Ilyanassa obsoleta

Invasion History

First Non-native North American Tidal Record: 1907
First Non-native West Coast Tidal Record: 1907
First Non-native East/Gulf Coast Tidal Record:

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General Invasion History:

Tritia obsoleta is native to the Northwest Atlantic, from the Bay of Chaleur, Quebec, in the Gulf of Saint Lawrence to western Florida (Bousfield 1960; Abbott 1974). It was first collected in San Francisco Bay in 1907, where it was introduced with Eastern Oysters (Dall 1907, cited by Carlton 1979) and is now widespread and abundant (Cohen and Carlton 1995; Cohen et al. 2005). It was also introduced to Willapa Bay, Washington and Boundary Bay, British Columbia by 1945 and 1952, respectively. It is established in both bays (Carlton 1979; Wonham and Carlton 2005).

North American Invasion History:

Invasion History on the West Coast:

Tritia obsoleta was first found in beds of newly planted Eastern Oysters (Crassostrea virginica) at Alameda, in San Francisco Bay by W. H. Dall in 1907 (Carlton 1979). It is now the most abundant gastropod on mudflats in San Francisco Bay. It ranges throughout the South and Central bays, and through San Pablo Bay to the Carquinez Straits (Carlton 1979; Nichols and Thompson 1985; Cohen and Carlton 1995; California Academy of Sciences 2008). In San Pablo Bay, it was found at sites from 9.5 to 25 PSU (Filice 1959, cited by Carlton 1979). Surprisingly, this snail has been collected in other California estuaries, but has not become established. Locations (listed from South to North) include: Monterey Bay, Moss Landing Harbor in 1971 (California Academy of Sciences 2008); Bolinas Lagoon in 1925 (Carlton 1979), Tomales Bay in 1930 (MacGinitie 1930, cited by Carlton 1979); Bodega Harbor in 1968 (Carlton 1979), and Humboldt Bay in 1932 (failed, Hanna 1966, Carlton 1979).

Tritia obsoleta was first collected in Willapa Bay, Washington (WA) and Boundary Bay, British Columbia in 1945 and 1952, respectively, but may have been introduced much earlier (1874-1905) with early oyster plantings. Collecting in these areas was sparse in the early 20th century (Carlton 1979). This snail is locally abundant in both bays (Cohen et al. 200; Sakamaki and Richardson 2006), but it has not been reported from Puget Sound, WA or from other locations in British Columbia (Cohen et al. 1998; Gillespie et al. 2007).

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Description

Ilyanassa obsoleta (Eastern Mud Snail) is a medium-sized marine and estuarine snail. Its shell is oval, and dextrally coiled, with a well-defined spire, usually eroded at the tip. The adult shell has 6-7 whorls. The parietal wall (inner lip) is thick and glazed with brown and gray. The columella has a spiral ridge near the base. The shell is weakly sculpted with numerous rows of small beads, running roughly at right angles to the growth lines. The outer lip has six small grayish teeth. The shell reaches 25-30 mm. The color of the shell is dark blackish-brown, often overgrown with algae or smeared with mud. The name Ilyanassa obsoleta is widely used, but a molecular analysis found that no genetic distinction between the genera Ilyanassa, Cyclope, and Tritia (Galindo et al. 2016). However, a later analysis restored the distinctness of the genus Ilyanassa (Yang et al. 2021). Escription from: Abbott 1974; Morris 1975; Gosner 1978; and McLean, in Carlton 2007.

Larval development of T. obsoleta is illustrated by Scheltema (1962).

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Taxonomy

Ecology

General:

Ilyanassa obsoleta is an estuarine-marine snail, especially associated with mudflats and other soft-sediment habitats. Sexes are separate and animals mature at about 12-14 mm, which in Massachusetts populations is reached in their third summer (Scheltema 1964). Fertilization is internal and eggs are laid in capsules, about 2.7 mm in height, containing about 40-150 eggs (Scheltema 1962). Small, medium, and large females laid means of 31, 55, and 79 egg capsules, respectively (Schwab 2012). The straw-colored capsules are distinctive, armed with spiny flanges, and are attached to the substrate. The eggs hatch into planktotrophic veligers, which take 10-22 days to develop at 17.5 to 25°C. The veligers settle and metamorphose at about 650-750 µm (Scheltema 1962).

Ilyanassa obsoleta spends much of its time on exposed intertidal mudflats where it experiences a wide range of temperatures and salinities. It tolerates temperatures as high as 30°C in water (Vernberg and Vernberg 1969) and probably higher in air when exposed. This snail responds to winter cold by migrating into shallow subtidal waters (Batchelder 1915: Murphy 1979). It tolerates salinities as low as 10 PSU (Carlton 1979; Cohen et al. 2001) and at least as high as 35 PSU, near the ocean, and maybe higher in salt marsh pools. The temperature range for larval development is 16.5 to 28°C and the salinity range is 21-33 (highest tested) PSU (Scheltema 1965).

