"Akentrogonid" Host Invasion and an Entirely New Type of Life Cycle in the Rhizocephalan Parasite Clistosaccus paguri (Thecostraca: Cirripedia)

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Abstract
Cyprid settlement and host invasion was studied in the rhizocephalan Clistosaccus paguri (Clistosaccidae) using transmission electron microscopy. Male and female larvae appear to be morphologically identical. The infestive kentrogon instar known from the Peltogastridae, Lernaeodiscidae, and Sacculinidae is absent in the Clistosaccidae. Instead, the cyprid itself uses an antennule to penetrate through the host integument. This is the same mechanism used by functional male cyprids to implant spermatogonia into juvenile female parasites, and it is the first demonstration of "akentrogonid" host invasion in the Rhizocephala. The primordial parasite is not preformed in the cyprid, but develops from injected, undifferentiated cells almost directly below the site of settlement. The full clistosaccid life cycle without kentrogons is described in detail, and compared to that found in the "classical" kentrogonid rhizocephalans. It is concluded that the traditional classification of the Rhizocephala into the suborders Kentrogonida and the Akentrogonida should be reconsidered, since the Clistosaccidae was previously included in the former.