Identification of Marine Fish Eggs: A Biochemical Genetics Approach

Abstract

We studied inter- and intra-specific variation at LDH (lactate dehydrogenase; EC 1.1.1.27) loci in 11 gadoid and 6 flatfish species regularly occurring in Norwegian coastal waters by means of isoelectric focusing and histochemical staining techniques. No two species showed identical locus A (white skeletal muscle predominating) zymograms. Polymorphism at LDH A was observed in haddock (Gadus aeglefinus), coalfish (G. virens), Norway pout (G. esmarkii), four-bearded rockling (Onos cimbrius), and dab (Limanda limanda). The allelic nature of observed LDH variants was confirmed by controlled crossings in cod (G. morhua, locus B), Norway pout (locus A), and haddock (locus A). Analyses of artificially fertilized eggs (in four species) and prenatal eggs from ripening ovaries (in 10 species) revealed a general predominance of locus A products. The maternal LDH A activity present in newly released eggs was overtaken by embryo-synthesized enzyme 1–2 d after fertilization. No unique embryonic LDH loci were observed. LDH A zymograms thus appear to be useful as a practically diagnostic tool for the identification of the pelagic fish eggs usually found in these waters. The identification method, applied to samples of pelagic eggs collected at the coast of northern Norway during March–May 1981 and 1982, showed that in a total of ~ 1500 eggs, the following species were represented: cod, Norway pout, haddock, coalfish, dab, plaice (Pleuronectes platessa), and long rough dab (Hippo-glossoides plates-soides). The species composition in these egg samples varied considerably with sampling location and sampling depth.Key words: fish egg identification, biochemical genetics, tissue enzymes, gadoids, flatfishes