Redescription of Prosaetes rhinodontis (Wright, 1876) (Crustacea: Copepoda: Siphonostomatoida), an enigmatic parasite of the whale shark, Rhincodon typus Smith (Elasmobranchii: Orectolobiformes: Rhincodontidae)

Abstract

The siphonostomatoid copepod Prosaetes rhinodontis (Wright, 1876) is redescribed in detail based on adult female specimens recently collected from the sieve-like gill rakers of a whale shark, Rhincodon typus Smith, held in captivity off the coast of Motobu-cho, Okinawa-jima Island, Japan. Comparisons with other caligiform copepod species previously identified as Dysgamus atlanticus Steenstrup & Lütken, 1861 and described as Echthrogaleus pectinatus Kirtisinghe, 1964 from a whale shark revealed that these two taxa are conspecific with P. rhinodontis. The latter is transferred herein from the Pandaridae to the Cecropidae based on the relatively slim shape of its maxilliped corpus, and an amended diagnosis of Prosaetes C. B. Wilson, 1907 is provided. Neotype material of P. rhinodontis was selected from a previous collection of 173 specimens removed from a whale shark caught alive off the coast of Yonabaru-cho, Okinawa-jima Island, Japan, and subsequently held in captivity, albeit briefly, in the Okinawa Churaumi Aquarium, Motobu-cho, Okinawa-jima Island, Japan. Asymmetrical clusters of P. rhinodontis females on the host’s gill rakers were observed in this study, which suggest that this aggregative behavior most likely does not facilitate their attachment to the host but rather is a strategy used to augment their reproductive fitness. We also postulate that P. rhinodontis grazes on the epithelium of the host’s gill rakers and is, in contrast to other cecropids, a relatively vagile species.