Abstract
Sex structure is very labile between populations and specific for each population because it is a result of genetic, ontogenetic and biocenotic influences on the mollusks. In this study, the age frequency distribution, age-sex structure, and sex ratio were assessed in the wild populations of the Yeso scallop Patinopecten yessoensis (Jay) observed at fifteen sites in the northwestern Sea of Japan (=East Sea). The sex ratio varied between the populations from 0.83:1 to 1.52:1 (males/females), with the mean sex ratio being 1.03 ± 0.05:1. Within a population, the proportions of males and females in term of number differed between age classes. Males were more numerous than females in the younger age classes, and females prevailed over males in the older age classes. It was found that in different scallop populations the sex change occurred at different ages. In the populations that predominantly consisted of young (two- to four-year-old) individuals, males prevailed over females in the age class 2 yr.; the equal male/female proportion was found in the age class 3 yr.; and in older age classes, females prevailed over males. Another pattern was observed in the populations that consisted mainly of middle-aged (five- to six-year-old) individuals. Here, the age-sex ratio became equal at an age of 4–6 years. In the old populations (mainly 6–12-year-olds) the equal male/female proportion was observed at an age of 8–10 years. Thus, the age of sex change was not uniform for the scallop populations. It depended on the age structure of the population and, thus, was socially controlled. The greater number of females in the older age classes suggests a protandric sex change.
Highlights
It is known that among marine scallops the sequential production of oocytes and sperm occur in the same individual
The size advantage hypothesis predicts an optimal size at sex change, below which all individuals are of the first sex, and above which all individuals belong to the second sex
It was found that the wild populations of Yeso scallop differed in age frequency distribution, as well as in sex ratio
Summary
It is known that among marine scallops the sequential production of oocytes and sperm occur in the same individual. The variations in the timing of sex change and the factors influencing the change remain poorly understood. The timing of sex change is often labile and is thought to be largely influenced by interactions with conspecifics. Field studies commonly find a considerable overlap between male and female sizes and, fail to determine a population-wide optimal size for sex change (Wright, 1989; Soong & Chen, 1991; Sewell, 1994). The variation in size at sex change, resulting in the overlap between the male and female sizes, is explained by the fact that animals change sex based on cues like age, which may not correlate exactly with size, and the same-sized bivalves may have different ages. The lack of a method to determine individual age, especially for specimens from wild habitats, makes it impossible to compare the results for different populations and same-aged mollusks
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