The vertical distribution of eggs of Maurolicus muelleri

Abstract

The vertical distribution of eggs of the sternoptychid Maurolicus muelleri from different localities worldwide was analysed on the basis of apparently conflicting literature and new data. Although the vertical distribution has been classified as "variable", most data from hydrographically comparable areas were similar. Differences at both larger and smaller geographical scales were mainly a consequence of environmental variability and to a minor extent the result of sampling limitations. In North Atlantic waters of average salinity and with a temperature range of 8–10°C, eggs generally occur deep because of their relatively high density. In less saline waters off New Zealand and also perhaps off Japan the same has been found. In highly saline waters such as those of the Mediterranean Sea, the eggs have a much shallower distribution. In areas without vertical motion of water masses, the depth of occurrence depends directly on the density of seawater. The equilibrium densities are not uniform worldwide and some density ranges are given. In areas with lower salinities, eggs show adaptations via a larger "perivitelline space", which results in reduced specific gravity. Eggs cannot compensate for high seawater densities. In upwelling ecosystems, eggs are found shallower than would be expected from the equilibrium density range present. Alternative hypotheses are discussed, that in upwelling systems spawning occurs shallower, or that eggs are transported upwards with upwelling waters. In the Benguela upwelling system, shallow distributions could coincide with high mortality of eggs.