Gonionemus is a genus of hydrozoan that uses adhesive discs near the middle of each tentacle to attach to eelgrass, sea lettuce or various types of algae instead of swimming. They are small (bell diameter to 25 mm) and hard to see when hanging on to swaying seaweed. Nevertheless they are capable of swimming when necessary. The bell is transparent, revealing the four orange to yellowish-tan gonads that lie along most of the length of the four radial canals. The pale yellow manubrium has four short frilly lips. Up to eighty tentacles line the bell margin, with about an equal number of statocysts. Copepods are a favored prey. Whereas Pacific Northwest Gonionemus vertens lacks a sting that is felt by people, the same species in the Russian Far-East is known to be venomous. This nearshore limnomedusa inhabits quiet waters of northern Japan and Kamchatka (Russia), and the area from Alaska's Aleutian Islands to northern California.
This marine hydrozoan is a small medusa which is common in warmer waters. The conspicuous stage in the dimorphic life cycle is the small medusa. The polypoid stage is present as a tiny solitary polyp which feeds on protozoan and other small plants and animals. The polyp stage closely resembles Hydra.
The medusae are active swimmers that propel themselves upward in the water column by rhythmic pulsations of the bell. Upon reaching the surface the bell relaxes, the tentacles become fully extended and any small fish or crustaceans encountered as the medusae slowly drift toward the bottom are ensnared. Occasionally the medusa use their adhesive pads to attach to seaweeds or other objects near the bottom, extends its tentacles and waits for prey to bump into them.
The manubrium hangs down from the centre of the subrellum. It bear the cross-shaped mouth and the four short oral lobes which grip the food. Digestion begins in the centre of the manubrium which communicates with the four radial canals and the ring canal. The velum is well developed and used in swimming. Having a velum is characteristic of the hydrozoan medusae.
The gonads are four yellowish structures embedded in the surface of the epidermis beneath the radial canals. The ovaries are more granular in appearance than the testes (sexes are separate). The gametes are shed into the sea, and the zygotes develop into ciliated planula larvae which grow into minute polyps. These polyps can bud off other polyps or medusae.
The tentacles of the medusa are hollow and are connected to the exumbrellar surface by a tentacular bulb where cnidoblasts are formed.
Read more about Gonionemus: Species