SUMMARY:Cyrtograpsus genus was traditionally considered to be composed of three species: C. angulatus, C. altimanus and C. affinis. However, recent studies have found solid evidence suggesting that C. affinis and C. altimanus belong to a single species and hypothesize that the morphological differences which caused this misclassification could be related to different ecophenotypes or life stages. Here we report a geometric morphometrics study on the carapace shape of Cyrtograpsus specimens from the Río de la Plata estuary (36°S) and the Nuevo Gulf (42.75°S), testing for shape differences between different sizes (allometry) in the two environments. We found that previous morphological descriptions of the two species were associated with different sizes of a continuous, statistically significant allometric shape variation, concluding that C. affinis is a junior synonym of C. altimanus. We also found significant differences in the carapace shape between estuarine and marine environments, suggesting an effect of the environmental variables on carapace shape and a potential adaptive value of this trait.Keywords: Cyrtograpsus altimanus, allometry, geometric morphometrics, Río de la Plata estuary, Nuevo Gulf, local adaptations, contrasting habitats. RESUMEN: Diferencias geográficas en la forma Del caparazón Del cangrejo Cyrtograpsus affinis (DecapoDa: VaruniDae) y sus implicaciones taxonómicas. -El género Cyrtograpsus fue tradicionalmente considerado como formado por tres especies: C. angulatus, C. altimanus y C. affinis. Trabajos recientes reportan sólida evidencia que sugiere que C. altimanus y C. affinis pertenecen a una única especie y proponen que las formas que originaron esta clasificación errónea pueden corresponderse con diferentes ecofenotipos o estadios sucesivos del ciclo de vida de una misma especie. Este trabajo presenta un estudio de morfometría geométrica sobre la forma del caparazón de especímenes del género Cyrtograpsus provenientes del estuario del Río de la Plata (36°S) y del golfo Nuevo (42.75°S), en el que se prueban las diferencias en la forma del caparazón relativas al tamaño (alometría) y a los distintos ambientes. Nuestros resultados muestran que las descripciones morfológicas previas de ambas especies están asociadas a diferentes regiones de una variación alométrica continua y estadísticamente significativa, y nos permite concluir que C. affinis es un sinónimo juvenil de C. altimanus. Además encontramos diferencias significativas en la forma del caparazón entre especímenes provenientes del ambiente estuárico y el marino, lo cual sugiere un efecto de ciertas variables ambientales sobre la forma del caparazón y un potencial valor adaptativo de esta característica.
The current study describes the variation in cheliped form of two species of squat lobsters that inhabit the continental margin off Chile: Cervimunida johni and Pleuroncodes monodon. We compared their cheliped form in the context of the reproductive strategy. The general tendency of form variation of both species is similar: chelipeds change, on average, from longer and narrower pollex with short manus to a relatively shorter and wider pollex with longer manus from small to large individuals, respectively. The degree of cheliped arching was greater in males than in females of similar carapace length, and only C. johni males showed fully arched morphology. The allometric trajectories (cheliped shape vs. carapace length) were largely aligned with vectors of mean shape difference in both species. Cheliped form variation of C. johni (extended mate-guarding) resembles the cheliped morphology reported for the related species Munida rugosa more than P. monodon (short mate-guarding). Our results are consistent with previous finding and suggest that the cheliped form variation (from straight and slender to a fully arched morphology) is, or has been, subject to sexual selection through male-male competition for mates. O presente estudo contém uma descrição da variação da forma dos quelípodos de duas espécies de lagostas que habitam a margem continental do Chile: Cervimunida johni e Pleuroncodes monodon. Foi comparada a forma dos quelípodos das espécies em relação à estratégia reprodutiva das espécies. Das espécies estudadas, C. johni apresenta um quelípodo arqueado, uma relação mais estreita entre a forma da quelípodo e comprimento da carapaça e um maior dimorfismo sexual. Apesar de que a tendência geral da variação da forma dos quelípodos de ambas as espécies é quase semelhante, a variação morfológica dos quelípodos C. johni ("extended mate-guarding") assemelha-se à relatada para M. rugosa mais do que assemelha-se P. monodon ("short mate-guarding"). Nossos resultados são consistentes com os resultados anteriores e sugerem que a variação na forma do quelípodo (de uma forma reta e fina para um formato arqueado) é ou foi sujeita à seleção sexual através da competição entre machos. AbstrAct
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