Sexually dimorphic skin glands in the invasive species Lithobates catesbeianus (Anura:Ranidae)

Males of most amphibian species possess specialized cutaneous glands, known as sexually dimorphic skin glands (SDSGs). SDSGs are usually clustered in specific body regions and are externally visible, but in some cases, external differences between males and females can be slight or absent, and the occurrence of SDSGs can only be disclosed by histological studies. Chemical signals produced by SDSGs markedly affect amphibian behaviour and reproduction, and therefore their occurrence, features, and location in the body could provide information on potential mechanisms of intraspecific communication in a particular species. In the present study, we perform light microscope (both histological and histochemical), and scanning electron microscope studies of skin samples from male and female adult specimens of the invasive bullfrog Lithobates catesbeianus, covering several body regions that could hold SDSGs. Most skin areas analysed showed only ordinary granular and mucous glands despite remarkable sexual dimorphism that could be externally observed. By contrast, the male nuptial pads contained exclusively SDSGs that were hypertrophied specialized mucous glands (SMGs), closely resembling breeding glands described in other anurans. Our histochemical study revealed that these SMGs contain heterogeneous populations of secretory cells, possibly involved in pheromone production. We discuss these characteristics of the SDSGs found in L. catesbeianus, as well as the surface specialization of the nuptial pads (achieved by scanning electron microscopy) in the light of their potential role in the chemical communication in this invasive species.