Invasion History

First Non-native North American Tidal Record: 1965
First Non-native West Coast Tidal Record: 1993
First Non-native East/Gulf Coast Tidal Record: 1965

General Invasion History:

The hydrozoan Moerisia lyonsi is only known from its type locality, Lake Qarun, Egypt (a saline lake in the Nile valley, near Cairo) and the East, Gulf and West coasts of North America (Boulenger 1908; Rees 1957; Calder and Burrell 1967). This medusa was probably introduced to Lake Qarun, as indicated by its occurrence on the introduced Ponto-Caspian hydroid Cordylophora caspia (Boulenger 1908). It is now extinct in the lake, as salinity has increased beyond its tolerance (Dumont 1999). Other species of Moerisia are known from the Caspian Sea, Japan, Hawaii, and the Mediterranean, in coastal and continental salt lakes (Calder 1971; Dumont 1994; Calder 2010). Rees and Gershwin (2000) suggest that coastal populations of Moerisia (including M. lyonsi, Lake Qarun and East and Gulf coast of US; Moerisia sp., San Francisco Bay; M. gangetica, Ganges River) may represent introductions, probably from a Ponto-Caspian source. Taxonomic studies are needed to resolve the systematics and distribution of the genus (Rees and Gershwin 2000). It is possible that M. lyonsi and other named forms represent a single, morphologically plastic species of Ponto-Caspian origin (Mills 1999), but genetic comparisons are needed (Meek et al. 2012).

North American Invasion History:

Invasion History on the West Coast:

In the San Francisco estuary, medusae and polyps identified as Moerisia sp. were discovered in the brackish Petaluma River, a tributary of San Pablo Bay, in 1993 (Mills and Sommer 1995), and were later found in the Napa River, another San Pablo tributary. In 1997, medusae were collected in Suisun Slough (Rees and Gershwin 2000). Extensive surveys of nonindigenous hydrozoans in Suisun Marsh, and the Napa and Petaluma rivers were made by Schroeter (2008) and Wintzer et al. (2011a; 2011b). Medusae of Moerisia were the most numerous of the three species of nonindigenous hydromedusae in Suisun Marsh, occasionally reaching densities of 100 m-3 (Schroeter 2008; Wintzer et al. 2011b). The San Francisco Bay hydrozoans have recently been found to be genetically identical to M. lyonsi from Chesapeake Bay (Meek et al. 2012).

Invasion History on the East Coast:

Moerisia lyonsi medusae and hydroids were first collected in 1965-68 in low-salinity waters of the James and Pamunkey Rivers, tributaries of Chesapeake Bay (Calder and Burrell 1967; Calder 1971). Subsequent reports of the species are sparse, but hydroids of M. lyonsi were found on settling plates from Baltimore Harbor in 1995 (Ruiz et al. unpublished). In 1992-93, M. lyonsi medusae invaded an experimental mesocosm system at the University of Maryland Horn Point Laboratory, and were also collected in the adjacent Choptank River (Purcell et al. 1999; Ma and Purcell 2005a,b). Polyps have also been collected from Delaware Bay (Sandifer et al. 1974). In South Carolina, medusae of M. lyonsi were reported as abundant in most areas of low salinity (Calder and Hester 1978). Medusae and polyps invaded a shrimp aquaculture system at Charleston, South Carolina in 1973 (Sandifer et al. 1974).

Invasion History on the Gulf Coast:

Polyps of Moerisia lyonsi were found in the Bonnet Carré spillway, on Lake Pontchartrain, Louisiana in 1975, but disappeared during unusually low salinities (Poirier and Mulino 1977).


The hydrozoan Moerisia lyonsi has a small polyp stage (~1-2 mm high), and a small, but more conspicuous medusa stage (3-8 mm in diameter). The colonial polyps grow from a long, branching stolon (up to 10 mm long), which may be draped over plant detritus, colonies of the hydroid Cordylophora caspia, or shells (e.g. Ischadium recurvum, Hooked Mussel). The stolon is covered with thin perisarc, and often encrusted with detritus. The zooids are spindle-shaped, with the greatest girth below the tentacles. There are 4-16 moniliform tentacles which arise below the hypostome. Frustule-like buds (often with 1 or 2 tentacles) develop below the tentacles- they grow outward, constrict, and drop off, becoming new polyps. The hydroid can also reproduce by a form of transverse fission, where the stalk constricts and breaks up into several buds, each of which can grow into a new colony. Medusa buds develop around the base of the tentacles, or at the base of the hypostome. As the buds grow, they are attached by a short stalk, and the radial canals and tentacle buds become visible, before the medusae are released (Rees 1957; Mills and Ress 2000; Rees and Gershwin 2000).