Ilyanassa obsoleta is an omnivore. Young snails in aquaria graze on algae growing on glass and probably feed by this method in the wild. As they grow, they switch to ingesting large quantities of sediment, together with organic matter and benthic diatoms. They also feed on decaying algae, such as Ulva, and are strongly attracted to carrion, such as dead fish and mollusks. However, they do not attack or feed on living bivalves (Scheltema 1964). This snail is eaten by fishes, crabs, and birds. It is also a host to many species of parasites. Blakeslee et al. (2012) found nine species of trematodes in I obsoleta from its native range, four each with birds or fishes as final hosts, and one infecting a turtle (Diamondback Terrapin, Malaclemys terrapin). Four of these parasites were found in I. obsoleta on the West Coast. (See 'Impacts' section for details.)

Food:

Detritus, Carrion, Algae

Consumers:

Fish, crabs, birds

Competitors:

Littorina littorea

Trophic Status:

Omnivore

Omni

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Habitats

General Habitat	Unstructured Bottom	None
General Habitat	Oyster Reef	None
General Habitat	Salt-brackish marsh	None
Salinity Range	Mesohaline	5-18 PSU
Salinity Range	Polyhaline	18-30 PSU
Salinity Range	Euhaline	30-40 PSU
Tidal Range	Subtidal	None
Tidal Range	Low Intertidal	None
Tidal Range	Mid Intertidal	None
Vertical Habitat	Epibenthic	None

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Tolerances and Life History Parameters

Minimum Temperature (ºC)	0	Experimental, limited winter freezing tolerance, response to cold by migrating to deeper water(Murphy 1979)
Maximum Temperature (ºC)	30	Snails from North Carolina were acclimated at this temperature. A temperature of 37 C resulted in ~50% mortality in 6 hours for unparasitized snails (Vernberg and Vernberg (1969).
Minimum Salinity (‰)	10	Field and Experimental, inactivity in adults inhibited below 12.5 PSU (Scheltema 1965). But animals abundant at 9-10 PSU (Carlton 1979; Cohen et al. 2001).
Maximum Salinity (‰)	35	Typical ocean salinity, probably experiences higher salinities in marsh and mudflat pools.
Minimum Reproductive Temperature	16.5	Temperature for larval development, experimental. No development at 11.5 C. Scheltema 1967)
Maximum Reproductive Temperature	28	Temperature for larval development, experimental, highest tested (Scheltema 1967)
Minimum Reproductive Salinity	20.9	Minimum salinity for completion of metamorphosis, not completed at 17.1 ppt (Scheltema 1965)
Minimum Duration	10	Experimental, larval duration, 25.2 C, beginning of creeping-swimming period (Scheltema 1967)
Maximum Duration	22	Experimental, larval duration,17.5 C, beginning of creeping-swimming period (Scheltema 1967)
Minimum Length (mm)	14	Size at maturity (Scheltema 1964)
Maximum Length (mm)	30	Shells in San Francisco Bay often reach 30 mm (Carlton 1979) but the usually reported maximum for the East Coast is 25 mm (Abbott 1974; Morris 1975).
Broad Temperature Range	None	Cold temperate-Warm temperate
Broad Salinity Range	None	Mesohaline-Euhaline

General Impacts

Tritia obsoleta has developed dense populations on the West Coast. The ecology of this species has been extensively studied on the East Coast, but much less is known about the ecology and impacts of this species on the West Coast. In San Francisco Bay (California), it has displaced the native California Hornshell (Cerithidea californica) from many habitats. In Willapa (Washington) and Boundary Bays (WA/British Columbia), where there are dense populations, and no native equivalents, it may have affected sediment characteristics and foodwebs. However, these impacts have not been studied. The introduction of T. obsoleta resulted in the transport of five digenetic trematode species to San Francisco Bay. Three of these parasites reached Willapa Bay, and two reached Boundary Bay. The adult hosts of these parasites are birds and fishes, but the effects of these parasites on the hosts are not known (Blakeslee et al. 2012).

Economic Impacts

Health: Outbreaks of 'Swimmers Itch' in San Francisco Bay were traced to the cercaria larvae of a blood-fluke (schistosome), Austrobilharzia variglandis, whose intermediate host is Tritia obsoleta. The final host of the trematode is birds, but the cercaria frequently penetrate the skin of bathing humans, where they cannot develop, but do cause itching and irritation. The trematode is believed to have been introduced to the San Francisco Bay with T. obsoleta (Grodhaus and Keh 1958).

