Parotoid macroglands in toad (Rhinella jimi): their structure and functioning in passive defence.

When toads (Rhinella) are threatened they inflate their lungs and tilt the body towards the predator, exposing their parotoid macroglands. Venom discharge, however, needs a mechanical pressure onto the parotoids exerted by the bite of the predator. The structure of Rhinella jimi parotoids was described before and after manual compression onto the macroglands mimicking a predator attack. Parotoids are formed by honeycomb-like collagenous alveoli. Each alveolus contains a syncytial gland enveloped by a myoepithelium and is provided with a duct surrounded by differentiated glands. The epithelium lining the duct is very thick and practically obstructs the ductal lumen, leaving only a narrow slit in the centre. After mechanical compression the venom is expelled as a thin jet and the venom glands are entirely emptied. The force applied by a bite of a potential predator may increase alveolar pressure, forcing the venom to be expelled as a thin jet through the narrow ductal slit. We suggest that the mechanism for venom discharge within all bufonids is possibly similar to that described herein for Rhinella jimi and that parotoids should be considered as cutaneous organs separate from the rest of the skin specially evolved for an efficient passive defence.

Parotoid macroglands in toad (Rhinella jimi): Their structure and functioning in passive defence

When toads (Rhinella) are threatened they inflate their lungs and tilt the body towards the predator, exposing their parotoid macroglands. Venom discharge, however, needs a mechanical pressure onto the parotoids exerted by the bite of the predator. The structure of Rhinella jimi parotoids was described before and after manual compression onto the macroglands mimicking a predator attack. Parotoids are formed by honeycomb-like collagenous alveoli. Each alveolus contains a syncytial gland enveloped by a myoepithelium and is provided with a duct surrounded by differentiated glands. The epithelium lining the duct is very thick and practically obstructs the ductal lumen, leaving only a narrow slit in the centre. After mechanical compression the venom is expelled as a thin jet and the venom glands are entirely emptied. The force applied by a bite of a potential predator may increase alveolar pressure, forcing the venom to be expelled as a thin jet through the narrow ductal slit. We suggest that the mechanism for venom discharge within all bufonids is possibly similar to that described herein for Rhinella jimi and that parotoids should be considered as cutaneous organs separate from the rest of the skin specially evolved for an efficient passive defence.

Parental investment by skin feeding in a caecilian amphibian

Although the initial growth and development of most multi-cellular animals depends on the provision of yolk, there are many varied contrivances by which animals provide additional or alternative investment in their offspring 1. Providing offspring with additional nutrition should be favoured by natural selection when the consequent increased fitness of the young offsets any corresponding reduction in fecundity2. Alternative forms of nutrition may allow parents to delay and potentially redirect their investment. Here we report a remarkable form of parental care and mechanism of parent-offspring nutrient transfer in a caecilian amphibian. Boulengerula taitanus is a direct-developing, oviparous caecilian3, the skin of which is transformed in brooding females to provide a rich supply of nutrients for the developing offspring. Young animals are equipped with a specialized dentition, which they use to peel and eat the outer layer of their mother's modified skin. This new form of parental care provides a plausible intermediate stage in the evolution of viviparity in caecilians. At independence, offspring of viviparous and of oviparous dermatotrophic caecilians are relatively large despite being provided with relatively little yolk. The specialized dentition of skin-feeding (dermatophagous) caecilians may constitute a pre-adaptation to the fetal feeding on the oviduct lining of viviparous caecilians.