Larval Development and Sex Ratio Variation of Polyascus Plana (Cirripedia: Rhizocephala), a Parasite of the Crab Grapsus Albolineatus, in Taiwan
Members of the Rhizocephala are parasitic barnacles in the Crustacea. The life cycle of the Rhizocephala is composed of planktonic larval stages and an endoparasitic adult stage. Most studies have used temperate species as models, and little is known about rhizocephalan ecology in subtropical and tropical waters. Because reproduction and larval development of the Rhizocephala can vary with the environment, tropical Rhizocephala may have different life history patterns than their temperate counterparts. We investigated the larval morphology of the tropical Polyascus plana Boschma, 1933 (parasitic on the crab, Grapsus albolineatus Lamarck, 1818 which inhabits exposed rocky shores) using light microscopy and scanning electron microscopy. Crabs with Polyascus externae were collected and maintained at 22–25 °C aerated seawater. Released nauplii were transferred into beakers containing 500 ml of aerated, filtered, and autoclaved seawater and the culture was maintained at 15, 20, and 25 °C. Larval development was completed in 66 hrs from hatching at 20 °C, similar to the subtropical sacculinids, Loxothylascus panopaei Gissler, 1884 and Loxothylacus texanus Boschma, 1933. The short larval development time of P. plana could be an adaptation to enhance successful settlement along exposed rocky shores. Naupliar morphology differed between the sexes. The hind body of female nauplii (stages I–IV) is more slender than that of males. The first seta and head shield of males have pores, but they are lacking in females. Male cyprids can be distinguished from females by a large posterior aesthetasc on segment III. Larval sex ratio was variable and there may be a seasonal component, with a higher percentage of females in summer and of males in winter. This seasonal variation in the larval sex ratio suggests that summer is the recruitment season for new parasites, and winter is the period for implantation of males into virgin externae.
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Document Type: Research Article
Publication date: 2009-05-01
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