Nomad Jellyfish

Type Description

"R . nomadica is a medium-sized rhizostomid medusa. It has a nearly hemispherical umbrella, thickest centrally, thinning gradually towards margin. Exumbrella minutely granulate, granules fewer and blunter near margin. Margin of umbrella divided into 64 rounded velar lappets. Ocular lappets small, lanceolate, one third as wide as velar lappets. Subumbrellar circular muscles well developed, uniform. Arm disc prismatic, about one third of bell diameter, oral pillars quadrate. Distal corners of oral pillars prominently tuberculate. Subgenital ostium at each interradius kidney-shaped, as wide as oral pillar. A pear-shaped, tuberculate papilla interradially on subumbrella, opposite ostial opening. Eight pairs of large, deeply bowed scapulets arise from adradial sides of armdisc; underside concave, smooth; upper side bearing frilled mouths and numerous filaments. Distal part of scapullets bearing long filaments, sometimes twice as long as scapulets. Each scapulet is divided, midlength, into two, with five appendages on each side. Eight adradial mouth arms stout, smooth, fused to midlength. Lower part of mouth arm divided into two triangular flaps, each flap distally tripartite and terminating in claw-like digitate processes. Ventrally, mouth arms bear numerous frilled mouths and long filaments. Lowermost end bearing a vermicular appendage terminating in a thin filament. Stomach cavity nearly octagonal. Gastrovascular system consisting of four perradial, four interradial and eight adradial canals interconnecting in a complex network of anastomosing vessels extending almost to bell margin. Four principal canals extend from lower part of stomach to radii entering each scapulet and mouth arm, then branching to numerous minute canals leading to mouths (Galil et al., 1990).
Colour in life icy blue. It can reach up to 100 cm umbrella diameter, but is commonly 30-50 cm in diameter. "

Habitat

R . nomadica is a neritic epipelagic, its swarms are recorded at a distance of 2-4 kms off shore. Occasionally it is swept nearer shore and into the intertidal zone.

Reproductive Information

R. nomadica has a two-stage life cycle consisting of the conspicuous, large, sexually reproducing swimming medusa stage and a benthic polyp stage (scyphistoma) that, because of its small size <2 mm), remains cryptic. Spawning occurs in summer. Fertilization is external and planulae are formed within hours. Settlement occurs within 3-4 days, and polyps developed within 3 weeks. Asexual reproduction occurs mostly through podocyst formation; polyps produce podocysts by growing stolons from the base of the disk. Polyps developed into polydisc strobilae within 45 days. The strobilation process is complete in 7 days; 5-6 ephyrae are formed on each strobila. Mature polyps may strobilate repeatedly. The development from the newly released ephyra (1.5-2 mm in diameter) to the young medusa (7-10 mm in diameter) may be completed within 2 months (Lotan et al., 1992).

Lifecycle

The fertilization of Rhopilema nomadica is external and planulae are formed within 12 h. Settlement of planulae is observed after 3 to 4 d, and polyps developed in 3 wk. Strobilation is completed within 7 d and 5 to 6 ephyrae are released into the water. The ephyrae of R. nomadica can be identified by the structure of the distal ends of their radial canals and the typical two-whorl arrangement of the nematocyst batteries on the exumbrella. The time course of development from a young polyp to the liberation of ephyra is 40 to 50 d. Ephyrae develop into young medusae within 2 mo. Asexual reproduction is mainly via podocyst formation.

Nutrition Information

R. nomadica are planktotrophic, meaning that they feed off of plankton for their nutritional and also for their survival needs.

General Impact Information

"Cultural/amenity: Negative
Economic/livelihood: Negative
Environment (generally): Negative
Human health: Negative"

General Management Information

As R. nomadica may be transported in shipping, efforts should be concentrated on fouling and ballast control. Coastal municipalities shovel tons of jellyfish off the beaches. Some have invested in booms and netting to create jellyfish-free zones at their most popular beaches. Marine biologists dislike the use of fixed nets or barriers around swimming areas as the waves amputate the filamentous tentacles of an ensnared jellyfish and carry the nematocysts-bearing filaments toward swimmers. Stray tentacles and even dead jellyfish can still be dangerous. 

General Pathway Information

Suez Canal

Notes

References