MEPS 254:199-211 (2003)  -  doi:10.3354/meps254199

ABSTRACT: Reproduction in the semelparous polychaete Nereis virens occurs at a time of year when seawater temperatures are lower than the optimum temperatures for fertilization and larval development and at a time when seawater temperatures are rising. This study assesses the possible role of intra-specific competitive interactions between settling N. virens post-larvae as a selective pressure for Œearly¹ reproduction. A series of laboratory competition experiments were conducted between 2 cohorts of N. virens post-larvae, under a range of environmental conditions. Significant pre-emptive competition between settling post-larvae was recorded at 15°C, whereby a survival advantage was gained via prior residence with early larval settlers out-competing later arrivals. This competitive interaction was reduced at lower temperatures and no significant effect was measured at 5°C. An increase in age difference between the 2 cohorts of post-larvae increased the competitive effect, whilst photoperiod regime and food availability had no significant effect on the outcome of these experiments. Intra-specific competition was greatest at the temperatures at which maximum fertilization and development are known to occur, but was reduced at the temperature range at which spawning occurs in the field. It is suggested, therefore, that pre-emptive competition between settling post-larvae forms a strong selective pressure for breeding early at a time of year when temperatures are below the optimum for fertilization and development.

KEY WORDS: Polychaeta · Pre-emptive competition · Nereis virens · Seasonal reproduction