Young medusae of M. lyonsi are bell-shaped (~ 0.5 mm in diameter) with four radial canals and short manubrium. There is a red ocellus on each tentacle bud, and a well-developed ring canal. The velum is well-developed. At about 1.1 mm, gonads start to develop. As the medusae grow, more tentacles are added, and the bell of the medusa becomes proportionately wider. Gonads extend outward, from the manubrium, along the radial canals, nearly to the ring canal. Quadrangular lips develop in larger specimens. Adult medusae had 29-64 tentacles and were 2.4-8.4 mm in diameter. (Description from: Boulenger 1908; Calder 1971; Purcell et al. 1999; Bouillon et al. 2004) 


Taxonomic Tree

Kingdom:   Animalia
Phylum:   Cnidaria
Class:   Hydrozoa
Subclass:   Hydroidolina
Order:   Anthoathecatae
Suborder:   Capitata
Family:   Moerisiidae
Genus:   Moerisia
Species:   lyonsi


Moerisia sp. (Rees & Gershwin, 2000)

Potentially Misidentified Species

Moerisia carinae
Mediterranean and Indo-Pacific

Moerisia gangetica
India (Dumont 1994)

Moerisia horii
Japan, Hawaii (Cooke 1977, cited by Calder 2010)

Moerisia inkermanica

Native to Black Sea, Azov Sea, eastern Mediterranean, Brazil, and China. Moerisia polyps from Barnegat Bay were morphologically closer to M. inkermanica from Brazil and China, than Moerisia lyonsi from California and Chesapeake Bay, although they matched the Chesapeake and California forms very closely genetically (Restaino et al. 2018)., The main difference between the two species is the number of tenatcles, 4 (usually) to 16 in M. lyonsi, and 24-32 in M. inkermannica (Svhuchert 2010).

Moerisia lacustris
Trinidad (Dumont 1994)

Moerisia maeotica
Black Sea (Naumov 1969)

Moerisia pallasi
Caspian Sea (Naumov 1969)



The hydrozoan Moerisia lyonsi has an epibenthic attached polyp stage and a planktonic medusa stage. The polyps are capable of asexual budding to form mobile frustules, which crawl away from the parent and grow into new polyps (Boulenger 1908; Calder 1971; Purcell et al. 1999; Ma and Purcell 2005a,b). The polyps can also form cysts during cold weather and other adverse conditions, which redevelop into new polyps when conditions improve. The cysts may be resistant to drying (Purcell et al. 1999), among other stresses. The polyps also produce medusa buds, which break off into free-swimming medusae. The rate of budding, both by polyp and medusa buds, increases with food supply (Purcell et al. 1999). Polyps grow and reproduce asexually at 10-29 ?C and 1-40 PSU salinity (Ma and Purcell 2005a). Development times of both types of buds decreases with increasing food levels, and are affected by temperature and salinity, but medusa budding is more sensitive to environmental change than polyp budding (Ma and Purcell 2005b).

Medusae tend to swim upwards and then drift downwards with extended tentacles. They do not linger near the bottom, as do the related Maeotias marginata or Craspedacusta sowerbii, or cling to vegetation, as does Gonionemus vertens. Their prey is primarily planktonic copepods. The distribution of M. lyonsi medusae in Chesapeake Bay was limited to salinities below 5-9.3 PSU, possibly due to predation by the ctenophore Mnemiopsis leidyi and the scyphomedusa Chrysaora quinquecirrha (Purcell et al. 1999; Ma and Purcell 2005a). Medusae of M. lyonsi probably have separate sexes. Fertilized eggs develop into planulae which settle on rocks, shells, vegetation, other hydroids (e.g. Cordylophora caspia), and man-made surfaces (Boulenger 1908; Barnes 1983; Calder 1971; Sandifer et al. 1974; Poirrier and Mulino 1977; Purcell et al. 1999).




Scyphomedusae (Chrysaora quinquecirrha)

Trophic Status:




General HabitatGrass BedNone
General HabitatMarinas & DocksNone
Salinity RangeOligohaline0.5-5 PSU
Salinity RangeMesohaline5-18 PSU
Salinity RangePolyhaline18-30 PSU
Salinity RangeEuhaline30-40 PSU
Tidal RangeSubtidalNone
Vertical HabitatEpibenthicNone
Vertical HabitatPlanktonicNone

Life History

Tolerances and Life History Parameters

Maximum Temperature (ºC)29Highest tested (Ma 2003)
Minimum Salinity (‰)1This range is based on field (Calder and Burrell 1967; Calder and Hester 1978) and experimental data (Ma and Purcell 2005a). This hydrozoan disappeared in Lake Pontchartrain LA as salinity dropped from 2.5 PSU to 0.3 PSU (Poirrer and Mulino 1977).
Maximum Salinity (‰)40Extensive reproduction occurred in aquaculture tanks at 16 ppt, but was not seen at 30 PSU (Sandifer et al. 1974). In laboratory experiments, asexual reproduction of polyps occurred at 1-40 PSU and 15-20 C. Medusae survived abrupt transfer from 10 to 1 and 26 PSU, though swimming was impaired at 1 ppt. Polyps can encyst under unfavorable conditions, but the tolerance of cysts has not yet been studied (Ma 2003).
Maximum Width (mm)8.4Diameter of medusae (Calder 1971; Purcell et al. 1999; Bouillon et al. 2004)
Broad Temperature RangeNoneWarm-temperate
Broad Salinity RangeNoneOligohaline-Mesohaline

General Impacts

Moerisia lyonsi has been introduced to several East and Gulf Coast estuaries, but available information on its abundance is limited. Field surveys from the Choptank River, Maryland found densities of medusae at 0.4-13 m-3, too low to significantly affect zooplankton populations (Purcell et al. 1999). However, in enclosed experimental and aquaculture ecosystems, M. lyonsi can reach densities where it becomes a significant predator (Sandifer et al. 1974; Petersen et al. 1998; Purcell et al. 1999).