Ecological Impacts

Competition: In San Francisco Bay, Tritia obsoleta has displaced the native snail Cerithidea californica (California Hornsnail) in tidal creeks and restricted them to salt pans (i.e. habitats with higher temperatures and more desiccation). These conditions are beyond the tolerance of T. obsoleta, but are tolerated (but are perhaps suboptimal) by C. californica. The mechanism of competition appears to be behavioral interference, in which T. obsoleta move over and bump into C. californica. The native snails respond by withdrawing into their shells, reducing the time available for foraging (Race 1982).

Predation: Tritia obsoleta has reduced recruitment of Cerithidea californica (California Hornsnail) in San Francisco Bay, by consuming C. californica's egg capsules (Race 1982).

Parasite/Predator Vector: The introduction of T. obsoleta resulted in the transport of five digenetic trematode species to the West Coast: Zoogonus lasius (as Z. rubellus); Lepocreadium setiferoides; Stephanostomum tenue; Himasthla quissetensis; and Austrobilharzia variglandis. The first three of these species have fishes as final hosts; the last two have birds (shorebirds, gulls, cormorants, ducks) as final hosts (Blakeslee et al. 2012). All of these parasites are present in San Francisco Bay, while three were found in Willapa Bay, and two in Boundary Bay. The two most widespread parasites, H. quissetensis and A. variglandis are bird-hosted as adults. Impacts of these parasites on their final hosts is unknown.

