Abstract
The Cooloola sedgefrog (Litoria cooloolensis) is one of a number of frog species endemic to the coastal sandy lowlands of east Australia exhibiting remarkable tolerance to dilute waters of low pH (< pH 3.5). To investigate the physiological and morphological underpinnings of acid tolerance in L. cooloolensis larvae, we compared Na+ balance, uptake and efflux rates, and gill and skin morphology in larvae reared in circum-neutral (pH 6.5) and pH 3.5 water. We hypothesised that L. cooloolensis larvae would be more resistant to ionregulatory disturbance and epithelial damage at low pH relative to acid-sensitive species. Net Na+ flux rates were not significantly different from zero in L. cooloolensis larvae reared at pH 3.5 and in acid-naïve animals maintained in pH 6.5 water. Animals reared at pH 6.5 and acutely exposed to pH 3.5, however, exhibited a net loss of Na+ due to inhibition of Na+ uptake. In contrast, L. cooloolensis larvae reared at pH 3.5 maintained Na+ balance at pH 3.5 and did not exhibit inhibition of Na+ uptake at this pH. Investigation of Na+ transport kinetics and the morphology of the gills and integument suggests tolerance of L. cooloolensis larvae to low pH may be attributed to a high capacity for branchial Na+ uptake, increased tight junction length and elevated mucus production at the gills and integument. These factors confer resistance to acid damage and disruption of ionic homeostasis which would otherwise result in the death of amphibian larvae exposed to waters of pH 4.0 and less.
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Abbreviations
- ASW:
-
Artificial soft water
- ICPOES/ICPMS:
-
Inductively coupled plasma optical emission spectroscopy/inductively coupled plasma mass spectroscopy
- MS222:
-
Ethyl 3-aminobenzoate methanesulfonate
- J in :
-
Rate of Na+ uptake
- J out :
-
Rate of Na+ efflux
- J net :
-
Net Na+ flux
- WBA:
-
Counts per minute of whole tadpoles
- SA:
-
Average specific activity
- m:
-
Dry mass of the tadpole
- t :
-
Time
- J max :
-
Maximum Na+ uptake rate
- K m :
-
Na+ concentration at 50% of Jmax
- DBM:
-
Dry body mass
- TEM:
-
Transmission electron micrograph
- LM:
-
Light micrograph
- SEM:
-
Scanning electron micrograph
- TJ:
-
Tight junction
- MR:
-
Mitochondrion-rich cell
- ECaC:
-
Epithelial Ca2+ channel
- ENaC:
-
Epithelial Na+ channel
- MSV:
-
Mucus secretory vesicle
- CCA:
-
Cell cross-sectional area
- TMSVA:
-
Total cross-sectional area of mucus secretory vesicles
- bl:
-
Basal lamina
- cap:
-
Capillary
- cc:
-
Ciliated cell
- ct:
-
Connective tissue
- d:
-
Desmosome
- ec:
-
Squamous epithelial cell of outer integument
- fp:
-
Filter plates
- ga:
-
Gill arch
- gt:
-
Gill tuft
- hd:
-
Hemi-desmosome
- mf:
-
Mucus fuzz
- mr:
-
Microridge
- mt:
-
Mitochondria
- n:
-
Nucleus
- pc:
-
Pavement cell
- rbc:
-
Red blood cell
- tj:
-
Tight junction
- v:
-
Velum
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All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by EAM and RLC. The first draft of the manuscript was written by EAM and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.
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Meyer, E.A., Franklin, C.E. & Cramp, R.L. Physiological and morphological correlates of extreme acid tolerance in larvae of the acidophilic amphibian Litoria cooloolensis. J Comp Physiol B 191, 159–171 (2021). https://doi.org/10.1007/s00360-020-01316-y
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DOI: https://doi.org/10.1007/s00360-020-01316-y