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DescriptionThis hydroid forms three dimensional colonies resembling a fir tree, giving rise to the common english name 'sea firs' often used in popular books for the hydroids. Small colonies start by branching in a flat plane and may be confused with Abietinaria filicula. The main stem gives rise to side branches of determinate length which branch dichotomously. They bear hydrothecae which are arranged subalternately along opposite sides of the branches, turned slightly towards the distal end of the colony. The outer part of each hydrotheca is turned slightly away from the branch. Gonothecae are borne abundantly on these side branches. Individual colonies of this species do not normally exceed 300mm in length, and typically may be about half this size. Sertularia cupressina is so similar to this species that its status as a separate species has been questioned. It grows to much greater length, and lives in more sand-scoured habitats. The shape of the side branches, hydrothecae and gonothecae is slightly different, but side by side comparison of specimens may be necessary to see this.