Bivalve-inhabiting hydroids of Mussels


Category 4 (Negligible Regulatory Significance in Canada)

Common, generally accepted names of the organism or disease agent

Bivalve-inhabiting hydroids of mussels.

Scientific name or taxonomic affiliation

Various species of hydroids (Cnideria, Hydrozoa, Leptomedusae) in the genera Eugymnanthea and Eutima that attach to the tissues within the mantle cavity of bivalves. These hydroids were originally included in the family Eutimidae (Rees 1967; Kubota 1978, 1983, 1984, 1985c, 1987b) but have recently been placed in the family Eirenidae (Bouillon 1985, Piraino et al. 1994, Kubota 2000, Govindarajan et al. 2005). Species reported from mussels including Eugymnanthea inquilina (=polimantii) in the Mediterranean Sea (Palombi,1935, Cerruti,1941, Crowell,1957, Piraino et al.,1994, Rayyan et al., 2002), Eugymnanthea (=Ostreohydra) japonica (initially identified as Eugymnanthea inquilina japonica), Eutima (=Eugymnanthea) cirrhifera and Eutima japonica (=Eucheilota intermedia a synonymy by Kubota 2000) in Japanese waters (Rees 1967; Kubota 1978, 1979, 1985b, 1987b, 1989, 1991, 2000). Kubota (2000) suggested the possibility of merging Eutima and Eugymnanthea into a single genus.

Geographic distribution

Reports of mussels containing hydroids are most common from the water surrounding Japan and Taiwan and the European coast of the Mediterranean Sea (with reports from Italy, France, Spain, Croatia, and Greece). However, no hydroids were found in 3960 M. galloprovincialis collected from 6 locations along the coast of Gaza between January and June in 2005 and 2006 (A. Rayyan, unpublished data). Hydroids were reported less frequently in other bivalves including Crassostrea gigas, Venerupis (=Ruditapes) philippinarum and Chlamys farreri from the coast of Japan and Ruditapes (=Tapes) decussatus, Cardium edule and Ostrea edulis from the Mediterranean Sea (Rees 1967; Kubota 1985c, 2000; Rayyan et al. 2002; Govindarajan et al. 2005). Other species of bivalve-inhabiting hydroids have also been described in Tivela mactroides from Brazil (Narchi and Hebling 1975), in Crassostrea rhizophorae from Puerto Rico (Mattox and Crowell 1951), and in several species of wood-boring bivalves from India (Santhakumari 1970). No bivalve-inhabiting hydroids have been reported from cold waters (Kubota 1987b) and none have been found in Canada.

Host species

Mytilus galloprovincialis, Mytilus edulis, Mytilus coruscus and other species of bivalves including oysters and other species as indicated above. Piraino et al. (1994), indicated that polyps of E. inquilina in R. decussatus from the Ionian Sea, SE Italy rarely produced medusoids and the medusoids that were produced had limited gonadal development suggesting that the life cycle was impaired in this species of hydroid from clams. Apparently, mussels played a key role in the stability and persistence of E. inquilina populations in that area.

Impact on the host

Bivalve-inhabiting hydroids attach to the mantle, foot, labial palps, body wall and infrequently on the gills of the mussel host in the Mediterranean (Kubota 1989, Rayyan et al. 2002) and more frequently on the gills of mussels from Japan (Kubota 1991, 2000). Piraino et al. (1994) indicated that E. inquilina was rarely found in mussels less than 40 mm in shell length. Often the polyps are found in clusters or patches in larger (older) M. galloprovincialis suggesting that the initial settlement of a single planula is followed by asexual budding of the resulting polyp to form a colony of clones even though the polyps are able to slowly move about the ciliated epithelium of the mantle or palps (Crowell 1957). The polyps asexually produce short-lived eumedusoids deprived of a manubrium in some species. The polyps become very small and often degenerate at the end of medusoid production but may regenerate after the medusoid is shed. Medusoids, which are usually separate sexes, are set free with ripe gonads ready to produce gametes (gonophores). The gametes unite and form planula larvae, which settle in a bivalve host and become polyps.

