Description and Complete Larval Development of a New Species of Baccalaureus (Crustacea: Ascothoracida) Parasitic in a Zoanthid from Tanabe Bay, Honshu, Japan

Abstract

An unidentified species of Zoanthus from Tanabe Bay, Honshu, Japan, is the host of an endoparasitic ascothoracidan crustacean, Baccalaureus falsiramus, new species. This is the first record of this zoanthid genus serving as the host of an ascothoracidan and the second species of Baccalaureus from Japan. The morphology of the adult females, nauplii, and ascothoracid larva is described based upon a detailed study combining light microscopy and SEM. The female of this new species is characterized by a coiled carapace but very short, more or less distally upturned thoracic horns, and very long, ventrally directed papillae for seminal receptacle ducts lateral to thoracopods II-IV. Much variability is recognized in the antennule, thoracopods, penis, and abdominal ornamentation. Larval specimens were individually reared in the laboratory. Six lecithotrophic naupliar instars with rudimentary endites on the antennae and mandibles are present before the ascothoracid larva. The nauplii swam for about one month without feeding until the metamorphosis to the ascothoracid larva. Naupliar instars II-VI have a sculpture of concentric cuticular ridges on the marginal area of the dorsal shield. A nauplius eye is present through all naupliar instars as well as in the ascothoracid larva. Setae are gradually added to the antennules, but the antennae and mandibles remain essentially unchanged after instar III; rudimentary maxillules appear in instar II. The ascothoracid larva is a "Tessmann's larva" similar to one recently described from Hawaiian plankton, but lacking central pores within the carapace reticulations. Morphological and developmental features of the nauplii and ascothoracid larva are discussed.