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Non-Indigenous Hydromedusae in California's Upper San Francisco Estuary

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Non-Indigenous Hydromedusae in California's Upper San Francisco Estuary SCI. MAR., 64 (Supl. 1): 73-86 SCIENTIA MARINA 2000 TRENDS IN HYDROZOAN BIOLOGY - IV. C.E. MILLS, F. BOERO, A. MIGOTTO and J.M. GILI (eds.) Non-indigenous hydromedusae in California’s upper San Francisco Estuary: life cycles, distribution, and potential environmental impacts* JOHN T. REES1,2 and LISA-ANN GERSHWIN3 1 Bay/Delta Shore Institute, California State University, Hayward, Building 7; 751 West Midway, Alameda Point, CA 94501 2 Department of Biological Sciences, California State University, Hayward, CA 94542 U.S.A. E-mail: [email protected] 3 Department of Integrative Biology, U.C. Museum of Paleontology, University of California, Berkeley, CA 94720 U.S.A. SUMMARY: Two species of hydromedusae, assumed to be native to the Black and Caspian Seas, were routinely collect- ed in Suisun Slough, California, at the Suisun City Marina, during late summer and fall of 1997. Suisun Slough connects directly with Suisun Bay, part of the biologically complex and commercially important upper San Francisco Estuary, and with San Francisco Bay via San Pablo Bay. Maeotias marginata (Modeer 1791), has been previously reported (as M. inexspectata) from the Petaluma River, another tributary entering San Pablo Bay, while an as-yet undetermined species of Moerisia represents a new distributional record for this genus in the eastern Pacific. Morphologies of the adults and imma- ture growth stages of medusae of both species are described. The polyp stages of both species were reared in the laborato- ry following spawning of adult medusae collected from the field. Both species are apparently representative of the robust and aggressive Black and Caspian Seas brackish water fauna, many species of which have been introduced into estuarine habitats worldwide. The potential of these planktonic predators to alter zooplankton communities and feed directly on lar- val and juvenile stages of threatened native and commercially valuable estuary fish species are all possible, but remain unin- vestigated. Key words: Maeotias marginata, Maeotias inexspectata, Moerisia sp., Moerisia lyonsi, Olindiidae, Limnomedusae, San Francisco Bay, Cnidaria INTRODUCTION October of 1997 and again in 1998. These two species are exotic introductions, and both are pre- Maeotias marginata (Modeer, 1791) (see Mills sumed members of the relict brackish-water Sarmat- and Rees, 2000) and Moerisia sp. are the first ic fauna native to the Black and Caspian Seas. Both hydromedusae to be reported from Suisun Bay, an of these hydrozoans have complex life cycles con- oligohaline, usually very low salinity portion of the sisting of asexual sessile polyps and sexual pelagic San Francisco Estuary situated between San Pablo medusae. Polyps of both species were obtained in Bay and the Sacramento–San Juaquin River delta. the laboratory following spawning of adult field- Both species were collected in Suisun Slough, a sin- collected medusae. On the west coast of North uous arm of Suisun Bay, during September and America, Maeotias marginata (as M. inexspectata) has been previously reported in the San Francisco *Received January 14, 1999. Accepted January 17, 2000. Estuary from the Petaluma River, which empties NON-INDIGENOUS HYDROMEDUSAE IN SAN FRANCISCO ESTUARY 73 into San Pablo Bay (Mills and Sommer, 1995), Large M. marginata were only occasionally col- while Moerisia sp. represents the first record of this lected in this manner, although immature stages of genus from the Eastern Pacific. the medusa were routinely dip-netted from the sur- The introduction of these medusae into Suisun face where they were visible. A complete series of Bay is significant, as no hydromedusae are known to growth stages of M. marginata and Moerisia sp. be native to this bay. Jellyfish are important, though were collected during the study period. Living little-appreciated and understudied, secondary con- medusae were further observed and photographed in sumers on the zooplankton and zoobenthos in many the laboratory. Adult Moerisia sp. medusae were fed other habitats. Under certain environmental condi- Artemia nauplii; M. marginata medusae ate bits of tions, hydromedusae can attain very high local minced, cooked shrimp. abundances (blooms) and under such circumstances A cursory search in the field for the polyps of are capable of severely depleting and altering the both species was made on local docks and other sub- structure of local zooplankton communities. Some strates, including scrapings from pilings and sides hydromedusae are also known to feed directly on and bottoms of floating docks. None were found, commercially valuable larval and juvenile fish. Here although the polyps of both species were subse- we document the life history stages of two intro- quently reared from mature medusae spawned in the duced species and report initial results of field work laboratory. Initially, all M. marginata medusae col- which aims to investigate the capabilities of these