Invasion History

First Non-native North American Tidal Record: 2009
First Non-native West Coast Tidal Record: 2009
First Non-native East/Gulf Coast Tidal Record:

General Invasion History:

The nereid polychaete Hediste diadroma is native to the northwest Pacific. It was described from the mouth of Omoigawa River, Kyushu, which flows into the East China Sea. Subsequently, it was found on the Pacific coast of Japan as far north as Hokkaido. We do not have specific records from the Sea of Japan, but it is known from the Korea Straits (Fukuoka Prefecture) and the Yellow Sea (Sato and Nakashima 2003). Introduced populations are known from Oregon to British Columbia in the northeast Pacific.

North American Invasion History:

Invasion History on the West Coast:

The nereid polychaete Hediste diadroma was discovered on the West Coast of North America in 2010. Its overall abundance and geographical distribution in North American waters is unknown, as are its ecological impacts. It is a potential competitor with the native H. limnicola and with introduced populations of Neanthes succinea in brackish West Coast estuaries.

In October 2009, Japanese researchers attempted to collect the native H. limnicola in Washington and Oregon, for a study of its unique reproductive biology, which included self-fertilizing hermaphroditism and viviparous development. They reared immature polychaetes, which all matured into epitokous adults in March-April 2010. The epitokes swarmed at the surface and released eggs which developed into planktotrophic larvae. These worms were identified by molecular methods as H. diadroma (Nishizawa et al. 2014). Hediste diadroma was found in Willapa Bay, and the Columbia, Necanicum, and Nehalem River estuaries. They were collected at salinities of 4-18 PSU (Nishizawa et al. 2014). Specimens collected by Leslie Harris, in California , Oregon, and Washington were examined morphologically by Hiroaki Tosuji in 2013. The California specimens were all H. limnicola, but some of the Oregon and Washington specimens were identified as H. diadroma (Leslie Harris, personal communication 6/6/14). A recent paper has expanded the range of H. diadroma from Puget Sound south to Coos Bay, Oregon, and Tomales Bay, California. At most intertidal sites, H. diadroma outnumbered the native H. limnicola, but at Coos Bay, there were 25 H. limnicola and H. diadroma, and at Tomales Bay, 23 H. limnicola and only 2 H. diadroma, possibly suggesting that the range is still expanding southward. Genetic comparisons of Japanese and Northwest Pacific populations suggests that the West Coast introductions came from several Japanese source populations (Tosuji and Furota 2016).  

Hediste diadroma has a prolonged larval period (~ 30 days) and a migratory life history. Adults move to the less saline parts of the estuary, probably by swimming, and later conduct reproductive swarming in the lower estuary. Thus, this worm has multiple opportunities for ballast water transport (Hanafiah et al. 2006; Tosuji and Sato 2006; Nishizawa et al. 2014).


Hediste diadroma is a nereid worm (commonly called a clam worm), characteristic of brackish waters. It has an elongate, cylindrical body, divided into up to 80-105 chaeta-bearing segments (chaetigers). The body is stout anteriorly and gradually tapers towards the pygidium (terminal segment of the body, including the anus). The dorsal surface is convex, while the ventral surface is relatively flat, with a mid-lateral groove. The head is shaped like a compressed pear, wider than its length. There are two pairs of eyes, in a trapezoidal arrangement, a pair of anterior antennae, and a pair of stout conical palps. The head is flanked by four pairs of tentacular cirri of unequal length. The muscular, extendable, proboscis protrudes below the head. It consists of two rings, terminating in a pair of amber-colored jaws, with 6-9 teeth each. The proboscis is marked by patches of amber-colored paragnaths (teeth), arranged in species-specific patterns (see Sato and Nakashima 2003) for descriptions of patterns in H. diadroma. These patterns and details of chaetal structure (acicula, homogomph, heterogomph, spiniger, falciger) are needed for identification of species (Blake and Ruff in Carlton 2007), but will not be dealt with here.

