The present paper deals with the morphology and population structure of Diopatra neapolitana collected in Izmir Bay between July 1997 and June 1998. The density of the species in the area ranged from 42 ind·m−2 (November 1997) to 198 ind·m−2 (March 1998), the biomass from 84 g·m−2 (November) to 408 g·m−2 (March). The coelom contained gametes in all months, except for January, indicating that it has a very long reproductive period in the area. The relationships among the biometrical measurements taken on specimens of Diopatra neapolitana from Izmir Bay were assessed by using regression and correlation analysis. A re‐description of the species is provided.

The present study reports the occurrence of two exotic species of polychaetes in Izmir Bay, the eastern Mediterranean: Streblospio gynobranchiata, newly reported for the Mediterranean, and Polydora cornuta. Their establishment in the area is most probably transport by ballast water since these two species greatly dominated polluted muddy bottoms near and in Alsancak Harbour. The maximum density and biomass of S. gynobranchiata in the area were 34,270 ind m−2 and 9·4 g wwt m−2, and those of P. cornuta were 3170 ind m−2 and 2·1 g wwt m−2, respectively. These two species accounted for more than 60% of total faunal populations in the majority of samples collected in winter. Both species are capable of reproducing in the area both in summer and winter. The morphological, ecological and distributional characteristics of these species as well as their impacts on the prevailing ecosystem are analysed and discussed.

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Temporal and spatial variation in soft‐bottom benthic communities following recovery from a pollution episode were studied between January and September 2004 in and around Alsancak Harbor, located in the polluted part of Izmir Bay (Aegean Sea, eastern Mediterranean). Samples were collected at seven stations by van Veen grab. Three additional stations were sampled by means of a beam trawl to take into account large mobile animals and for a better estimate of the local biodiversity. A total of 231 species belonging to 10 zoobenthic groups were found. Polychaetes contributed 90% of the total faunal populations and mollusks 87% of the total biomass in the area. Community parameters varied significantly among stations and sampling periods; number of species ranged from 2 to 79 per 0.1 m2 grab sample; density from 20 to 81,720 ind·m−2; biomass from 0.1 to 4190 g·m−2; Shannon–Wiener diversity index (log2 base) from 0.4 to 4.4; and Pielou's evenness index from 0.11 to 1.0. Collections indicate that a number of species, including those sensitive to pollution, have colonized the area where azoic conditions had been previously reported. A total of six exotic species, Streblospio gynobranchiata, Polydora cornuta, Hydroides dianthus, Hydroides elegans, Anadara demiri and Fulvia fragilis, probably transferred to the area via ballast water or hull fouling, dominated soft or hard substrata in and near Alsancak Harbor. The first two species accounted for more than 70% of the total population in the area, while A. demiri contributed the most to the biomass (93%, at station 7).

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