Glaucus atlanticus

Glaucus atlanticus, Forster, 1777 © Taro Taylor

The family Glaucidae has just one genus, Glaucus and two species: Glaucus atlanticus and Glaucus marginatus.

Glaucus atlanticus:

  • can be up to 40mm in size
  • has single rows of up to 84 cerata - outgrowths from the sides of the body

The cerata hold stinging cells (nematocysts), which are taken from the Portuguese Man o' War and other hydrozoans they eat and are stored in special sacs called cnidosacs.

This species closely resembles the only other species in the family, Glaucus marginatus. Both species float upside down under the water surface so that the blue colours are actually the ventral surface.

Glaucus marginatus

Glaucus marginatus, Bergh, 1860 © Taro Taylor

The main differences between the two species are:

  • Glaucus marginatus has multiple rows of cerata and up to 137 cerata in total.
  • The tail (metapodium) of Glaucus atlanticus is much longer than that of Glaucus marginatus.
  • Glaucus atlanticus is the larger of the two species - Glaucus marginatus only reaches 17mm in length.
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