Rayyan et al. (2004) observed that E. inquilina infestations increased with mussel size and the condition index was lowest among mussels with the greatest number of hydroids especially for mussels also infected with Urastoma cyprinae and Mytilicola intestinalis. Rayyan et al. (2004) also noted that the meat of mussels with large numbers of hydroids had an unpleasant smell. Although one report indicated that the polyps may cause loss of cilia and the presence of granules in the epithelial cells of the host mussel, another report indicate no evidence of effect on mantle tissues by the attachment of the hydroid pedal disc (Piraino et al. 1994) . However, the relationship between the hydroid and its mussel host was interpreted as commensal (living together with some degree of harmony) with the hydroids receiving some food and possibly providing the mussel with some protection against other invaders (Rees 1967, Kubota 1983, Piraino et al. 1994). Specifically, Piraino et al. (1994) noted the selective ingestion of bucephalid trematode sporocysts by E. inquilina inhabiting mussels in southern Italy. However, Kubota (1983) noted that pea crabs (Pinnotheres spp.) were sometimes found in mussels inhabited by hydroids but the hydroids were not found attached to the crabs.

Diagnostic techniques

Gross and Wet Mount Observations: Athecate (no periderm (= perisarc)) on the entire body; only a thin membrane covers the medusa-bud (Figure 2), solitary polyps (about 0.7 up to 3.5 mm in length from hydrosome to disc and up to 0.29 mm in width), lacking hydrorhizae and gonothecae, possessing a basal disc for attachment to the molluscan tissue, a single whorl of 17 to 30 distal filiform tentacles (up to 7.0 mm in length) around a shallow hypostome and producing on the column near the base either a polyp bud or a single medusoid (leptomedusae). The colour of the polyp is variable, appearing from white, to yellow, orange and sometimes even red (Rayyan et al. 2002). A second polyp can arise as a bud near the base of the first. When the bud has developed into a second polyp, the two polyps will split the basal portion of the column and the basal disc and separate.

Figure 1. Polyp of Eugymnanthea inquilina showing a tiny undifferentiated bud (arrow) close to the pedal disk.

Figure 2. An E. inquilina polyp with a developing medusoid bud that is surrounded by a membrane (arrow).

The morphology of the medusoid is required for species identification (Kubota 1985b). One day old medusoids of E. inquilina are 0.55- 0.90 mm in width at the umbrella. The medusoid has eight marginal statocysts, and neither manubrium nor marginal tentacles (Figure 3). There are four radial canals, each bearing proximally a large oval gonad, and terminating distally in small rudimentary tentacular bulbs. The statocyst contain a variable number of statoliths 0-7, but the majority of statocysts have more than two statoliths (Figure 4) (Rayyan et al. 2002, Govindarajan et al. 2005). The usual absence of a manubrium and greater number of statoliths per statocysts in the medusa stage, can be used to distinguish E. inquilina from Eugymnanthea japonica (which usually has a slender manubrium and usually one and rarely two or three statoliths per statocyst) (Kubota 1989, Kubota 1991, Govindarajan et al. 2005). Eutima cirrhifera has lateral cirri present on either side of the tentacle bulbs. Eutima japonica (=Eucheilota intermedia) has many (up to 41) cirri, 4 tentacles and up to 10 statoliths per statocyst (Kubota 1984).

Figure 3. The mature medusoid of E. inquilina (1 day old). Note no manubrium. The arrows indicate the locations of three of the eight statocysts.

Figure 4. Three statocysts (arrows) of E. inquilina, two containing five statoliths and one containing three.

DNA Probes: The sequence of polymerase chain reaction (PCR) product (about a 600 base pair segment) of the 16S rDNA was used to discriminate between the two species of Eugymnanthea in mussels, E. inquilina and E. japonica (Govindarajan et al. 2005).

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. For taxonomic purposes, the medusoids can be examined live or preserved in buffered 10% formalin.

Bioassay: The mesoglea test is based on the adhesion and spread of cells on the mesoglea (extracellular matrix) and is a tool to discriminate between hydrozoan species. The mesoglea is excised from a live polyp and incubated in calcium and magnesium free water with agitation to allow the cells to disassociate from the matrix. The mesoglea is then washed in distilled water, placed on a glass coverslip and allowed to air-dry. Tissue from the test sample is then excised, washed in filtered sea water, applied to the dried mesoglea and kept moist. After about 1 hour, the mesoglea is examined for cell adhesion. Alternately, tissue fragments from one individual are grafted onto the mesoglea from another individual. The tissue fragments should only adhere and spread if the mesoglea is from a closely related (i.e., conspecific) individual. However, the mesoglea test may not have sufficient resolution to differentiate between closely related species (Govindarajan et al. 2005).

Methods of control

No known methods of prevention or control. The prevalence of infestation seems to be greater in confined locations with high bivalve populations than in open waters.


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Citation Information

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

Date last revised: July 2009
Comments to Susan Bower

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