Bivalve-inhabiting hydroids of Oysters


Category 4 (Negligible Regulatory Significance in Canada)

Common, generally accepted names of the organism or disease agent

Bivalve-inhabiting hydroids of oysters, bivalve gill cnidarians.

Scientific name or taxonomic affiliation

Various species of hydroids in the genera Eugymnanthea and Eutima (Cnideria, Hydrozoa, Leptomedusae) that attach to the tissues within the mantle cavity of bivalves. These hydroids were originally included in the families Eucopiidae (Mattox and Crowell 1951) and Eutimidae (Rees 1967; Kubota 1985a) but are now placed in the family Eirenidae (Govindarajan et al. 2005). Species reported from oysters include Eugymnanthea inquilina japonica from Crassostera gigas as well as other bivalves (Kubota 1985a), Eugymnanthea inquilina from Ostrea sp. (Rees 1967), Eugymnanthea ostrearum from Crassostrea rhizophorae (Mattox and Crowell 1951), and Eutima sp. from Crassostrea virginica (Mulholland and Friedl 1996, Winstead et al. 2004). However, these and other species of hydroids have been reported more frequently in mussels. Kubota (2000) suggested the possibility of merging Eutima and Eugymnanthea into a single genus.

Geographic distribution

Oysters containing hydroids have been reported from the coast of Japan in Crassostrea gigas, from the Mediterranean Sea in Ostrea sp., from Puerto Rico in Crassostrea rhizophorae, and on the Gulf and Atlantic coasts of Florida, USA in Crassostrea virginica. No bivalve-inhabiting hydroids have been reported from cold waters (Kubota 1987) and none have been found in Canada.

Host species

Crassostrea gigas, Crassostrea rhizophorae, Crassostrea virginica, Ostrea sp. and other species of bivalves as indicated on the page that describes hydroids in mussels.

Impact on the host

Bivalve-inhabiting hydroids attach to the mantle, foot, gills and labial palps of oysters. Polyps of E. ostrearum (up to 210 in a 4.5 cm long oyster), attached singly or in large groups mainly to the inner surface of the dorsal region of the mantle of C. rhizophorae, were found in all seasons in Puerto Rico (Mattox and Crowell 1951). Polyps of Eutima sp. (hundreds, even thousands per C. virginica), most obvious in histological section of the gills, appeared to be seasonal with highest prevalence during spring and early summer (February to June) in areas around Florida (Winstead et al. 2004). Numbers of Eutima sp. dropped drastically after oysters were exposed to freezing temperatures during low tides and during a summer of unusually heavy rainfall and reduced salinity (Mulholland and Friedl 1996). No pathology was reported to be associated with inhabitation of oysters by polyps.

Diagnostic techniques

Gross and Wet Mount Observations: Athecate (no periderm (= perisarc)) on the entire body; only a thin membrane covers the medusa-bud, solitary polyps (about 1.5 mm and up to 3.5 mm in length from hydrosome to disc and up to 0.35 mm in width), lacking hydrorhizae and gonothecae, possessing a basal disc for attachment to the molluscan tissue, a single whorl of up to 35 distal filiform tentacles with a slight intertentacular membrane around a shallow hypostome and producing on the column near the base either a polyp bud or a single medusoid (leptomedusae). Eugymnanthea inquilina japonica has been reported to have a maximum of 28 tentacles and produce up to five medusoid (medusa-buds) per specimen and polyps of this species vary in colour from orange to dark brown (Kubota 1985a). During the late stages of the development of medusoids of E. ostrearum, there is a striking reduction in the size of the polyp. The polyp losses its tentacles by resorption and diminishes to less than one quarter of its original size (Mattox and Crowell 1951).

