The ecology of Donax trunculus Linnaeus and Donax semistriatus Poli from the Mediterranean coast of Israel

The ecology of two species of Donax D. trunculus Linnaeus and D. semistriatus Poli, from the shallow water in the sandy beach of Haifa Bay, northern Israel, was studied during 20 months (June 1983-January 1985). D. trunculus was the dominant species and was found at high densities during the entire year; D. semistriatus was less abundant and sometimes disappeared from the area. The recruitment period of D. trunculus was from July to September while for D. semistriatus several recruitments occurred during the period sampled. Shell length increment was determined. There was a rapid increase in the shell length of D. trunculus during the first 2.5 months of the animal's life. By the end of this period, most of the animals attained ≈ 50 % of the maximal size. Changes in the body weight, energy contents of the soft parts and oocyte size of D. trunculus were determined monthly. These coincided with the values of recruitments and growth found in the field. Abiotic variables, such as temperature, salinity, POC in the water and OC in the sand, were analysed. Predation by Natica josephinus and mortality for other reasons were determined; the percentage of empty shells of D. semistriatus was very high, indicating that this species is susceptible to environmental stresses. There was no preference of species by the predator and the proportion of drilled shells was similar in both species.