Zooplankton were collected from four marine ecosystems namely the North Sea (Helgoland - Germany), Banyuls-sur-Mer (Mediterranean sea - France), Abu Qir Bay (Mediterranean Sea- Egypt) and the northern estruarine harbour of the Arabian Gulf (Saudi Arabia). Collection tools used involved primarily the filtration of water by net, collecting the water in bottles/ water samplers or by pumps. Artificial heterologous inseminations on ascidians were tried in the laboratory and the larval stages have been described and identified. Collected zooplaktons were prepared for both macroscopic or / and scanning electron microscopy. All zooplankton were stained with Evans stain or Nile blue or Borax carmine to observe their internal structures since they are mostly transparent. Others were dissected with micro-needles and incised to ease their identification. Marine Species Identification Portal has been applied: http://species-identification.org/index.php//.Six species of Bryozoa were identified namely Bugula neritina (Linnaeus, 1758) and itsbarrel shapedlarva, Electra crustulenta (Pallas, 1766),Bowerbankia gracilis (Leidy, 1855) and its coronated larva, Hippaliosina depressa (Busk, 1854), Nolella dilatata (Marcus, 1940)and Reptadeonella violacea (Johnston, 1847). Two hydrozoan cnidarians were identified namely Obelia geniculata (Linnaeus, 1758) and Pennaria disticha (Goldfuss, 1820). Planula larva of Hydrozoa and the anthozoan Actinodendron sp. were collected from the Mediterranean sea. Two rotifers were identified namely Paraseison annulatus (Claus, 1876) and Seison nebaliae (Grube, 1861).The nematode Anisakis simplex and its third stage larva were extracted from the branchial chambers of ascidians whereas free nematode toothless larval stage has been collected from nekton. Four polychaetes were identified namely Harmothoe sp.,(scale worm), Pomatocerous triqueter (Linnaeus, 1758), Nemidia lawrencii (McIntosh, 1874) with synoneme Nemidia torelli and Notomastus latericeus (Sars, 1851). The copepod Megacyclops viridis (Jurine, 1820) and the gammarid Gammaropsis sp. with Naupli, zoaeaand megalopods were found in the nekton. The the isopod Caecocassidias patagonica (Kussakin, 1967) has been collected from the benthos. The scaphopod Dentalium vulgare (da Costa, 1778) and the bivalve Microgloma turnerae (Sanders and Allen,1973) were found in the benthos. Veliger and glochidia larvae were collected from the nekton. Two species of brittle star namely Amphiura sp and Ophiomastix annulosa were collected from the benthos. Echinoplutei with 8 arms were found in the nekton. Nine ascidian larvae were identified namely larvae of Styela plicata (Lesuaer, 1823), Phallusia mammilata (Cüvier 1815), Corella parallelogramma (Müller,1776), Diplosoma migrans (Menker und Ax. 1970), Halocynthia roretzi (Drasche), Microcosmus claudicans (Savigny,1816), Molgula manhattensis (Dekay, 1843), Ascidiella aspersa (Müller, 1776), and Cnemidocarpa mollis (Stimpson,1852). The abundance and distribution of all plankton studied varied considerably according to seasons and habitats. The findings of this work, the density of each genus or / and species in the four study localities and the presence or absence of a certain zooplankton in the different seasons of the year (faunal composition) will be statistically analyzed in another publication.