The morphology of the medusoid is required for species identification (Kubota 1985b). The young medusoids of E. ostrearum are oviod, slightly higher than wide with a maximum height of 3.0 mm. There are four simple radial canals and a circular canal. Four simple periradial tentacles are born on conspicuous hollow bases and eight adradial tentacular bulbs develop by the fifth day after liberation. There are eight adradial statocyst each with 2-6 statoliths. The manubrium extends to the margin of the well developed velum. The mouth is surrounded by four simple lips. At the time of liberation, there are no gonads but gonadal development was observed on the oral surface of the radial canals after five days (Mattox and Crowell 1951). The medusoids of E. inquilina japonica have a mean umbrellar height of 0.89 mm (0.59-1.1 mm range) and width of 1.1 mm (0.86-1.4 mm range), a manubrium with no function, usually 4 radial canals, 4 to 9 marginal warts and 4 to 9 statocysts each with usually 1 but from 0 to 3 statoliths. Eighty percent of the meducoids of E. inquilina japonica had 8 marginal warts and 8 statosysts. Gonads are present at the time of liberation and medusoids become spent immediately after liberation or at most within several days after liberation and had a maximum life span of 9 days at 24 to 27°C with no further gonadal development (Kubota 1985a).

Histology: Polyps of Eutima sp. in C. virginica were usually observed attached to gill epithelium by the pedal disc with no apparent damage to the ciliated epithelium (Winstead et al. 2004).

Culture: Polyps can be carefully removed from the host tissue and maintained in small trays or dishes filled with natural seawater at conditions ambient to the location that the host mussel was obtained. The hydroids (original polyps and subsequent medusoids) are fed newly hatched Artemia spp. (brine shrimp) larvae and the water aerated and changed daily. Critical temperatures and salinities for Eutima sp. from C. virginica were between 10 to 33°C and 8% to 34%, respectively (Winstead et al. 2004). For taxonomic purposes, the medusoids can be examined live or preserved in buffered 10% formalin.

Methods of control

No known methods of prevention or control.


Govindarajan, A.F., S. Piraino, C. Gravili and S. Kubota. 2005. Species identification of bivalve-inhabiting marine hydrozoans of the genus Eugymnanthea. Invertebrate Biology 124: 1-10 (for electronic version see:

Kubota, S. 1985a. Systematic study on a bivalve-inhabiting hydroid Eugymnanthea inquilina japonica Kubota from central Japan. Journal of the Faculty of Science, Hokkaido University, Series VI, Zoology 24: 70-85.

Kubota, S. 1985b. Systematic study on a bivalve-inhabiting hydroid Eucheilota intermedia Kubota from central Japan. Journal of the Faculty of Science, Hokkaido University, Series VI, Zoology 24: 122-143.

Kubota, S. 1987. The origin and systematics of four Japanese bivalve-inhabiting hydroids. In: Bouillon, J., F. Boero, F. Cicogna, P.F.S. Cornelius (eds.) Modern trends in the Systematics, Ecology, and Evolution of Hydroids and Hydromedusae. Oxford University Press, Oxford. pp. 275-287.

Kubota, S. 2000. Parallel, paedomorphic evolutionary processes of the bivalve-inhabiting hydrozoans (Leptomedusae, Eirenidae) deduced from morphology, life cycle and biogeography, with special reference to taxonomic treatment of Eugymnanthea. Scientia Marina 64 (Supl.1): 241-247 (for electronic version see:

Mattox, N.T. and S. Crowell. 1951. A new commensal hydroid of the mantle cavity of an oyster. The Biological Bulletin (Woods Hole, Mass.) 101: 162-170.

Mulholland, D.S. and F.E. Friedl. 1996. Distribution and population dynamics of a hydrozoan inquiline symbiont of the eastern oyster. Journal of Shellfish Research 15: 496-497. (Abstract)

Rees, W.J. 1967. A brief survey of the symbiotic associations of Cnidaria with Mollusca. Proceedings of the Malacological Society of London 37: 213-231.

Winstead, J.T., A.K. Volety and S.G. Tolley. 2004. Parasitic and symbiotic fauna in oysters (Crassostrea virginica) collected from the Caloosahatchee River and estuary in Florida. Journal of Shellfish Research 23: 831-840.

Citation Information

Bower, S.M. (2009): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Bivalve-inhabiting hydroids of Oysters.

Date last revised: July 2009
Comments to Susan Bower

Date modified: