts, and natural system integrity. The Swedish Museum of Natural History (Stockholm, Sweden) houses a significant collection of parasitic Platyhelminthes, including unstudied material collected by Theodor Odhner. Herein, we provide novel morphometrical and anatomical data for Microcotyle donovani Van Beneden & Hesse, 1863, type species of Microcotyle Van Beneden & Hesse, 1863 from the type-host Labrus bergylta off Sweden, Northeast Atlantic, and we describe M. odhnaturiks n. sp. from the gills of L. bergylta off Sweden, both found in T. Odhner’s collection. Microcotyle odhnaturiks n. sp. differed from Microcotyle spp. from Atlantic waters and labrid hosts by morphometrical (number and size of clamps, number of rows of testes) and anatomical characters (extension of the caeca, organization of the germarium). An anatomical comparison, particularly of the ovarian region, indicates significant differences between M. odhnaturiks n. sp., M. donovani , and other Microcotyle species, confirming the presence of two distinct Microcotyle species on L. bergylta . Microcotyle odhnaturiks n. sp. was differentiated from M. donovani by having a complex, inverted question mark–shaped germarium, caeca not confluent posteriorly, and by lacking conical expansions on rims of buccal suckers, among other features. The discovery raises the number of Microcotyle species parasitizing labrid fishes to four and marks the first Microcotyle species reported from Sweden and Scandinavian waters. We also discuss hosts and distribution of Microcotyle spp. in Atlantic waters.