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Regional Impacts

NEP-V	Northern California to Mid Channel Islands		Ecological Impact	Competition
Tritia obsoleta has displaced the native snail Cerithidea californica in San Francisco Bay from tidal creeks. The native snails actively withdraw from T. obsoleta, and are mostly restricted to salt pans, which are frequently subject to high temperatures and desiccation (Race 1982).
NEP-V	Northern California to Mid Channel Islands		Ecological Impact	Predation
Tritia obsoleta actively feeds on the egg capsules of the native snail Cerithidea californica in San Francisco Bay (Race 1982)
P090	San Francisco Bay		Ecological Impact	Competition
Tritia obsoleta has displaced the native snail Cerithidea californica in San Francisco Bay from tidal creeks. The native snails actively withdraw from T. obsoleta, and are mostly restricted to salt pans, which are frequently subject to high temperatures and desiccation (Race 1982).
P090	San Francisco Bay		Ecological Impact	Predation
Tritia obsoleta actively feeds on the egg capsules of the native snail Cerithidea californica in San Francisco Bay (Race 1982)
NEP-V	Northern California to Mid Channel Islands		Ecological Impact	Parasite/Predator Vector
The introduction of T. obsoleta resulted in the transport of 5 digenetic trematode species to San Francisco Bay: Zoogonus lasius (as Z. rubellus); Lepocreadium setiferoides; Stephanostomum tenue; Himasthla quissetensis; Austrobilharzia variglandis. The first 3 of these species have fishes as final hosts; the last 2 have birds (shorebirds, gulls, cormorants, ducks) as final hosts (Blakeslee et al. 2012). The effect of these parasites on their final hosts is unknown.
P090	San Francisco Bay		Ecological Impact	Parasite/Predator Vector
The introduction of T. obsoleta resulted in the transport of 5 digenetic trematode species to San Francisco Bay: Zoogonus lasius (as Z. rubellus); Lepocreadium setiferoides; Stephanostomum tenue; Himasthla quissetensis; Austrobilharzia variglandis. The first 3 of these species have fishes as final hosts; the last 2 have birds (shorebirds, gulls, cormorants, ducks) as final hosts (Blakeslee et al. 2012). The effect of these parasites on their final hosts is unknown.
P270	Willapa Bay		Ecological Impact	Parasite/Predator Vector
The introduction of T. obsoleta resulted in the transport of 3 digenetic trematode species to Willapa Bay: Zoogonus lasius (as Z. rubellus); Himasthla quissetensis; Austrobilharzia variglandis. The first of these species has fishes as its final hosts; the last 2 have birds (shorebirds, gulls, cormorants, ducks) as final hosts (Blakeslee et al. 2012). The effect of these parasites on their final hosts is unknown.
NEP-IV	Puget Sound to Northern California		Ecological Impact	Predation
The introduction of T. obsoleta resulted in the transport of 3 digenetic trematode species to Willapa Bay: Zoogonus lasius (as Z. rubellus); Himasthla quissetensis; Austrobilharzia variglandis. The first of these species has fishes as its final hosts; the last 2 have birds (shorebirds, gulls, cormorants, ducks) as final hosts (Blakeslee et al. 2012). The effect of these parasites on their final hosts is unknown.
P297	_CDA_P297 (Strait of Georgia)		Ecological Impact	Predation
The introduction of T. obsoleta resulted in the transport of 2 digenetic trematode species to Boundary Bay: Himasthla quissetensis; Austrobilharzia variglandis. These species have birds (shorebirds, gulls, cormorants, ducks) as final hosts (Blakeslee et al. 2012). The effect of these parasites on their final hosts is unknown.
NEP-III	Alaskan panhandle to N. of Puget Sound		Ecological Impact	Parasite/Predator Vector
The introduction of T. obsoleta resulted in the transport of 2 digenetic trematode species to Boundary Bay: Himasthla quissetensis; Austrobilharzia variglandis. These species have birds (shorebirds, gulls, cormorants, ducks) as final hosts (Blakeslee et al. 2012). The effect of these parasites on their final hosts is unknown.
P090	San Francisco Bay		Economic Impact	Health
Tritia obsoleta is an intermediate host of the digenetic trematode (blood-fluke, schistosome) Austrobilharzia variglandis. The final host of the the trematode is birds but the cercaria frequently infect the skin of bathing humans, causing Swimmer's Itch (Grodhaus and Keh 1958).
NEP-V	Northern California to Mid Channel Islands		Economic Impact	Health
Tritia obsoleta is an intermediate host of the digenetic trematode (blood-fluke, schistosome) Austrobilharzia variglandis. The final host of the trematode is birds, but the cercaria frequently infect the skin of bathing humans, causing Swimmer's Itch (Grodhaus and Keh 1958).
CA	California		Ecological Impact	Competition
Tritia obsoleta has displaced the native snail Cerithidea californica in San Francisco Bay from tidal creeks. The native snails actively withdraw from T. obsoleta, and are mostly restricted to salt pans, which are frequently subject to high temperatures and desiccation (Race 1982)., Tritia obsoleta has displaced the native snail Cerithidea californica in San Francisco Bay from tidal creeks. The native snails actively withdraw from T. obsoleta, and are mostly restricted to salt pans, which are frequently subject to high temperatures and desiccation (Race 1982).
CA	California		Ecological Impact	Parasite/Predator Vector
The introduction of T. obsoleta resulted in the transport of 5 digenetic trematode species to San Francisco Bay: Zoogonus lasius (as Z. rubellus); Lepocreadium setiferoides; Stephanostomum tenue; Himasthla quissetensis; Austrobilharzia variglandis. The first 3 of these species have fishes as final hosts; the last 2 have birds (shorebirds, gulls, cormorants, ducks) as final hosts (Blakeslee et al. 2012). The effect of these parasites on their final hosts is unknown., The introduction of T. obsoleta resulted in the transport of 5 digenetic trematode species to San Francisco Bay: Zoogonus lasius (as Z. rubellus); Lepocreadium setiferoides; Stephanostomum tenue; Himasthla quissetensis; Austrobilharzia variglandis. The first 3 of these species have fishes as final hosts; the last 2 have birds (shorebirds, gulls, cormorants, ducks) as final hosts (Blakeslee et al. 2012). The effect of these parasites on their final hosts is unknown.
CA	California		Ecological Impact	Predation
Tritia obsoleta actively feeds on the egg capsules of the native snail Cerithidea californica in San Francisco Bay (Race 1982), Tritia obsoleta actively feeds on the egg capsules of the native snail Cerithidea californica in San Francisco Bay (Race 1982)
CA	California		Economic Impact	Health
Tritia obsoleta is an intermediate host of the digenetic trematode (blood-fluke, schistosome) Austrobilharzia variglandis. The final host of the trematode is birds, but the cercaria frequently infect the skin of bathing humans, causing Swimmer's Itch (Grodhaus and Keh 1958)., Tritia obsoleta is an intermediate host of the digenetic trematode (blood-fluke, schistosome) Austrobilharzia variglandis. The final host of the the trematode is birds but the cercaria frequently infect the skin of bathing humans, causing Swimmer's Itch (Grodhaus and Keh 1958).
WA	Washington		Ecological Impact	Parasite/Predator Vector
The introduction of T. obsoleta resulted in the transport of 3 digenetic trematode species to Willapa Bay: Zoogonus lasius (as Z. rubellus); Himasthla quissetensis; Austrobilharzia variglandis. The first of these species has fishes as its final hosts; the last 2 have birds (shorebirds, gulls, cormorants, ducks) as final hosts (Blakeslee et al. 2012). The effect of these parasites on their final hosts is unknown.

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