Colobonema

Colobonema sericeum Vanhoffen, 1902
Phylum Cnidaria / Class Hydrozoa / Order Trachymedusae / Family Rhopalonematidae
colobonema

On rare occasions you may encounter this abundant midwater jelly near the surface. The conical bell, diameter up to 5 cm, lacks pigment and is transparent. A wide velum lines the inside of the bell margin. With side lighting, birefringence of the bell muscles can create a beautiful array of colors as seen in the photo. The stomach has 4 lips and lacks a gastric peduncle. Look for 8 radial canals on which are attached the gonads. The bioluminescent tentacles (usually 32) readily break free from the bell when the jelly is disturbed, probably as a defense against predators. This species is commonly collected in midwater trawls, but invariably the tentacles have been shed. Only surface-collected specimens will show the full complement of tentacles as in the photo here. Colobonema is a strong swimmer with relatively powerful pulsing of the bell. As is typical of Trachymedusae, this species is holoplanktonic (lacks an attached polyps phase), with each fertilized egg developing directly into a medusa. It is among the more widespread and abundant of the deep-water Trachymedusae, with a maximum depth range between 500 and 1500 meters. The distribution range does not extend into Arctic and Antarctic waters.

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