The occurrence of Rhopilema verrilli (Cnidaria: Scyphozoa: Rhizostomeae) on Galveston Island, Texas and a discussion on its distribution in U.S. waters.

A specimen of the rhizostome scyphomedusa Rhopi/ema verrilli was stranded qn the beach at Galveston Island, Texas, In the spring of 1985. Two young specimens have since been collected In the marshes on the north side of Galv,eston bordering West Galveston Bay. This species Is relatively rare throughout most of Its range and has only been reported once before in the western Gulf of Mexico. Seasonal and geographic distributions from literature records are discussed in relation to temperature, salinity, currents, and life history. Occurrences along the U.S. East Coast Indicate progressively later seasonal appearances and decreased abundances from Georgia to New England. Occurrences In the Gulf of Mexico are almost all limited to the Mississippi Sound area. Collection records suggest that R. verrilli has two centers of distribution, one in southern U.S. East Coast waters and one in the northeastern Gulf of Mexico, and that occurrences outside these centers are due to transport by currents. We believe that R. verrllli's apparent rarity in the western Golf of Mexico results primarily from a combination of wind· driven current patterns, timing of volume of Mississippi·Aatchafalaya River discharge, and temperature·salinlty tolerances of the species acting against transport into, and survival in, the western Gulf. On 26 March 1985, we were informed by the Galveston Beach Patrol that a "giant" jellyfish had washed ashore on East Beach, Galveston Island, Texas. When we located the stranded specimen (which was quite large compared to the usual beached jellyfish), we were unable to make a field identification and returned the organism to our laboratory. We subsequently identified the jellyfish as Rhopilema verrilli (Fewkes 1887), a member of the rhizostome order to which the locally very abundant "cannonball" or "cabbagehead" jellyfish (Stomo/ophus meleagris L. Agassiz 1860) also belongs. Although it appears formidable because of its large size, R. verrilli does not present a stinging threat to humans (Calder 1972a:214). 19 Examination of the literature and information from Dr. Dale Calder, Royal Ontario Museum (personal communication) indicated that this was the second specimen reported from the Gulf of Mexico west of the Mississippi Delta. Two juvenile specimens which appear to be R. verrilli were collected in the marshes bordering West Bay (part of the Galveston Bay complex) in 1986 and 1987. Rhopilema verrilli is known only from New England to Georgia on the U.S. East Coast and the northern Gulf of Mexico (Gasner 1971, Calder 1972a, 1973, Phillips 1972, Kraeuter and Setzler 1975, Larson 1976). Apparently the species is rare to uncommon throughout most of its range and is limited to the coast and the mouths of estuaries (Larson 1976, Calder 1 Harper and Runnels: The Occurrence of Rhopilema verrilli (Cnidaria: Scyphozoa: Rhizos Published by The Aquila Digital Community, 1990 20 Harper, D.E., Jr., and R.J. Runnels 1977). In the Gulf of Mexico, prior to 1953, specimens of R. verrilli had been collected in Mobile Bay, and a single specimen had been collected at Port Aransas (although the species was considered common, in Chandeleur Sound by Martin Burkenroad) (Hedgpeth 1954:277). Subsequent records in the Gulf of Mexico are all from the Mississippi Sound area, east of the Mississippi Delta (Phillips 1972, Burke 1975a, b). Specimens were not collected during oceanographic cruises in the eastern Gulf of Mexico (CollardJ977) or from the central and western Gulf of Mexico (Phillips 1972, Berkowitz 1976, Park and Turk 1980, Wormuth, McEachran and Pequegnat 1980). A number of check1 ists and reports from various coastal regions of the Gulf of Mexico were reviewed and found not to mention R. verrilli (Cross and Parks 1937, Behre 1950, Walker 1953, Humm 1953, Tabb and Manning 1961, Richmond 1962, 1968, Dawson 1966, Gillespie 1971, Perret 1971, Christmas 1973, Fotheringham, 1980). Life history studies of Rhopi/ema verrilli indicate that development is accelerated at higher temperatures (20 C vs. 10-12 C) (Cargo 1972, Calder 1973, Burke 1975b) .. Planulae swam actively when liberated, settled within 7-10 days and produced scyphistomae (Calder 1973). However, swimming planulae may be suppressed, with polyps budding directly from gonadal tissue (Cargo 1972). Scyphistomae typically produce monodiscous strobilae, although 2 or 3 ephyrae sometimes develop. Ephyrae were liberated within 7 days of first indication of strobilization when cultures were kept at 20 C (Calder 1973). Calder's (1973) work indicated that development to the medusa stage may occur in less than a month, whereas Burke (1975b) indicated development required 90 days. Polyps of R. verrilli and Stomolophus meleagris are very similar (Calder 1982: 156) as are the ephyrae, and the most reliable method of distinguishing between the stages of these two species is examination of the nematocysts (Calder 1977, 1983). Burke (1975b) conducted substrate preference tests and found that polyps did not develop normally on particles smaller than silt-size (250 1-1). He also reported that polyps were not found on fouling plates or artificial reefs placed in the Mississippi Sound area. Calder eta/. (1977:30) reported polyps of R. verrilli from oyster dredge collections in Little River Inlet, South Carolina. No report of podocysts was found.