Nanism (dwarfism) in fish: a comparison between red mullet Mullus barbatus from the southeastern and the central Mediterranean

The gradient of environmental conditions from west to east in the Mediterranean results in very low primary productivity in the eastern area of this sea. This impoverishment is expressed also in higher trophic levels and has been accounted for by several faunistic phenomena. One of these is 'Levantine nanism' (dwarfism); this is characterized by smaller body size of specimens in the Levantine basin compared with conspecifics in the western Mediterranean. Nanism has been hypothesized for various taxonomic groups in the Mediterranean, but no quantitative study has yet been carried out to confirm it. In the present study male and female red mullet Mullus barbatus from trawl surveys carried out along the Mediterranean coast of Israel and the Strait of Sicily were sampled. Each fish was sexed, measured for total length (TL) and aged by otolith readings. ANCOVA analyses indicated that the TL of both males and females from Israel was significantly smaller at increasing ages than conspecifics of the same age and sex from Sicily. In addition, preliminary examination of sexual maturity of M. barba- tus of both sexes indicated that the Israeli fish sexually mature at a smaller size than conspecifics of the same sex from Sicily. These findings can be explained by low productivity in the Levantine basin com- pared with the western Mediterranean. The low and unpredictable food supply in the southeastern Mediterranean may result in a form of r-strategy of the marginal eastern populations of this species that leads to early reproduction and smaller body size. The average higher water temperature may also partly explain Levantine nanism, as it may cause more intensive metabolic processes in the south- eastern population, resulting in earlier sexual maturity and cessation of growth.