Effects of salinity and two food regimes on survival, fecundity and sex ratio in two groups of Dinophilus gyrociliatus (Polychaeta : Dinophilidae)

The effects of salinity on survival, fecundity and sex ratio were studied in Dinophilus gyrociliatus; a small polychaete with a short lifespan, semicontinuous reproduction and progamic sex determination. The experiment was performed on two groups, originally collected as one sample, from the port of Genoa in November 1995. The specimens were separated into two groups, differing only in their diet: the first was fed with frozen spinach and the other with Tetramin Mikromin. The experiment was performed in 1998 after numerous generations had been maintained under laboratory conditions. The maximum expectation of life at birth varied with salinity level, and both groups showed a better tolerance towards a salinity below 35 psu than above 35 psu. The influence of both salinity and diet on fecundity had statistical significance even though the greatest differences were due to diet; animals fed with Tetramin had a two- or even threefold higher fecundity than those fed with spinach. Diet and salinity also had a statistically significant influence on the sex ratio. These results, as well as all the experimental observations carried out up to date, indicate that in D. gyrociliatus the determination of the sex ratio has a strong phenotypic component.