Description: The main stems of the colony are bright yellow and relatively thick, being a composite of several fibres, adhering to each other. The side branches are approximately alternate and support the hydrothecae. The hydrothecae are regularly alternate, one to each internode. As in Sertularella polyzonias they are bulbous in the basal region, and the margin is quadridentate. The operculum consists of four triangular flaps. The gonothecae are oval shaped, and the upper portion has transverse ridges whilst the lower portion is smooth. The aperture is bidentate.

Habitat: This hydroid is normally found in the circalittoral zone, attached to bedrock in areas with slight to moderate tidal streams.

Distribution: Common throughout the British Isles

Similar Species: Sertularella gayi is sometimes confused with Sertularella polyzonias, but is much more robust in its overall appearance. The gonothecae are also quite different, in S. gayi they are much smaller, they lack a tubular aperture, and the aperture usually has only two denticles whilst four are present in S. polyzonias. Another species to consider is Abietinaria abietina.

Key Identification Features:

• Younger branches yellow to green in colour
• Hydrothecae alternate on main stem

Distribution Map from NBN: Sertularella gayi at National Biodiversity Network mapping facility, data for UK.

iNaturalist: Sertularella gayi at iNaturalist World Species Observations database.

WoRMS: Sertularella gayi at World Register of Marine Species. Accepted name: Sertularella gayi (Lamouroux, 1821). AphiaID: 117902.

Classification: Biota; Animalia; Cnidaria; Medusozoa; Hydrozoa; Hydroidolina; Leptothecata; Sertularioidea; Sertularellidae; Sertularella