This chapter reviews the current knowledge of the marine (including deep-sea species) and freshwater fishes of the Gulf of Guinea oceanic islands. Some biogeographic and conservation considerations are also presented. A total of 1045 species are likely present in the region, including 107 Elasmobranchii (37 confirmed, 65 potential, and 5 erroneous), one confirmed Holocephali, and 937 Actinopteri species (515 confirmed, 385 potential, 32 erroneous, and 5 questionable). Most of the coastal species are shared with the surrounding African continental shelf, while several species are amphi-Atlantic (present in both sides of the Atlantic Ocean), and some species have sister-species in the western Atlantic. A total of 15 species are endemic to the region, and 2 are introduced. Further studies are still needed to better understand the ichthyofauna of the Gulf of Guinea oceanic islands and help policymakers better define conservation and protection plans.

The Squalius alburnoides hybrid complex has diploid, triploid and some rare tetraploid male and female forms, with triploid females predominating in most populations. The reproductive traits of diploids and triploids from a population in a tri- butary of the Guadiana River, Portugal, were compared, during a two-years period. Few differences were found in the growth and reproductive traits of diploid and trip- loid females. Marginal differences were found in the longevity, with a few triploid females living up to six years compared with a maximum of five years for diploid females and a maximum of four years for diploid males. Both diploid and triploid females exhibited rapid growth in the first two years of life followed by rapid drop off linked to attaining sexual maturity. Reproductive effort increased with age but was mainly linked to number of eggs produced and not to egg size, since oocyte size did not vary with age or length of fish in either ploidy forms. Both forms exhibited a pro- tracted spawning period between March and June, suggesting multiple-spawning be- haviour. Triploid females had a slightly smaller oocyte diameter that may be due to the production of reduced (haploid) oocytes while diploid females produced diploid (unre- duced) oocytes The potentially higher production of diploid oocytes (which become triploid eggs after fertilisation) by diploid females could at least partially account for the higher percentage of triploids in natural populations. In addition, the production of bigger (diploid) oocytes, with possibly higher energetic content could lead to bigger fish larvae, which tend to have a higher survival rate. The time of spawning of both diploids and triploids appears to be synchronised, although differences were found in