The parapodia of the first two chaetigers are uniramous, but all the following parapodia are biramous. The first two chaetigers do have a dorsal cirrus and smaller lobes, called ligules. The subsequent anterior parapodia are divided into two branches, which are in turn divided into the ligules. The dorsal branch is called the notopodium. It has a dorsal cirrus, which does not extend beyond the ligules. The dorsal ligule is large and triangular, the lowest one is smaller, and the middle one is smallest, about 1-2 to 2/3 the size of the ventral ligule. The middle ligule is called the pre-chaetal lobe, with a bundle of thin chaetae. In posterior chaetigers, this ligule is reduced to a small protrusion. The dorsal ligule is widest in anterior chaetigers, and narrower on middle and posterior chaetigers, while the dorsal cirri also diminish in posterior chaetigers. The neuropodium has a pre-chaetal ligule, a broader median post-chaetal ligule, with a bundle of thicker chaetae, a ventral (post-chaetal) ligule, and a ventral cirrus. The pre-chaetal and post-chaetal ligules are separate on the first 10 chaetigers, and are then fused. The ventral ligule and cirrus decrease in size in the posterior direction. Japanese specimens are up to 150 mm in length, with 60-110 chaetigers (Sato and Nakashima 2003), but the largest North American specimen was 43 mm (Nishizawa et al. 2014). This worm is reddish or greenish-brown in life, and fades to cream-color in preservation (Sato and Nakashima 2003). Male Hediste spp. are yellow and white, with a prominent dorsal blood vessel, while females are green (Hanafiah et al. 2006, see photo).

Hediste diadroma undergoes a dramatic morphological change (epitoky) when breeding, with both sexes changing into a 'heteronereis' form. The body wall becomes thin and transparent, with greenish eggs in mature females and white sperm in males. The eyes are slightly enlarged. The ligules of the parapodia become lengthened in both sexes. Chaetae are modified in the chaetigers of the middle region of the body. The mature worms form large swarms, actively swimming at the surface, discharging eggs and sperm (Sato and Nakashima 2003). The planktotrophic larval stages were described and illustrated by Tosuji and Sato (2006).

Morphological identification of sexually immature Hediste spp. is very difficult. Tosuji and Sato (2012) describe a molecular method for identification of these polychaetes using multiplex PCR with species-specific primer sets. This method was used to identify H. diadroma specimens from Oregon estuaries (Nishizawa et al. 2014).


Taxonomic Tree

Kingdom:   Animalia
Phylum:   Annelida
Class:   Polychaeta
Subclass:   Palpata
Order:   Aciculata
Suborder:   Phyllodocida
Family:   Nereididae
Genus:   Hediste
Species:   diadroma


Potentially Misidentified Species

Hediste atoka
Hediste atoka is native to the Northwest Pacific, and is morphologically quite similar to H. japonica and H. diadroma but lacks a reproductive swarming stage (Sato and Nakashima 2003).

Hediste japonica
Hediste japonica is native to a limited range in the Northwest Pacific, around Kyushu, Japan. Hediste diadroma was formerly considered conspecific with H. japonica (Sato and Nakashima 2003).

Hediste limnicola
Hediste limnicola is native to the Northeast Pacific estuaries (British Columbia to California). Morphologically it is very similar to H. diadroma during most of its life, but is very different in its mode of reproduction and development. It is hermaphroditic, self-fertilization and viviparous, and has at least one population in a freshwater lake (Blake and Ruff 2007; Nishizawa et al. 2014).

Neanthes succinea
Neanthes succinea is probably native to the North Atlantic and is widely introduced. The posterior parapodial lobes are elongated, while the parapodial lobes are much shorter in H. limnicola and H. diadroma (Blake and Ruff 2007; Nishizawa et al. 2014).



Hediste diadroma is a nereid worm, characteristic of brackish waters, and the middle and upper reaches of estuaries. Sexes are separate. Reproduction involves a dramatic metamorphosis (called epitoky), in which the parapodia are modified for swimming, the eyes are enlarged, the body wall thins, and the body is swollen with eggs or sperm (Kikuchi and Yasuda 2009; Sato and Nakashima 2013). In Japan, the animals swarm in the mouths of estuaries at night on high tides, from November through April. Water temperatures at this time are 5-15°C (Hanafiah et al. 2006). Immature animals, collected in Washington and Oregon estuaries in late October, metamorphosed in March and early April when kept in the laboratory at 18°C (Nishizawa and Sato 2014). Eggs and sperm are shed into the water, and the adults die afterwards (Kikuchi and Yasuda 2009).

