SEX DIFFERENCIATION OF Carcinus maenas BY ANALYSIS OF CARAPACE GEOMETRICAL MORPHOMETRY

LEDESMA, FEDERICO M.; VAN DER MOLEN, SILVINA; BARÓN, PEDRO J.

La Serena, Chile

Congreso; The Crustacean Society Mid Year Meeting 2007; 2007

The Crustacean Society and Universidad Católica del Norte

The European green crab Carcinus maenas is a widely spread invasive species introduced to coastal waters of Eastern and Western North America, Australia, Japan, South Africa, and recently to Eastern South America (Patagonia). Analysis of size (carapace width: CW) frequency distribution (SFD´s) may help to understand the size structure of the invasive population throughout the invaded geographical ranges, to differentiate the growth instars and to draw conclusions about the dispersal dynamics of the species. Collection of C. maenas carapaces on the intertidal wrack of the Patagonian shore has provided abundant material to build SFD´s. However, since growth rates (and consequently size at instar) differ between sexes, finding a method to differentiate crab sex through the observation of carapace morphology was necessary to allow the construction of independent SFD´s for each sex. Aiming to find such a method, carapaces of live C. maenas males (N = 20, CW range: 59-94 mm) and females (N = 20, CW range: 40-82 mm) were dissected, thoroughly cleaned, dried and photographed from several angles. Profiles of the projections of carapace external surfaces on a transverse plane were obtained by image digitalization. The form of male and female profiles was statistically analyzed using a Fourier Analysis geometric morphometry technique. Additionally, the 40 carapaces corresponding to crabs of known sex were blindly-assigned to either sex by two independent observers. In males, profiles showed marked concavities on the lateral ends, corresponding to the latero-upwardly pointing lateral spines, while in females the lateral ends of the profiles were rect, corresponding to laterally pointing lateral spines. Significant statistical differences between male and female profiles resulted from the Fourier Analysis. One hundred percent correct sex assignation by the two independent observers proved that carapace morphology differences are conspicuous enough to differentiate crab sex by direct observation of carapaces. Carcinus maenas is a widely spread invasive species introduced to coastal waters of Eastern and Western North America, Australia, Japan, South Africa, and recently to Eastern South America (Patagonia). Analysis of size (carapace width: CW) frequency distribution (SFD´s) may help to understand the size structure of the invasive population throughout the invaded geographical ranges, to differentiate the growth instars and to draw conclusions about the dispersal dynamics of the species. Collection of C. maenas carapaces on the intertidal wrack of the Patagonian shore has provided abundant material to build SFD´s. However, since growth rates (and consequently size at instar) differ between sexes, finding a method to differentiate crab sex through the observation of carapace morphology was necessary to allow the construction of independent SFD´s for each sex. Aiming to find such a method, carapaces of live C. maenas males (N = 20, CW range: 59-94 mm) and females (N = 20, CW range: 40-82 mm) were dissected, thoroughly cleaned, dried and photographed from several angles. Profiles of the projections of carapace external surfaces on a transverse plane were obtained by image digitalization. The form of male and female profiles was statistically analyzed using a Fourier Analysis geometric morphometry technique. Additionally, the 40 carapaces corresponding to crabs of known sex were blindly-assigned to either sex by two independent observers. In males, profiles showed marked concavities on the lateral ends, corresponding to the latero-upwardly pointing lateral spines, while in females the lateral ends of the profiles were rect, corresponding to laterally pointing lateral spines. Significant statistical differences between male and female profiles resulted from the Fourier Analysis. One hundred percent correct sex assignation by the two independent observers proved that carapace morphology differences are conspicuous enough to differentiate crab sex by direct observation of carapaces.