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information on the biology of species and the ecology of habitats found around the coasts and seas of the British Isles

Northern hatchet shell (Thyasira gouldi)

Distribution data supplied by the Ocean Biogeographic Information System (OBIS). To interrogate UK data visit the NBN Atlas.

Summary

Description

A small, rather globular, white burrowing bivalve. The two halves of the shell are the same shape and the shell itself is thin and fragile. This species is very similar to Thyasira flexuosa, the wavy hatchet shell.

Recorded distribution in Britain and Ireland

At the head of Loch Etive, west coast of Scotland. Formerly also found in Lochs Linnhe, Eil and Sunart. Recorded also from Shetland (further detail lacking). Presence in Scottish waters forms the extreme southern end of the geographic range.

Global distribution

A pan-arctic distribution from waters of the Commonwealth of Independent States along the north coast of Norway, around the coast of Greenland. On American coasts as far south as Cape Cod on the east and California on the west coast.

Habitat

Inhabits a small chamber in the top few centimetres of soft mud or sand-mud sediments at the head of some sea lochs. Habitat generally characterized by the presence of organic matter.

Depth range

15-25

Identifying features

  • A small, white, globular bivalve.
  • The shell is equivalved but thin and fragile.
  • The hinge line is without teeth.
  • The inside of the shell is white and muscle scars indistinct.
  • The gills are thick, fleshy and dark brown in colour.

Additional information

The larger eggs and characteristic sperm are useful features for separating Thyasira gouldi from Thyasira flexuosa.

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Biology review

Taxonomy

PhylumMollusca
ClassBivalvia
OrderLucinoida
FamilyThyasiridae
GenusThyasira
Authority(Philippi, 1845)
Recent Synonyms

Biology

Typical abundanceModerate density
Male size rangeup to 8mm
Male size at maturity4mm
Female size range4mm
Female size at maturity
Growth formBivalved
Growth rate1mm/year
Body flexibilityNone (less than 10 degrees)
Mobility
Characteristic feeding methodActive suspension feeder, Non-feeding
Diet/food source
Typically feeds onsuspended organic matter and digestion of mutualistic bacteria.
Sociability
Environmental positionInfaunal
DependencyMutualist (Mutualism).
a bacterium.
SupportsHost

the parasitic copepod Axinophylus thyasirae and a mutualistic bacterium.

Is the species harmful?No

Biology information

Thyasira gouldi has been found up to 1,500 individuals per square metre but typically below 500. Such abundances may no longer exist in Scottish waters. It is difficult to define an adult size range as there appears no specific point where juveniles become adult. Values provided are roughly maximum size where size refers to shell length. Large numbers of a mutualistic bacterium live sub-cuticularly in the gills of Thyasira gouldi (and several other thyasirids). The bacteria are chemoautotrophic and oxidise sulphur in order to assimilate carbon dioxide. Carbon isotope ratios indicate that digestion of these bacteria contributes considerably to the nutrition of this species. Although the bacteria utilise sulphur the bivalves inhabit sediment with very little free sulphide. The relationship is not thought to be obligate but the presence of the bacterium is very beneficial to the brachiopod. Eleven percent of a population of Thyasira gouldi in Loch Etive was infected with the parasitic copepod Axinophylus thyasirae Blacknell & Ansell, 1975). This parasite inhabits the mantle cavity and causes lower body weights and indirect castration. The female parasites reach sizes of 4.5 mm and there can be up to five parasites per host causing massive restriction of the cavity and interfering with feeding currents.

