Scanning electron microscopical description and biology of three new endoparasitic Ceratosomicola species from tropical Indo-Pacific nudibranch hosts

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Abstract

The Splanchnotrophidae are bizarre copepod endoparasites of shell-less opisthobranch gastropods which were rarely found in the Indo-Pacific. The genus Ceratosomicola Huys, 2001 included only the type species, C. sacculata (O’Donoghue, 1924), from tropical Western Australia. The present study points towards a much higher diversity: three new species, C. coia spec. nov., C. delicata spec. nov and C. mammillata spec. nov., are described from chromodoridid nudibranch hosts from Sulawesi, Indonesia. The parasite morphology was examined in detail using SEM. All female Ceratosomicola possess 3 pairs of elongate body processes, 4 pairs of cephalic appendages (two pairs of antennae, one pair of mandibles and one pair of maxillae) and highly reduced thoracopods. Major specific differences refer to the body shape, presence and number of dorsal and ventral bulges, and the structure of labrum, labium and thoracopods. The new species are sexually dimorphic, dwarf males of Ceratosomicola are herein described for the first time. Morphological structures are discussed comparatively; both intraspecific and intersexual variability is addressed. The genus Ceratosomicola is regarded as monophyletic, radiation may have occurred in and south of the Indonesian Archipelago, by host switching. All four Ceratosomicola species appear to be host specific, with females showing positional preferences inside their hosts. Biological adaptations of splanchnotrophids to special hosts and aspects of co-evolution are discussed.