Immune and hormonal modulation in the postprandial period of bullfrogs (Lithobates catesbeianus)

Mammals show immune up-regulation and increased plasma and local (gastrointestinal tract) concentrations of some immunoregulatory hormones, such as corticosterone and melatonin, after feeding. However, little is known about the endocrine and immune modulation in the postprandial period of ectothermic animals. This study investigated the effects of feeding on endocrine and immune responses in the bullfrog (Lithobates catesbeianus). Frogs were fasted for 10 days and divided into two groups: fasted and fed with fish feed (5% of body mass). Blood and gastrointestinal tract tissues (stomach and intestine) were collected at 6, 24, 48, 96, and 168 h to measure neutrophil/lymphocyte ratio, plasma bacterial killing ability, phagocytosis of blood leukocytes, plasma corticosterone and melatonin; and stomach and intestine melatonin. Feeding increased plasma corticosterone at 24 h and decreased at 168 h; and increased neutrophil/lymphocyte ratio at 6, 24, and 96 h. We also observed decreased bacterial killing ability 48 h after feeding. Stomach melatonin increased after 17-days fasting. We show that feeding activates the hypothalamic-pituitary-interrenal axis and promotes transient immunosuppression, without stimulating an inflammatory response. Increased CORT may mobilize energy to support the digestive processes and melatonin may protect the stomach during fasting. We conclude feeding modulate secretion of immunoregulatory hormones, increasing plasma CORT levels in the beginning followed by a decrease in the end of meal digestion; and systemic immune cell redistribution, increasing NL ratio during almost all meal digestion in bullfrogs. Also, fasting modulate secretion of melatonin in the stomach.

Distribution Change of Invasive American Bullfrogs (Lithobates catesbeianus) by Future Climate Threaten Endangered Suweon Treefrog (Hyla suweonensis) in South Korea

The American Bullfrog (Lithobates catesbeianus) has been imported into South Korea in earnest for food since the 1970s and introduced into nature due to release and escape. Accordingly, the influx and spread of American Bullfrogs are expected to have a direct impact on native species, but few related studies have been conducted on this. We predicted changes in the potential distribution and future distribution based on climate change scenarios to analyze how those changes affect critically endangered Suwon treefrogs. Suwon treefrog sites (63.9%, 78/122) overlapped with the distribution of Bullfrogs. According to the prediction of the future distribution of Bullfrogs, the overlapping of American Bullfrogs and Suwon treefrog will remain similar to the current level in the Representative Concentration Pathway (RCP) 4.5 scenario. On the other hand, in the RCP 8.5 scenario, the number of overlapping sites will increase to 72.1% (88/122) due to the spreading of the American Bullfrogs. The results show that climate change directly affects the distribution expansion of the American Bullfrogs but also indirectly can lead to an increased threat to Suwon treefrogs. In conclusion, our results strongly suggest why climate change should be actively addressed in terms of the spread of invasive species and the protection of endangered species.

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.

Histopathological changes in the gills of zebrafish (Danio rerio) and bullfrog tadpoles (Lithobates catesbeianus) caused by the use of formaldehyde

Formaldehyde is a carcinogenic and aggressive agent mainly to epithelial tissues. However, for rearing aquatic organisms its use is common for the treatment of fungi and parasites, and the use of incorrect doses can harm the health and life of these animals. The fish species Danio rerio and the tadpoles of the species Lithobates catesbeianus are internationally recognized for use in aquatic toxicology tests. We aimed identify the effects caused by formaldehyde on the gills of these two experimental models used in aquatic toxicology, verifying the susceptibility of both species. Acute and chronic tests with formaldehyde were conducted for experiment. We found that the formaldehyde in the higher concentration caused injuries to the gills of both fish and tadpoles, with a loss and displacement of epithelium, vascular congestion, telangiectasia and lamellar epithelial lifting/edema (possible aneurysm), hyperplasia and hypertrophy of epithelial cells, lamellar fusion in addition to the proliferation of mucus-secreting cells and chloride cells. Despite structural differences, the histological changes caused by chronic exposure to formaldehyde in sublethal concentrations were similar in both organisms and we recommend reviewing its use in prophylaxis and in prolonged treatments with this chemical.