To study how teleosts have modified their immunoglobulin structures to adapt to the Antarctic environment, we investigated Trematomus bernacchii IgM at protein and gene levels. Serum IgM was purified, characterized and quantified. The nucleotide and deduced amino acid sequences of IgL and IgH were determined. Upon comparison with other teleosts, T. bernacchii IgH showed a higher hydrophilic character and the presence of two remarkable insertions, probably increasing the flexibility of the molecule. To investigate the amplitude of the antibody repertoire, a VH library was constructed. The analysis of rearranged VH/D/JH transcripts revealed a high occurrence of the RGYW motif and a clear bias in the usage of serine codons. Diversity generated by insertions/deletions occurred more often than in other species. VH sequences fell into only two gene families referred to as Trbe VH I and Trbe VH II. The deduced amino acid sequence of the membrane region was found to be the longest and an F/Y substitution was observed at a highly conserved position. Models of the transmembrane helices of T. bernacchii and other teleost Ig were constructed by molecular dynamics. Finally, the analysis of the antibody specificity revealed the presence of antibodies specific to the proteins of nematode parasites.