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The first appearance of IgM in lymphocytes varies considerably among fish species. Generally, the first appearance of B-lymphocytes and immunoglobulins is late in marine species compared to fresh water species, and larvae have reached about 20-30 mm in length when IgM is first expressed. Rainbow trout and channel catfish show the first appearances of surface IgM at about 1 week after hatching. Marine species like the sea bass, spotted wolffish and cod show IgM positive lymphocytes 1-10 weeks after hatching. Transfer of maternal antibody to eggs and embryos has been demonstrated in several species. Examples are plaice, tilapia, carp, sea bass and salmon, but not cod. The ontogeny of complement component C3 has been studied in Atlantic halibut (Hippoglossus hippoglossus L.), Atlantic cod (Gadus morhua L.) and the spotted wolffish (Anarhichas minor O.). By Western blotting experiments C3 was found in unfertilised eggs in the spotted wolffish indicating a maternal transfer. RT-PCR analysis revealed C3 mRNA transcripts from 290 degrees in spotted wolffish eggs. Using immunohistochemistry and in situ hybridisation, C3 was found in liver, brain, kidney and muscle of cod larvae 2 days post-hatching and in intestines, pancreas, heart and gills at different stages of larval development. Also, C3 was detectable in halibut larvae in yolk sac, muscle, liver, brain, chondrocytes, spinal chord, eye, heart, intestines and kidney. These studies suggest that complement may play a role in generation of different organs, not only in the defence against invading pathogens. Lysozyme is a bactericidal enzyme present in mucus, lymphoid tissue and serum of most fish species, but not in cod and wolffish. The enzyme has been detected in oocytes, fertilised eggs and larval stages of fish species like coho salmon, sea bass and tilapia. The activity of other enzymes like the cathepsins has been described in eggs and larvae of sea bass, cod and salmonids. Cathepsins may have a bactericidal role in the skin of fish. Lectins are carbohydrate-binding proteins that interact with pathogenic surface structures that result in opsonization, phagocytosis or activation of complement. Lectins have been isolated from the eggs of various fish species.

Non-specific mechanisms are important in the defence of all multicellular animals against pathogenic microorganisms. Macrophages and granulocytes play a central role in this respect. It is thus pertinent to develop methods for obtaining and cultivation of macrophages and assessing their functions in the spotted wolffish, a cold water species of current interest for the aquaculture industry. Kidney macrophages from the spotted wolffish ( Anarhichas minor Olafsen) were isolated by density sedimentation using Percoll. The cells were highly phagocytic and possessed typical macrophage morphology evaluated by transmission and scanning electron microscopy. Using electron microscopic analysis, the size of the macrophages, collected from the Percoll density interface, was 5–9 μm. The viability in vitro was highest (87.1%) when the cells were kept at 13 °C with the addition of synthetic serum replacement (SSR-2) when measured 24 h after seeding. One day old cells were not significantly activated by addition of bacterial lipopolysaccharide (LPS) for 24 h when measured by reduction of nitroblue tetrazolium compared to control cells. The cells were negative in respect to synthesis and contents of complement component C3.

Eggs and larvae from the spotted wolffish (Anarhichas minor Olafsen) were analysed to study the ontogeny of the complement component C3. By Western blotting experiments, the results revealed that C3 was present already in unfertilised eggs and that there was a decrease in C3 content towards hatch, whereas the results using reverse transcription-polymerase chain-reaction showed a continuous increase of C3 mRNA content towards hatch. These findings indicate a transfer of maternal complement component C3 to the offspring where the content of C3 decreases and the level of C3 mRNA increases towards hatch. Immunohistochemical experiments revealed that only the liver of spotted wolffish larvae contained complement component C3, suggesting that the liver is the main site of production.

The spotted wolffish Anarhichas minor Olafsen is a promising new species in aquaculture in the cold waters of northern Norway. In this paper, some basic immunological studies of this marine species are reported. Of comparative interest are the cDNA sequences of the immunoglobulin transcript and the antibody responses to model antigens. Of more practical importance are the humoral immune responses and antibody specificities to potentially pathogenic bacteria. Full length cDNA clones encoding the immunoglobulin heavy and light chains in the spotted wolffish were sequenced demonstrating variable degrees of similarity to other teleost fish species. Also in the spotted wolffish the CH4 domain was deleted in the transmembrane form of the immunoglobulin heavy chain (IgH) as a receptor on B cells, with the transmembrane exon spliced directly to the CH3 domain. The antibody responses to various antigens like hapten-carrier molecules, protein antigens and bacterial pathogens were relatively high, but with some interesting exceptions. Anti-hapten responses to NIP and FITC were high while anti-DNS responses were low, but more surprisingly, there was hardly any B-cell response to the carrier molecule LPH. On the other hand, protein antigens like CGG and BSA were highly immunogenic in the spotted wolffish as were the bacterial antigens Vibrio anguillarum, V. salmonicida and Aeromonas salmonicida.