Fertilized eggs develop into trochophores which hatch out of jelly-like capsules in 26-40 hours. By 3 days, they develop into metatrochophores with beginnings of segmentation and protruding chaetae. By 4 days, the larvae became nectochaetes, with 3 chaetigers. This stage is prolonged, and can last up to 12 days. During the 4-chaetiger stage, at 16-30 days, the larvae begin to alternate between crawling on the bottom and swimming, but settlement is a gradual process, without a clear boundary. Larvae develop in waters of 22-30 PSU salinity (Tosuji and Sato 2006). Immature worms presumably migrate up the estuary by swimming and/or tidal transport, and are most abundant in the mesohaline reaches of estuaries (4-19 PSU, Sato and Nakashima 2003; Hanafiah et al. 2006; Kikuchi and Yasuda 2006; Tosuji and Sato 2006; Nishizawa et al. 2014).

Immature H. diadroma are most abundant in the upper, mesohaline reaches of estuaries, on lower intertidal mudflats and mud bottoms, salt marshes, oyster beds, and in southern Japan, mangroves (Sato and Nakamura 2003; Hanafiah et al. 2006; Tosuji and Sato 2006; Kanaya et al. 2008). The worms live in U-shaped burrows, and are usually regarded as surface-deposit feeders, feeding on detritus and surface microalgae (Kanaya et al. 2008). However, they are capable of secreting a mucous net, and using body movements to generate a downward current, trapping particles in the net, which is then swallowed. Toba and Sato (2013) observed this filter-feeding behavior in all three Japanese species of Hediste, and note reports of its occurrence in the North Atlantic H. diversicolor as well. In a marine dominated Japanese lagoon, isotopic measurements indicated that Hediste spp. fed mostly on benthic diatoms, while in a river-dominated lagoon, terrestrial plant detritus was the predominant food (Kanaya et al. 2008). At a mudflat site on Ise Bay, Japan, H. diadroma was a major food item for shorebirds (Iwamatsu et al. 2011). Other likely predators include shrimps, fishes, and crabs.


Detritus, benthic microalgae. Phytoplankton


Fishes, crabs, shrimps, birds


Trophic Status:

Deposit Feeder



General HabitatUnstructured BottomNone
General HabitatOyster ReefNone
General HabitatSalt-brackish marshNone
General HabitatMangrovesNone
Salinity RangeOligohaline0.5-5 PSU
Salinity RangeMesohaline5-18 PSU
Salinity RangePolyhaline18-30 PSU
Salinity RangeEuhaline30-40 PSU
Tidal RangeSubtidalNone
Tidal RangeLow IntertidalNone
Vertical HabitatEndobenthicNone
Vertical HabitatEpibenthicNone
Vertical HabitatNektonicNone

Tolerances and Life History Parameters

Minimum Temperature (ºC)NoneNone
Maximum Temperature (ºC)NoneNone
Minimum Salinity (‰)4Field, Oregon, Nishizawa et al. 2014
Maximum Salinity (‰)34Field, Japan (spawning adults, Tosuji and Sato 2006)
Minimum Reproductive Temperature5Field, Japan (Hanafia et al. 2006)
Maximum Reproductive Temperature20Field, Japan (Hanafiah et al. 2006)
Minimum Reproductive Salinity22.5Experimental (Tosuji and Sato 2006)
Maximum Reproductive Salinity30Experimental (Tosuji and Sato 2006)
Maximum Duration30Experimental, 15 C (Tosuji and Sato 2006)
Minimum Length (mm)25Spawning adults (Sato and Nakashima et al. 2003)
Maximum Length (mm)150Spawning adults (Sato and Nakashima et al. 2003)
Broad Temperature RangeNoneWarm temperate-Cold temperate
Broad Salinity RangeNoneOligohaline-Euhaline

General Impacts

The nereid polychaete Hediste diadroma was discovered on the West Coast of North America in 2009. Its overall abundance and geographical distribution in North American waters is unknown, as are its ecological impacts. It is a potential competitor with the native H. limnicola and with introduced populations of Neanthes succinea in brackish West Coast estuaries.

Ecological Impacts

Competition: A recent paper indicates that Hediste diadroma has replaced H. limnicola in the more saline intertidal portions of estuaries, while H. limnicola remained dominant at salinities below 5 PSU and in subtidal habitats (Tosuji and Furota 2016).