Habitat preferences

Physiographic preferencesSea loch / Sea lough
Biological zone preferencesLower circalittoral, Lower infralittoral, Upper circalittoral
Substratum / habitat preferencesMud, Muddy sand, Sandy mud
Tidal strength preferences
Wave exposure preferences
Salinity preferencesReduced (18-30 psu)
Depth range15-25
Other preferencesNo text entered
Migration PatternNon-migratory / resident

Habitat Information

Geographic distribution was probably more general during the last glaciation and remaining populations are relicts. The populations in Lochs Linnhe and Eil have been killed by the discharge of pulp-mill effluent. The population in Loch Etive has also decreased massively between 1984 and 1989. It is possible that this decrease has been brought about by a viral infection of the mutualistic bacteria living on the gills of Thyasira gouldi. Digestion of the bacteria provides considerable nutrient input. This species can burrow up to ten times its shell length (max. 8cm) and uses its vermiform foot to create channels deeper into the sediment. A mucus lined inhalant tube is made up to the surface from the living chamber. Little information is available about preferred water flow rates but are probably quite low being at the head of a sea loch. Wave exposure preferences are also likely to be sheltered. Typical depths in Scottish waters are 15-25 metres but the species has been found down to a few hundred metres depth. Optimal salinity levels are 25-30 psu. Thyasira gouldi appears to be restricted to locations where bottom waters remain cool throughout the year as a result of salinity stratification.

Life history

Adult characteristics

Reproductive typeGonochoristic (dioecious)
Reproductive frequency Annual protracted
Fecundity (number of eggs)100-1,000
Generation timeInsufficient information
Age at maturityInsufficient information
SeasonJanuary - December
Life spanInsufficient information

Larval characteristics

Larval/propagule type-
Larval/juvenile development Direct development
Duration of larval stageNot relevant
Larval dispersal potential <10 m
Larval settlement periodNot relevant

Life history information

The sexes are separate and fertilization probably occurs in the mantle or suprabranchial cavity. Egg development is temperature dependent being (in the laboratory) around 50 days at 10 degrees C and 37 days at 16 degrees C. There is no synchronisation of reproduction and spawning occurs throughout the year. Eggs are white, oval and about 260 microns long. Up to 750 eggs are produced with each spawning. No information is available on the mechanism of spawning or the number of spawnings per year. Fertilised eggs are 'pumped' out of the inhalant tube and being quite dense, sink down onto and stick to the sediment about 1 cm from the opening. Consequently eggs are rarely dispersed by water currents. No information is available about lifespan but given the known growth rate and maximum size achieved it must be at least 5-10 years.

Sensitivity reviewHow is sensitivity assessed?

