Functional and morphological plasticity of crocodile (Crocodylus porosus) salt glands

The estuarine crocodile, Crocodylus porosus, inhabits both freshwater and hypersaline waterways and maintains ionic homeostasis by excreting excess sodium and chloride ions via lingual salt glands. In the present study, we sought to investigate the phenotypic plasticity, both morphological and functional, in the lingual salt glands of the estuarine crocodile associated with chronic exposure to freshwater (FW) and saltwater(SW) environments. Examination of haematological parameters indicated that there were no long-term disruptions to ionic homeostasis with prolonged exposure to SW. Maximal secretory rates from the salt glands of SW-acclimated animals (100.8±14.7 μmol 100 g–0.7 body mass h–1) were almost three times greater than those of FW-acclimated animals (31.6±6.2 μmol 100 g–0.7 body mass h–1). There were no differences in the mass-specific metabolic rate of salt gland tissue slices from FW- and SW-acclimated animals(558.9±49.6 and 527.3±142.8 μl O2g–1 h–1, respectively). Stimulation of the tissue slices from SW-acclimated animals by methacholine resulted in a 33%increase in oxygen consumption rate. There was no significant increase in the metabolic rate of tissues from FW-acclimated animals in response to methacholine. Morphologically, the secretory cells from the salt glands of SW-acclimated animals were larger than those of FW-acclimated animals. In addition, there were significantly more mitochondria per unit volume in secretory tissue from SW-acclimated animals. The results from this study demonstrate that the salt glands of C. porosus are phenotypically plastic, both morphologically and functionally and acclimate to changes in environmental salinity.

A gharial from the Oligocene of Puerto Rico: transoceanic dispersal in the history of a non-marine reptile

The Indian gharial (Gavialis gangeticus) is not found in saltwater, but the geographical distribution of fossil relatives suggests a derivation from ancestors that lived in, or were at least able to withstand, saline conditions. Here, we describe a new Oligocene gharial, Aktiogavialis puertoricensis, from deltaic-coastal deposits of northern Puerto Rico. It is related to a clade of Neogene gharials otherwise restricted to South America. Its geological and geographical settings, along with its phylogenetic relationships, are consistent with two scenarios: (i) that a single trans-Atlantic dispersal event during the Tertiary explains the South American Neogene gharial assemblage and (ii) that stem gharials were coastal animals and their current restriction to freshwater settings is a comparatively recent environmental shift for the group. This discovery highlights the importance of including fossil information in a phylogenetic context when assessing the ecological history of modern organisms.

Cholinergic and adrenergic innervation of lingual salt glands of the estuarine crocodile, Crocodylus porosus

Many marine reptiles and birds possess extrarenal salt glands that facilitate the excretion of excess sodium and chloride ions accumulated as a consequence of living in saline environments. Control of the secretory activity of avian salt glands is under neural control, but little information is available on the control of reptilian salt glands. Innervation of the lingual salt glands of the salt water crocodile, Crocodylus porosus, was examined in salt water-acclimated animals using histological methods. Extensive networks of both cholinergic and adrenergic nerve fibres were identified close to salt-secreting lobules and vasculature. The identification of both catecholamine-containing and cholinergic neurons in the salt gland epithelium and close to major blood vessels in the tissue suggests the action of the neurotransmitters on the salt-secreting epithelium itself and the rich vascular network of the lingual salt glands.

Morphology of the cloaca in the estuarine crocodile, Crocodylus porosus, and its plastic response to salinity

The cloacal complex of Crocodylus porosus is composed of three chambers (proctodaeum, urodaeum, and coprodaeum) separated by tight, muscular sphincters. The proctodaeum is proximal to the cloacal vent and houses the genitalia. The urodaeum is the largest chamber, is capable of storing large quantities of urine, and is lined with an epithelium with the capacity for transepithelial water and ion exchange. The coprodaeum, the most orad cloacal chamber, is a small, only marginally expandable chamber that has an epithelium composed almost entirely of mucus-secreting cells. The coprodaeum and lower intestine are reported to be the site(s) for urine modification in birds and bladderless lizards. A radiographic trace of urine storage in C. porosus kept for 2 months under hyperosmotic conditions showed no signs of retrograde movement of urine into the coprodaeum or rectum. Instead, urine was stored in the urodaeum of C. porosus. Examination of the mucosal surface of the urodaeum by SEM showed a plastic response to environmental salinity, with a possible increase in surface area in animals kept in hyperosmotic water compared with animals from fresh water. We propose the urodaeum as the primary site for postrenal modification of urine in C. porosus.