Monday, May 11, 2015

Age by Association: Barnacles may tell us when blue crabs shed their last shell

Guest post from Christine Ewers-Saucedo at the University of Georgia:

Determining the age of blue crabs is challenging. Size is not a good predictor, as blue crabs only grow when they molt – and whether a crab molts or not depends on various factors, such as gender, nutritional status and temperature. However, some of the animals that can attach to blue crabs grow continuously, such as the conspicuous barnacle Chelonibia testudinaria. Its size provides an estimate for the minimum time since molting. But growth rates, crucial to convert size to age, were not available until now.

Large female barnacle with several dwarf males attached.
We recently published a study that determined growth rates for this barnacle on blue crabs and two other host species, loggerhead sea turtles and horseshoe crabs. The motivation for this study was the peculiar sexual system of this barnacle in which large hermaphrodites (with both male and female reproductive organs) coexist with dwarf males. The existence of dwarf males in hermaphroditic systems is baffling: hermaphrodites reproduce as both male and female, so how can males, which are lacking the female function, compete with them for reproductive success? Theoretically, high mortality rates, low growth rates or a low number of competing hermaphrodites (small mating groups) should allow the maintenance of males. We found that mortality rates are indeed higher than in most purely hermaphroditic barnacle species. Given their commensal life style on blue crabs and other animals, this is not surprising. They can only survive as long as the host is alive and keeps its shell.

            While the main point of this study was to explore barnacle reproductive strategies, it also provided application new tool for blue crab researchers. We can now determine the age of barnacles based on their size. This may help improve our understanding of the age structure of blue crabs in the field. This is particularly important for understanding the “age” of mature female crabs. Once mature, females stop growing and retain their carapace (shell) for the rest of their life. They also undergo a spawning migration from low salinity estuaries to high salinity coastal areas where barnacles are more common. Thus, barnacles provide an opportunity to determine how long individual females have been in high salinity spawning areas.

Ewers-Saucedo, C., M. D. Arendt, J. P. Wares, and D. Rittschof. 2015. Growth, mortality, and mating group size of an androdioecious barnacle: implications for the evolution of dwarf males. Journal of Crustacean Biology 35:166-176.

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