Physical pressures

 IntoleranceRecoverabilitySensitivityEvidence/Confidence
High Very low / none Very High Low
The adults bury in the sediment so sediment loss would result in death. Populations in Scotland are isolated from each other. Development is direct without a planktonic stage, eggs are deposited next to the adult so larval dispersal is very limited. The populations are at the southern extreme of the species range and so may be already stressed by temperature or salinity variation. Recovery may not be possible.
Low Very high Very Low Low
The species is an active burrower down to 8cm. Smothering with a further 5 cm sediment should not cause mortality. The bivalve would be able use its vermiform foot to burrow back up towards the surface. Spawning occurs throughout the year so once the factor is removed, feeding, burrowing and reproduction can resume as normal.
Low Very high Very Low Low
Changes in siltation would probably not have a great effect on Thyasira gouldi. Feeding depends on the presence of suspended organic matter - decreases in siltation may reduce the food available, increases may facilitate feeding. Large increases in siltation may block up the inhalant tube which would require energetic expenditure to clear. Spawning occurs throughout the year so once the factor is removed, feeding, burrowing and reproduction can resume as normal.
No information
High Very low / none Very High Low
The species in entirely subtidal and breathes using gills. It is likely to be highly intolerant of desiccating influences. Populations in Scotland are isolated from each other. Development is direct without a planktonic stage, eggs are deposited next to the adult so larval dispersal is very limited. The populations are at the southern extreme of the species range and so may be already stressed by temperature or salinity variation. Recovery may not be possible.
High Very High Low
The species in entirely subtidal and breathes using gills. It is likely to be highly intolerant of emergence. Populations in Scotland are isolated from each other. Development is direct without a planktonic stage, eggs are deposited next to the adult so larval dispersal is very limited. The populations are at the southern extreme of the species range and so may be already stressed by temperature or salinity variation. Recovery may not be possible.
No information
Low Very high Very Low Low
Increases in water flow may cause sediment movement, blocking the inhalant tube requiring energetic expenditure to clear. Spawning occurs throughout the year so once the factor is removed, feeding, burrowing and reproduction can resume as normal.
No information
High Very High Moderate
The populations in the British Isles are at the southern extreme of the geographical distribution. The surviving relict populations are restricted to areas where the bottom waters remain cool all year round. Any increases in temperature are likely to cause death. Populations in Scotland are isolated from each other. Development is direct without a planktonic stage, eggs are deposited next to the adult so larval dispersal is very limited. The populations are at the southern extreme of the species range and so may be already stressed by temperature or salinity variation. Recovery may not be possible.
No information
Tolerant Not relevant Not sensitive Low
This species is infaunal and as such will be tolerant of changes in turbidity.
No information
High Very High Low
Thyasira gouldi lives in rather wave sheltered areas at the heads of sealochs. Increases in wave exposure may disrupt the sediment in which they live, cause continual displacement and physical damage to the shells which are thin and fragile. Populations in Scotland are isolated from each other. Development is direct without a planktonic stage, eggs are deposited next to the adult so larval dispersal is very limited. The populations are at the southern extreme of the species range and so may be already stressed by temperature or salinity variation. Recovery may not be possible.
No information
Tolerant Not relevant Not sensitive Very low
This species probably has very limited facility for detection of vibrations.
Tolerant Not relevant Not sensitive Low
This species is infaunal and probably has very limited facility for visual perception.
High Very High Low
Thyasira gouldi is a small species and the shell is thin and fragile. They occupy the top few centimetres of the sediment and physical disturbance by a passing scallop dredge is likely to cause death Therefore, an intolerance of high has been recorded.
Populations in Scotland are isolated from each other. Development is direct without a planktonic stage, eggs are deposited next to the adult so larval dispersal is very limited. The populations are at the southern extreme of the species range and so may be already stressed by temperature or salinity variation. Recovery may not be possible.
Low Very high Very Low Low
Thyasira gouldi are active burrowers, displacement will probably result in the bivalve burrowing back down into the sediment and creating a new living chamber and mucous lined inhalant siphon. There would be an energetic cost to this and feeding would be impaired. Spawning occurs throughout the year so once the factor is removed, feeding, burrowing and reproduction can resume as normal.

Chemical pressures

 IntoleranceRecoverabilitySensitivityEvidence/Confidence
No information No information No information Not relevant
Insufficient
information
Heavy metal contamination
No information No information No information Not relevant
Insufficient
information
Hydrocarbon contamination
No information No information No information Not relevant
Insufficient
information
Radionuclide contamination
No information No information No information Very low
Insufficient
information
Changes in nutrient levels
High Very High Moderate
Organic enrichment from pulp mills is believed to have been the cause of the death of two populations in west Scotland sealochs. Similar species such as Thyasira flexuosa may be considerably more tolerant of nutrient enrichment (densities of up to 4000 per square metre have been recorded in areas or organic enrichment). Populations in Scotland are isolated from each other. Development is direct without a planktonic stage, eggs are deposited next to the adult so larval dispersal is very limited. The populations are at the southern extreme of the species range and so may be already stressed by temperature or salinity variation. Recovery may not be possible.
Intermediate Moderate Moderate Moderate
The species inhabits waters of reduced salinity with 25-30 psu being optimal. Adults exposed to lower than optimal salinities produced non viable or slow developing eggs. There is insufficient information regarding the effects of salinity on adults. Development is direct without a planktonic stage, eggs are deposited next to the adult so larval dispersal is very limited. Immigration from other populations is highly unlikely. Reproduction occurs throughout the year, development is direct with quite high fecundity (750 eggs / spawning) High densities of the species are known to occur (up to 1500 per square metre). Growth however is rather slow (1mm/yr) and maturity is only reached at around 4 mm. Recruitment from this population would allow the population to recover in five to ten years.
No information
Intermediate Moderate Moderate Low
The bivalve burrows in often black, anoxic sediment but the area surrounding the mollusc is oxygenated through water movement down the inhalant tube. If the water inhaled becomes de-oxygenated (for instance because of isolation of water masses ) some mortality is likely.. Development is direct without a planktonic stage, eggs are deposited next to the adult so larval dispersal is very limited. Immigration from other populations is highly unlikely. Reproduction occurs throughout the year, development is direct with quite high fecundity (750 eggs / spawning) High densities of the species are known to occur (up to 1500 per square metre). Growth however is rather slow (1mm/yr) and maturity is only reached at around 4 mm. Recruitment from the local population would allow the population to recover in five to ten years.

