The aim of this paper is to present the first record of ctenophore species Pleurobrachia pileus (O. F. Müller, 1776) in the coral reef as was recently found in Iraqi marine waters. The specimens were collected from two sites, the first was in Khor Abdullah during May 2015, and the second site was located in the pelagic water of the coral reef area, near the Al-Basrah deep sea crude oil marine loading terminal. Three samples were collected at this site during May 2015, February and March 2018 which showed that P. pileus were present at a densities of 3.0, 2.2 and 0.55 ind./ m3 respectively. The species can affect on the abundance of other zooplankton community through predation.
The results of examining the stomach contents revealed that they are important zooplanktivorous species; their diets comprised large number of zooplankton as well as egg and fish larvae. The calanoid copepods formed the highest percentage of the diet, reaching 47%, followed by cyclopoid copepods 30%, and then the fish larvae formed 20% of the diet.
The current investigation showed that the density of zooplankton decreased significantly in the second site of all the groups, but the highest was in Calanoida group (324 ind./ m3) coincident with the presence of noticeable numbers of P. pileus (0.55-3.0 ind./ m3), in the same site compared to the density of Calanoida in the first site, which amounted to 991ind./ m3; as well, the increase was noticeable in relation to the rest of the other groups of zooplankton during the same period; such a state confirms the effect of predation by P. pileus on the zooplankton community. The study recommended that more attention should be given to investigate this group of zooplankton, and specific care should be exerted in preserving the specimens collected from the sea.