Economic impacts

Fisheries- The hydrozoan Moerisia lyonsi has been a pest in closed-system aquaculture tanks used to rear Macrobrachium spp. (shrimp) larvae in South Carolina. The medusae are capable of killing and/or eating decapod larvae larger than themselves (Sandifer et al. 1974). This occurrence and its spread in an experimental mesocosm system at Horn Point, Maryland (Peteren et al. 1998; Purcell et al. 1999) suggest that this species might be a pest in brackish-water aquaculture systems in the Chesapeake Bay region.

Ecological Impacts

Predation- Moerisia lyonsi together with the other introduced hydrozoans in the brackish portions of the San Francisco estuary (Blackfordia virginica, Cordylophora caspia, and Maeotias marginata), has the potential to alter planktonic food webs through predation on zooplankton. Predation on fish larvae occurred, but at a very low rate, less than 1% of food composition (Schroeter 2008; Wintzer et al. 2011c). This estuary has no native brackish-water hydrozoans (Rees and Gershwin 2000).

Competition- Moerisia lyonsi is a potential competitor with larval fishes, particularly economically important Striped Bass (Morone saxatilis) and endangered Delta Smelt (Hypomesus transpacificus) (Schroeter 2008; Wintzer et al. 2011b; Wintzer et al. 2011c). Temporal and dietary overlap was greatest with Delta Smelt and introduced Threadfin Shad (Dorosoma petenense), but minimal with Morone saxatilis, and threatened Longfin Smelt (Spirinchus thaleichthys) (Wintzer et al. 2011c).

Regional Impacts

CAR-VIICape Hatteras to Mid-East FloridaEconomic ImpactFisheries
Hydroids can be pests in brackish closed-circuit aquaculture tanks.
S080Charleston HarborEconomic ImpactFisheries
Hydroids can be pests in brackish closed-circuit aquaculture tanks.
NEP-VNorthern California to Mid Channel IslandsEcological ImpactCompetition
Moerisia lyonsi is a potential competitor with larval fishes, particularly economically important Striped Bass (Morone saxatilis) and endangered Delta Smelt (Hypomesus transpacificus) (Schroeter 2008; Wintzer et al. 2011b; Wintzer et al. 2011c). Temporal and dietary overlap was greatest with Delta Smelt and introduced Threadfin Shad (Dorosoma petenense), but minimal with Morone saxatilis and threatened Longfin Smelt (Spirinchus thaleichthys) (Wintzer et al. 2011c).
P090San Francisco BayEcological ImpactCompetition
Moerisia lysoni is a potential competitor with larval fishes, particularly economically important Striped Bass (Morone saxatilis) and endangered Delta Smelt (Hypomesus transpacificus) (Schroeter 2008; Wintzer et al. 2011b; Wintzer et al. 2011c). Temporal and dietary overlap was greatest with Delta Smelt and introduced Threadfin Shad (Dorosoma petenense), but minimal with Morone saxatilis and threatened Longfin Smelt (Spirinchus thaleichthys) (Wintzer et al. 2011c).

Regional Distribution Map

Bioregion Region Name Year Invasion Status Population Status
MED-V None 1908 Def Unk
NA-ET3 Cape Cod to Cape Hatteras 1965 Def Estab
CAR-VII Cape Hatteras to Mid-East Florida 1973 Def Estab
CAR-I Northern Yucatan, Gulf of Mexico, Florida Straits, to Middle Eastern Florida 1975 Def Estab
M130 Chesapeake Bay 1965 Def Estab
M090 Delaware Bay 1974 Def Unk
S080 Charleston Harbor 1973 Def Estab
G170 West Mississippi Sound 1975 Def Estab
S090 Stono/North Edisto Rivers 1978 Def Estab
NEP-V Northern California to Mid Channel Islands 1993 Def Estab
P090 San Francisco Bay 1993 Def Estab
M070 Barnegat Bay 2014 Def Estab

Occurrence Map

OCC_ID Author Year Date Locality Status Latitude Longitude
34127 Schroeter 2008 1981 1981-01-01 Suisun Marsh Def 38.0638 -121.9927
34128 Mills and Sommer 1996; 1997 1997-01-01 Petaluma River Def 38.1104 -122.4874
34129 Mills and Rees 2000 2000 2001-01-01 Napa River Def 38.2504 -122.2844


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