Characterisation of proteins in epidermal mucus of discus fish ( Symphysodon spp.) during parental phase

We conducted a series of experiments to investigate protein content and properties of epidermal mucus from discus fish ( Symphysodon spp.), a cichlid species demonstrating parental feeding of larvae with mucus secretion. Observation on the ontogenic development of larval biting behavior showed that feeding rate on parental mucus increased until 15 days of free-swimming period, followed by a decreasing trend, presumably coinciding with the onset of full capacity for exogenous feeding. Analysis of total protein content showed a higher amount in parental mucus (0.73 ± 0.03 mg per ml) as compared to juvenile mucus (0.63 ± 0.01 mg per ml). Electrophoretic pattern also showed similar proteins with differences in levels of expression between parental and juvenile fish mucus. Mass spectrometry analysis revealed the increased expression of a discus type II epidermal keratin in parental mucus. Free amino acid analysis also showed the presence of several essential amino acids such as lysine and phenylalanine in mucus of parental fish.