The population dynamics of Pandalus gracilis was investigated in the southeastern coastal area of Korea between May 1998 and April 2000. Of the 4,127 specimens, 57% were identified as females, 39% as males, and 4% as transitional hermaphrodites. The number of females was greater than that of males and transexuals. A significant correlation was observed between the number of transitional hermaphrodites and ambient seawater temperature. Growth parameters were estimated using the modified von Bertalanffy growth function model incorporating seasonal variation in growth using the program ELEFAN. Females grew faster and reached a larger sizeat-age than males (K=0.65/y and L∞=17.86 mm carapace length [CL] for females; K=0.51/y and L∞=14.70 mm CL for males). Mean size and age (95% confidence limits) at sex transition, calculated from growth parameters, were 7.07 mm carapace length and 1.05 years, respectively. The reproductive strategies of pandalid shrimps are discussed in terms of the type of sex transition.
The pandalid shrimp
The genus
Despite its ecological and economic importance,few studies have examined the biological and ecological aspects of
Recently, Bergstrom (2000) published a comprehensive review on the biological aspects of
Monthly samples of
Sex was determined by the shape of the endopodite of the first pleopods and the presence or absence of the masculine appendix of the second pleopods.
Based on morphological characteristics of the endopod of the second pleopods, transitional stages between male and female phases were identified to determine whether
Length-frequency distributions (LFDs) were constructed for males, females, and pooled data using 1 mm intervals of CL. Growth was described using the modified von Bertalanffy growth function (VBGF) (Pauly and Gaschutz, 1979):
where
Growth curves were estimated from the LFDA using the program ELEFAN in FiSAT (Gayanilo et al., 1997), which uses a nonparametric method to fit the modified VBGF through modes. The Rn value gives an estimator of the goodness of fit. ELEFAN estimates the growth parameters (
Growth performance was compared using a growth performance index (
A chi-square test was used to test for differences in the sex ratio. Differences in the size-frequency distributions of the population between the 2 sampling years and the size-frequency distributions of the population between sexes were determined using a Kolmogorov-Smirnov two-sample test (Sokal and Rohlf, 1995). Statistical correlation analysis of Pearson product moments was accomplished using MINITAB 15. Kimura’s likelihood ratio test (Haddon, 2001) was used to investigate differences in growth between the sexes.
All specimens of
Between May 1998 and April 2000, 4,093 individuals (1,557 females and 2,536 males) collected from the study area were used for growth estimations. CL length ranged from 3.16 mm to 16.91 mm. These lengths were subdivided into 1 mm CL size classes prior to growth analysis. The LFDs showed that the population had two modal size groups per year, with similar patterns in both sexes. Apparent shifts were seen in the modal length of cohorts with time. In both sexes, small newly recruited shrimp appeared during the period between June and August. This pulse could be followed until September 1999, and disappeared in October 1999. The best fit to the length-frequency data identified a distinct new cohort, which became apparent in June or July in each year.
The VBGF parameters estimated by ELEFAN for each sex and for pooled data are summarized in Table 1. The values of
The growth curve showed strong seasonal oscillations of 26% for males, 81% for females, and 63% for the combined data. The slow-growth phase occurred in April for males (
von Bertalanffy growth parameter estimationby the ELEFAN analysis of length frequency data formales females and pooled data
In this study, females had faster growth rates than males, which indicates differences in growth patterns between the sexes. Similar results were found in
Fished species may be dioecious or hermaphroditic.Sexual reproduction can provide an advantage because it creates and perpetuates genetic diversity and allows the formation of novel genetic combinations that provide adaptations to ever-changing environments (Williams, 1975; Bell, 1982; Michod and Levin, 1988). All elasmobranchs have separate sexes throughout their lives and population sex ratios are usually close to 1:1. Most temperate teleost fishes are dioecious, but almost 50% of fished families in the tropics contain hermaphroditic species (Sadovy,1996). Sequential hermaphrodites may be protogynous,in which adult females change to adult males, or protandrous, whereby adult males change to adult females. The fished families Lethrinidae, Scaridae, Labridae, Sparidae, and Serranidae contain protogynous species. Protandrous hermaphroditism has been observed in Sparidae, Centropomidae, and Platycephalidae.
The sex ratios of hermaphroditic species are affected by fishing because they can be size-related.In protogynous species, for example, males are larger than females; therefore size-selective fishing leads to a female-dominated population. Why does sex change occur? Selection should favor sex changes with increasing size or age when one sex gains a relatively greater reproductive advantage with size or age (Warner, 1975, 1988; Charnov, 1993; Reynolds, 1996). Thus, if male reproductive advantage increases faster than that of females, a female to male change will be favored. Conversely, if female reproductive advantage increases faster than that of males, then a male to female change will be favored. For species that change sex from male to female, a lower disparity between the sexes in sexual selection exists due to dispersed breeding resources, which cannot easily be monopolized by a single male.
The rate of gain with size or age is therefore slower for males and is exceeded by the gains that females receive per unit size due to fecundity advantages. The initial sex is always more abundant in fishes that change sex, resulting in a male bias for protandrous species and a female bias for protogynous species (Choat and Robertson, 1975). Some decapod crustaceans, such as pandalid and crangonid shrimps,which are commercially exploited, are protandrous hermaphrodites (Tiews, 1970; Bergstrom, 2000). These shrimps exhibit a wide range of reproductive strategies because of the characteristics of sex transition.
In this study, a significant correlation was observed between seawater temperature and the number of transitionals, which indicates that the rate of sex transition from male to female can be affected by temperature. Previous studies suggested that several factors affected sex changes in pandalid shrimps (Koeller et al., 2000; Chiba et al., 2003; Wieland, 2004; Skuladottir et al., 2005). According to Wieland (2004) and Skuladottir et al. (2005), the change in size at sex transition in
petition leads to a delayed sex change.
In conclusion,