Biological pressures

 IntoleranceRecoverabilitySensitivityEvidence/Confidence
Intermediate Moderate Moderate Moderate
Viral infection of the mutualist bacterium living on the bivalve's gills has been suggested as the reason for a major decline in the Loch Etive population. Development is direct without a planktonic stage, eggs are deposited next to the adult so larval dispersal is very limited. Immigration from other populations is highly unlikely. Reproduction occurs throughout the year, development is direct with quite high fecundity (750 eggs / spawning) High densities of the species are known to occur (up to 1500 per square metre). Growth however is rather slow (1mm/yr) and maturity is only reached at around 4 mm. Recruitment from this population would allow the population to recover in five to ten years.
No information No information No information Not relevant
Insufficient
information
Not relevant Not relevant Not relevant Moderate
It is extremely unlikely that this species will be extracted - it has no economic or interest value and is protected by the Wildlife and Countryside Act and is covered by a UK Biodiversity Action Plan.
Not relevant Not relevant Not relevant Moderate
The larva has no known obligate partner species that are likely to be extracted. Benthic trawls or dredges for other species may damage or destroy the shells but the species is protected by the Wildlife and Countryside Act and is covered by a UK Biodiversity Action Plan.

Additional information

Importance review

Policy/legislation

Wildlife & Countryside ActSchedule 5, section 9
Scottish Biodiversity List
Features of Conservation Importance (England & Wales)

Status

Non-native

Importance information

-none-

Bibliography

  1. Anonymous, 1999g. Northern hatchet shell (Thyasira gouldi). Species Action Plan. In UK Biodiversity Group. Tranche 2 Action Plans. English Nature for the UK Biodiversity Group, Peterborough., English Nature for the UK Biodiversity Group, Peterborough.

  2. Blacknell, W. M. & Ansell, A. D., 1974. The direct development of bivalve Thyasira gouldi (Philippi). Thalassia Jugoslavica, 10, 23-43.

  3. Blacknell, W. M. & Ansell, A. D., 1975. Features of the reproductive cycle of an arctic bivalve from a Scottish sea loch. Marine Ecology, Pubblicazioni Della Stazione Zoologica Di Napoli I, 39, Suppl., 26-52.

  4. Bowden, J. & Heppel, D., 1973. Revised list of British Mollusca. 2. Unionacea - Cardiacea. Journal of Conchology, 26, 237-272.

  5. Dando, P.R. & Southward, A.J., 1986. Chemoautotrophy in bivalve molluscs of the Genus Thyasira. Journal of the Marine Biological Association of the United Kingdom, 60, 915-929.

  6. Howson, C.M. & Picton, B.E., 1997. The species directory of the marine fauna and flora of the British Isles and surrounding seas. Belfast: Ulster Museum. [Ulster Museum publication, no. 276.]

  7. Southward, E.C. & Southward, A.J., 1991. Virus-like particles in the bacteria symbiotic in bivalve gills. Journal of the Marine Biological Association of the United Kingdom, 71, 37-45.

  8. Southward, E.C., 1986. Gill symbionts in the Thyasirids and other bivalve molluscs. Journal of the Marine Biological Association of the United Kingdom, 66, 889-914.

  9. Tebble, N., 1966. British Bivalve Seashells. A Handbook for Identification, 2nd ed. Edinburgh: British Museum (Natural History), Her Majesty's Stationary Office.

Citation

This review can be cited as:

Jackson, A. 2007. Thyasira gouldi Northern hatchet shell. In Tyler-Walters H. and Hiscock K. (eds) Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. Available from: http://www.marlin.ac.uk/species/detail/1149

Last Updated: 30/08/2007