Competitive advantage of the invasive copepod Oithona davisae over the indigenous copepod Oithona nana in the Marmara Sea and Golden Horn Estuary

Abstract

Around 2010, the alien copepod Oithona davisae was introduced to the Marmara Sea and Golden Horn Estuary to live alongside the native species, Oithona nana. By October 2015, the population number, including nauplii, of the introduced species (213,600 ind m−3) was higher than that of the native species (54,900 ind m−3) and close to that of O. davisae in Sevastopol Bay of the Black Sea (342,000 ind m−3) where it appeared 14 years ago. A high percentage of O. davisae females with ovisacs (up to 30% of total female number) carrying up to 15.1 ± 2.1 eggs in the sac and the predominance of nauplii in the population (up to 95% of the total population number) indicate that this alien copepod has become a permanent component of the Marmara Sea and Golden Horn Estuary ecosystem. Our experiments showed that in comparison with widely euryhaline O. davisae, O. nana is a stenohaline species. It has a salinity tolerance in the range of 13–27 psu and, in spite of having a larger body, a lower body density (1.037 ± 0.007 g cm−3) and sinking speed. Carbon-specific respiration rates of both species were close (0.32–0.38 μL O2 μgC−1 day−1) and similar to that of O. davisae from the Black Sea during this period but the frequency and speed of routine jumps, and the maximum escape reaction speed (up to 23.3 ± 1.7 cm s−1) in O. davisae from the Marmara Sea were significantly higher. This indicates the higher adaptive potential of O. davisae in this situation compared to that of O. nana.