Why selection favors protandrous sex change for the parasitic isopod, Ichthyoxenus fushanensis (Isopoda: Cymothoidae) |
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Authors: | Min-Li Tsai Jan-Jung Li Chang-Feng Dai |
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Institution: | (1) Institute of Oceanography, National Taiwan University, Taipei, 106, Taiwan;(2) National Museum of Marine Biology and Aquarium, Che-Cheng, Pingtung, Taiwan |
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Abstract: | A flesh burrowing parasitic isopod, Ichthyoxenus fushanensis, was found infecting the body cavity of a freshwater fish, Varicorhinus bacbatulus, in pairs. The marked sexual size dimorphism, with much larger females than males, and the presence of penes vestige on mature
females suggest a protandrous sex change in I. fushanensis. Here we investigate the question of why selection favors protandrous sex change for I. fushanensis, by analyzing the interactions among clutch size, female size, male size, and their host size. The number of manca, the first
free-living juvenile stage released, per brood was closely related to the size of the female. Excluding the effects of interaction
among causal variables, the negative correlation of male size alone on clutch size suggests that a small male did not limit
an individual's mating and fertilization success. When the effect of host size is removed statistically, there exists a significant
negative relationship between the sizes of paired males and females. This indicates that the resources available from host
fish are limited, and that competition exists between paired male and female resulting in a trade-off of body size. Due to
the very low success rate of hunting for a host of mancas, a female with larger body size and higher fecundity has a fitness
advantage. To augment the clutch size, a productive combination is a smaller male and a larger female in a host. The constraints
of the limited resources and the trade-off between the sizes of paired male and female may favor I. fushanensis to adopt the reproductive strategy of protandrous sex change resulting in a larger female and hence more mancas. The pattern
of the interactions among male, female, and the number of mancas, may be considered as a selective force for I. fushanensis protandrous sex change, where the available resources are constrained by the size of the host.
This revised version was published online in July 2006 with corrections to the Cover Date. |
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Keywords: | Ichthyoxenus fushanensis parasite protandry sequential hermaphroditism sex change |
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