lected were males, as previously reported by Mills hydrozoans to invade and impact sensitive estuarine and Sommer (1995) from the Petaluma River. In environments. Knowledge of the basic biology of early September 1998, a few female M. marginata these cnidarians, as well as other aggressive medusae were collected in the Napa River (which invaders of estuarine habitats, is needed if appropri- empties into San Pablo Bay), and primary polyps ate preventative measures are to be instituted were obtained by spawning adult male and female towards limiting negative impacts of alien introduc- medusae in the laboratory. tions in vulnerable and economically important estuarine habitats around the world. Laboratory Culture Adult medusae were taken to the laboratory, MATERIALS AND METHODS sorted by sex using a dissecting microscope, and placed in 10 cm diameter glass rearing bowls filled On information that unidentified jellyfish had with water collected from Suisun Slough. Labora- been observed in Suisun Bay, one of us (L.G.) tory culture techniques followed those described undertook a field excursion to the marina adjacent to by J.T. Rees (1979) for Eutonina indicans. Release downtown Suisun City, located at the end of Suisun of eggs was observed, and development and subse- Slough, in early September 1997. At that time adult quent settlement of planulae took place directly on jellyfish, subsequently identified as Maeotias mar- the bottoms of the bowls. Primary and adult polyps ginata, were seen in abundance at the water surface. thrived on newly hatched Artemia nauplii. Newly- Through routine plankton tows taken alongside the released medusae obtained from polyps were trans- marina dock, a second hydromedusan species, later ferred to small (5 cm diameter) plastic culture con- identified as Moerisia sp., was found to be sporadi- tainers and reared to maturity. All cultures were fed cally abundant at the marina. Growth stages of both once daily. Culture water was changed every sec- medusae were collected, photographed, and pre- ond day prior to feeding. Culture temperature served. ranged from 18–21o C; salinity was maintained at about 10 psu (practical salinity units). No aeration Field Collections was used in polyp or medusa culture containers. Salinities were measured in both the field and lab- Medusae of M. marginata and Moerisia sp. were oratory with a hand-held refractometer or a specif- collected weekly at the Suisun City Marina from mid- ic gravity meter calibrated with a refractometer. September to October 29, 1997, by taking horizontal Nematocysts of both species were examined using plankton tows with a 0.25 m diameter, 500 µm-mesh a compound microscope at 500 and 1000X in net at about 0.5 m depth over a 1-2 meter bottom. A squash preparations; measurements given are for secchi disc was used to determine water clarity. unexploded nematocysts. 74 J.T. REES and L.-A. GERSHWIN RESULTS Maeotias marginata (Modeer, 1791) Photographic documentation of the life history M. marginata medusae were collected in the field stages of Maeotias marginata and Moerisia sp. from in 1997 and 1998 between late August and early Suisun Slough is presented in Figures 1–6 and November. Despite numerous plankton tows, almost nematocysts are shown in Figure 7. Salinites and all medusae captured were adults 25 mm in diame- temperatures at the field site ranged from 7–10 psu, ter or greater, indicating that the peak period of and 18–19o C, respectively, throughout the study release of the medusa from the hydroid had been period. Turbidity at the site was high and secchi disc earlier in the year. A few very small M. marginata readings were always 25 cm or less. medusae were collected in October, indicating that FIG. 1. – Maeotias marginata, adult medusae, 3.0–5.0 cm in bell diameter. (a,b) Lateral and oral views: note thickness of the mesoglea, con- ical subumbrellar profile, curtain-like gonads, centripetal canals, and numerous tentacles. (c) Lateral view of typical resting posture of medusa on the bottom. (d) Medusa at left shows maximum contraction of the bell while swimming. (e) Oral view of typical resting position on bot- tom. (f) Magnification of bell rim showing tentacles with rings of nematocysts, oval-shaped pigmented areas at bases of tentacles, and statocysts between the tentacle bases. NON-INDIGENOUS HYDROMEDUSAE IN SAN FRANCISCO ESTUARY 75 occasional medusae were still being released from cysts, each with a single inclusion, were located the hydroid at this time. The biggest adults were 50 alongside the ring canal. No sensory clubs were pre- mm in bell diameter, among the largest reported for sent among the tentacles, confirming the observa- this species, including those collected and described tions of Mills and Sommer (1995). The large size, from native habitat in the Sea of Azov and other with bell wider than high, large number of tentacles, areas in the Black Sea (Kramp, 1961; Mills and presence of very long centripetal canals, and a Sommer, 1995). Size class measurements taken in brownish or reddish bell rim render adult medusae late September and October showed that the of M.
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