Strobilation, budding and initiation of scyphistoma morphogenesis in the rhizostome Cassiopea andromeda (Cnidaria: Scyphozoa)

Abstract

Scyphistomae of Cassiopea andromeda Forskål, 1775 containing symbiontic zooxanthellae did not develop medusae at a constant temperature of ∼20°C, but monodisc strobilation was initiated after transfer of the polyps to ∼24°C. After release of the ephyrae and regeneration of the hypostome and tentacular region, the recovered polyps either produced vegetative buds or entered a new strobilation phase. Formation of motile, planula-like buds was not found to be indicative of unfavourable environmental conditions. Intensity of budding was positively correlated with available food and with increase of temperature. Budding was negatively correlated with the number of polyps maintained per dish and with the conditioning of the sea water. Under optimal feeding and temperature conditions, polyps could simultaneously produce chains of buds at 2 to 4 budding regions. Settlement and development of buds into scyphistomae was suppressed in pasteurized sea water and in pasteurized sea water containing antibiotics, but polyps developed from buds in the presence of algal material taken from the aquarium, debris or egg shells of Artemia salina, or on glass slides which had been incubated in used A. salina culture medium. Several species of marine bacteria were detected after staining these slides. One, a Gramnegative coccoid rod, which was identified as a nonpathogenic Vibrio species, was isolated, cultivated as a pure strain, and was proved to induce the development of C. andromeda buds into polyps. Millipore filter-plates coated with Vibrio sp. cells grown in suspension culture were ineffectual, but diluted filtrate initiated polyp morphogenesis. The inducing factor is obviously not a constituent of the bacterial cell surface, but is a product of growing Vibrio sp. cells released into the medium. This product was found to be relatively heat-stable and dialyzable. As to the basic mechanism involved in the induction of polyp formation, it is suggested that the inducing factor (s) acts bimodally by inducing pedal disc development and by eliminating a head inhibitor originating from the basal end of the bud. The life history, and various aspects of medusa-formation and of vegetative reproduction in scyphozoans are reviewed and discussed with particular reference to rhizostome species. Special attention has been paid to some reports of larval metamorphosis controlled by marine bacteria.