Variability of mandible shape in the freshwater glacial relict Eurytemora lacustris (Poppe, 1887) (Copepoda, Calanoida, Temoridae)

Abstract

The mandibles of copepods are used to classify the feeding modes of certain species, because these appendages have specific shapes. The number, shape, location, and structure of the mandibular teeth are traits that are related to the feeding preferences of copepods. Therefore, the mandibles have been used as parameters in order to determine the role of the corresponding species in the trophic network. In this study, we aimed to determine the morphological variability of the mandible shape in a rare calanoid, Eurytemora lacustris (Poppe, 1887). The size of the mandibles of E. lacustris is closely related to its body size; however, considering the proportions of the width of the mandibles to that body size, young individuals have relatively larger mandibles. Despite the large body size of males, the females are characterized by larger mandibles than those of males. The teeth of males were found to be smaller, because of intensive abrasion. Moreover, the season and habitat affected the size of the mandibles and also the values of the edge index (EI). The food conditions in the studied lakes fluctuated within seasons, which led to changes in the shape of the dental crowns. Using this index (EI), E. lacustris was classified as an omnivorous species; however, the range of calculated values varied considerably. We note that the values of the EI decrease along with maturation, which suggests that with increase in age, the copepod becomes more herbivorous; however, this contradicts the paradigm that copepods become omnivorous during maturation. All those morphological differences, furthermore, indicate the high phenotypic plasticity of the species, which was found to be related to maturity, sex, phenology, and the environmental conditions of the habitat.