Morphological diversity in the parotoid macroglands of neotropical true toads (Anura: Bufonidae) and its association with the release of glandular secretion

Abstract

The parotoid macrogland in toads of the family Bufonidae is a complex structure mainly related to a defensive function. It presents considerable variability among species regarding their morphology, chemical nature of the glandular content, and secretion release mechanisms. This macrogland is formed by a group of large syncytial glands (LSGs) that like any other dermal gland, comprise a pore, a secretory duct, a neck, and adenomere. Previous works have shown that the neck is a region with a high morphological variability among species. Here, we study the secretory duct and neck of LSGs from parotoids in nine bufonids using light and electron microscopy, linking the morphological variation to what is currently known about secretion discharge mechanisms in this family. Our results show that differences in the LSGs are mainly associated with pore shape, duct-neck length, the presence of epithelial plug, and the presence/absence of two accessory glands, namely intraepithelial and duct glands. Besides, two types of duct glands were identified according to whether their secretion is released to the epidermal surface or into the most external portion of the LSGs neck. Summarizing this information, we defined four morphological patterns for LSGs necks, two of them lacking associated duct glands, and the others differing on the presence of one or two types of duct glands. We found a possible relationship between gland morphology and the secretion release mechanisms in different toad species, where the length of the duct and neck, and the presence or absence of a conical plug, could be relevant to the release mechanism in form of droplets or spray.