Ruhnkebothrium Caira & Bueno & Jensen 2021, GEN. NOV.

Field: Agricultural and Biological Sciences (Life Sciences)

RUHNKEBOTHRIUM GEN. NOV. ZooBank registration: 9518EC43-EC41-4B3B-AF5C- C0C03F180D 9F. Diagnosis: Worms euapolytic, acraspedote. Scolex with four bothridia; cephalic peduncle and myzorhynchus lacking; neck present. Bothridia consisting of small, simple anterior loculus and expansive, highly folded posterior loculus. Scolex with slender gladiate or cyrillionate spinitriches and capilliform filitriches; slender band of papilliform filitriches on distal surface of bothridial rim. Neck and strobila scutellate. Immature proglottids wider than long; mature proglottids longer than wide. Testes numerous, extending throughout most of proglottid; post-ovarian field absent. Vas deferens minimal. Genital pores lateral, irregularly alternating; genital atrium shallow. Cirrus sac narrowly oblong or pyriform, containing coiled cirrus; cirrus armed with spinitriches. Vagina weakly sinuous, extending from ootype along midline of proglottid to anterior margin of cirrus sac, then laterally to open into genital atrium anterior to cirrus; vaginal sphincter absent; seminal receptacle present. Ovary H-shaped in frontal view, tetralobed in cross-section; ovarian margins lobulate or digitiform. Vitellarium follicular; follicles in two lateral bands; each band consisting of multiple columns of follicles, extending length of proglottid, usually interrupted dorsally and ventrally by terminal genitalia, not interrupted by ovary. Uterus medial, ventral, sacciform, extending from anterior margin of ovary to level of cirrus sac. Excretory vessels 4, arranged in one dorsal and one ventral pair on each lateral margin of proglottid. Parasites of hammerhead sharks (Sphyrnidae Gill). Cosmopolitan. Type species: Ruhnkebothrium mattisi sp. nov. Additional species: Ruhnkebothrium bajaense sp. nov. Etymology: This genus honours Dr Tim Ruhnke, whose keen insight into the taxonomic complexity of the Phyllobothriidea has led to key advancements in the systematics of the members of this order. Bothrium is derived from the Greek βοθριων, a well or pit. The gender is neuter. Provisional name: New genus 10 of Caira et al. (2014) and Ruhnke et al. (2017). Remarks: Among the 18 genera of Phyllobothriidea considered valid (see Ruhnke et al., 2017; Caira et al., 2020b), Ruhnkebothrium differs from all but one in that, rather than bearing bothridia that are essentially oval in form, it bears bothridia that are narrow anteriorly and extensive and highly folded posteriorly. In addition, rather than an apical sucker, each bothridium bears an anterior loculus. In both respects, the bothridia of Ruhnkebothrium resemble those of Thysanocephalum Linton, 1890. The scolex of Thysanocephalum was considered historically to consist of four small bothridia followed by an elaborately folded structure referred to varyingly as a ‘pseudoscolex’ (e.g. Linton, 1892: p. 544) or a ‘metascolex’ (e.g. Euzet, 1959: p. 136). However, Caira et al. (1999) determined that the entire structure constitutes the scolex, which bears four extensive bothridia, each of which consists of a narrow anterior loculus and a broad, highly folded posterior loculus. Ruhnkebothrium is easily distinguished from Thysanocephalum, in that its uterus extends only to the cirrus sac, rather than to the anterior margin of the proglottid and in that the surface of its neck bears scutes rather than elaborate leaf-like folds (Caira et al., 1999). In addition, unlike Thysanocephalum, the proximal and distal bothridial surfaces of Ruhnkebothrium bear slender, simple gladiate or cyrillionate spinitriches rather than serrate gladiate spinitriches.