December 2023

Costa Nostrum's 2023 Calendar

Pennaria disticha, also known as the Christmas tree hydroid, is a species of athecate hydroid in the family Pennariidae. Colonies are common in the Mediterranean Sea growing on rocks close to the surface. Colonies of Pennaria disticha consist of numerous much-branched stems up to 30 cm high. The branching system is alternate. The polyps are tiny and are supported by a hydrocaulus, a fairly stiff hollow tube with a perisarc (sheath) made of chitin and protein. This is dark brown or blackish, but is often colonised by algae and diatoms giving it a muddy appearance. The whitish polyps are tinged with red. Each polyp has ten to eighteen slender, filiform tentacles at its base and up to twelve tentacles with knobbed tips surrounding its terminal mouth.