Regional Impacts

NEP-IIIAlaskan panhandle to N. of Puget SoundEcological ImpactCompetition
The native Hediste limnicola appears to have been displaced from higher-salinity intertidal habitats, but continues to be dominant in the inner oligohaline-fresh water parts of estuaries (Tosuji and Furota 2016).
NEP-IVPuget Sound to Northern CaliforniaEcological ImpactCompetition
The native Hediste limnicola appears to have been displaced from higher-salinity intertidal habitats, but continues to be dominant in the inner oligohaline-fresh water parts of estuaries (Tosuji and Furota 2016).

Regional Distribution Map

Bioregion Region Name Year Invasion Status Population Status
NEP-IV Puget Sound to Northern California 2009 Def Estab
P270 Willapa Bay 2009 Def Estab
P260 Columbia River 2009 Def Estab
P255 _CDA_P255 (Necanicum) 2010 Def Estab
P250 Nehalem River 2009 Def Estab
P210 Yaquina Bay 2009 Def Estab
NWP-3b None 1906 Native Estab
NWP-3a None 1986 Native Estab
NWP-4b None 0 Native Estab
NWP-4a None 0 Native Estab
P290 Puget Sound 2012 Def Estab
NEP-III Alaskan panhandle to N. of Puget Sound 2012 Def Estab
P280 Grays Harbor 2012 Def Unk
P220 Siletz Bay 2012 Def Estab
P200 Alsea River 2014 Def Estab
P190 Siuslaw River 2012 Def Estab
P180 Umpqua River 0 Def Estab
P110 Tomales Bay 2013 Def Unk
NEP-V Northern California to Mid Channel Islands 2013 Def Unk

Occurrence Map

OCC_ID Author Year Date Locality Status Latitude Longitude


Blake, James A.; Ruff, R. Eugene (2007) The Light and Smith Manual: Intertidal invertebrates from Central California to Oregon (4th edition), University of California, Berkeley CA. Pp. 309-410

Hanafiah, Zazili; Sato, Masanori; Nakashimi, Hidetoshi; Tosuji, Hiroaki (2006) Reproductive swarming of sympatric nereidid polychaetes in an estuary of the Omuta-Gawa River, in Kyusu, Japan, with reference to simutaneous swarming of tow Hediste species, Zoological Science 23: 205-217

Iwamatsu, Shinichiro; Suzuki, Akiko; Sato, Masanori (2007) Nereidid polychaetes as the major diet of migratory shorebirds on the estuarine tidal flats at Fujimae-Higata in Japan, Zoological Science 24(7): 676-685

Kanaya, Gen; Takagi, Shigeto; Kikuchi, Eisuke (2008) Spatial dietary variations in Laternula marilina (Bivalva) and Hediste spp. (Polychaeta) along environmental gradients in two brackish lagoons, Marine Ecology Progress Series 359: 133-144

Kikuchi, Eisuke; Yasuda, Koji (2006) Comparison of the life cycles of two sympatric estuarine polychaetes, Hediste diadroma and H. atoka (Polychaeta: Nereididae), in the Nanakita River estuary, northeastern Japan, Limnology 7: 103-115

Nishizawa, Ryogo; Sato, Masanori; Furota, Toshio; Tosuji, Hiroaki (2014) Cryptic invasion of northeast Pacific estuaries by the Asian polychaete, Hediste diadroma (Nereididae), Marine Biology 161: 187-194

Sato, Masanori; Nakashima, Akiyuki (2003) A review of Asian Hediste species complex (Nereididae, Polychaeta) with descriptions of two new species and a redescription of Hediste japonica (Izuka, 1908), Zoological Journal of the Linnean Society 137: 403-445

Toba, Yuji; Sato, Masanori (2013) Filter-feeding behavior of three Asian Hediste species (Polychaeta: Nereididae)., Plankton & Benthos Research 8(4): 159-165

Tosuji, Hiroaki; Furota, Toshio (2016) Molecular evidence for the expansion of the Asian cryptic invader Hediste diadroma (Nereididae: Annelida) into the Northeast Pacific habitats of the native H. limnicola, Zoological Science 33(2): 162-169

Tosuji, Hiroaki; Sato, Masanori (2006) Salinity favorable for early development and gamete compatibility in two sympatric estuarine species of the genus Hediste (Polychaeta: Nereididae) in the Ariake Sea, Japan, Marine Biology 148: 529-539

Tosuji, Hiroaki; Sato, Masanori (2012) A simple method to identify Hediste sibling species (Polychaeta: Nereididae) using multiplex PCR amplification of the mitochondrial 16S rRNA gene, Plankton & Benthos Research 7(4): 195-202