Abstract.The ecology and systematics of the hydroid population ofPosidonia oceanicameadows have been studied in two different zones of the Western Mediterranean. A new sampling method has been employed and the results obtained have been compared with those of a standard method. The distribution of the species and several morphological variations are pointed out. Depth‐related faunistic zones are evidenced. The key‐species of thePosidoniahydroid population arc individua
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