Gravel sea cucumber (Neopentadactyla mixta)

Feeding tentacles of Neopentadactyla mixta protruding from gravel.

Photographer: Sue Scott Copyright: Sue Scott

Neopentadactyla mixta feeding tentacles.

Photographer: Dr Keith Hiscock Copyright: Dr Keith Hiscock

Feeding tentacles of Neopentadactyla mixta

Photographer: Paul Newland Copyright: Paul Newland

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

Researched by	Angus Jackson	Refereed by	Dr Andrew C. Campbell
Researched by Angus Jackson	Refereed by Dr Andrew C. Campbell
Authority	(Östergren, 1898) Deichmann, 1944
Authority (Östergren, 1898) Deichmann, 1944
Other common names	-	Synonyms	Pseudocucumis mixta (Östergren, 1898) Deichmann, 1944
Other common names -	Synonyms Pseudocucumis mixta (Östergren, 1898) Deichmann, 1944

Summary

Description

Neopentadactyla mixta is a cylindrical sea cucumber up to about 20 cm long and creamy yellow in colour. The body tapers towards either end and there are a bunch of profusely branching tentacles around the mouth. The tentacles are usually light grey but may be darker, appearing almost transparent when fully extended. The gravel sea cucumber lives within the substratum so the only parts visible are the tentacles, when extended.

Recorded distribution in Britain and Ireland

All up the west coast of Scotland to Orkney and Shetland. A few isolated records from the east coast. Also from SW England, SW and NW Wales. In Ireland, particularly Kilkieran Bay, round the north and north-east, also the SE and SW corners.

Global distribution

South, west and north coasts of the British Isles, the Faeroe Islands, the west coast of Norway (Molde) and the Atlantic coasts of France.

Habitat

The gravel sea cucumber lives within coarse, typically mobile shell sand, gravel or maerl where water flow is quite strong.

Depth range

15-70

Identifying features

Found in coarse gravel or maerl with only the buccal tentacles showing above the surface.
There are 20 tentacles, 10 large, 5 intermediate and 5 small.
The tentacles are thicker at the base than the tip and branch in a highly arborescent fashion.
There are numerous calcareous deposits in the skin.

Additional information

Various descriptions of the tentacles have been made regarding the layout and size of the rings. One of the most recent states that there are twenty in total arranged pentaradially around the mouth. There are ten large outer tentacles arranged as five pairs, then within these, five single intermediate tentacles and then the inside ring is of five small tentacles. Tube feet are usually confined to the radii and may be crowded in the middle of the body.

Listed by

- none -

Biology review

Taxonomy

Level	Scientific name	Common name
Phylum	Echinodermata	Starfish, brittlestars, sea urchins & sea cucumbers
Starfish, brittlestars, sea urchins & sea cucumbers
Class	Holothuroidea	Sea cucumbers
Sea cucumbers
Order	Dendrochirotida

Family	Phyllophoridae
Genus	Neopentadactyla
Authority	(Östergren, 1898) Deichmann, 1944
Recent Synonyms	Pseudocucumis mixta (Östergren, 1898) Deichmann, 1944

Biology

Parameter	Data
Typical abundance	High density
Male size range	Up to 25cm
Male size at maturity
Female size range	Medium-large(21-50cm)
Female size at maturity
Growth form	Cylindrical
Growth rate	No information found
Body flexibility
Mobility	Burrower
Characteristic feeding method	Passive suspension feeder
Diet/food source	Omnivore
Typically feeds on	Seston
Sociability	Solitary
Environmental position	Infaunal
Dependency	No text entered.
Supports	Host Melanella alba.
Is the species harmful?	Data deficient

Biology information

In suitable habitat, densities have been recorded as high as 400 per square metre. In 1973, the population in Kilkieran Bay appeared to be increasing.
Most sea cucumbers are gonochoristic although some species are hermaphrodite.
The values for length apply to body length excluding the tentacular crown. When extended, the tentacular crown can be up to a quarter of the body length and have a spread of 140 square cm. The gravel sea cucumber is an infaunal burrower and is only visible when the tentacles are projected above the surface. The body is generally held in a u-shape within the sediment with the tentacles held in the water column and the terminal anus just at the surface.
Food particles are trapped using special adhesive areas at the tips of the tentacles. To ingest food, a tentacle is inserted into the mouth, the buccal membrane constricts and the tentacle withdrawn, scraping off any adherent food particles.
Melanella alba, a eulimid gastropod is a temporary ectoparasite on Neopentadactyla mixta, piercing the skin and feeding on the internal organs.

Habitat preferences

Parameter	Data
Physiographic preferences	Open coast, Strait or Sound, Sea loch or Sea lough, Ria or Voe
Biological zone preferences	Lower infralittoral, Upper circalittoral
Substratum / habitat preferences	Gravel / shingle, Maerl
Tidal strength preferences	Moderately strong 1 to 3 knots (0.5-1.5 m/sec.), Weak < 1 knot (<0.5 m/sec.)
Wave exposure preferences	Exposed, Moderately exposed, Sheltered, Very sheltered
Salinity preferences	Full (30-40 psu)
Depth range	15-70
Other preferences	No text entered
Migration Pattern	Diel, Seasonal (feeding)

Habitat Information

Although not necessarily representative of all populations, Neopentadactyla mixta exhibits regular daily and seasonal movements within the substratum. In the Kilkieran Bay population, individuals withdraw further into the sediment between 1 or 4 hours after sunrise and remain in the substratum for 1 or 2 hours, re-emerging over a period of up to four hours. In September/October the entire population withdraws into the substratum and re-emerges in March/April. They remain buried in aerobic conditions at depths of up to 60 cm with tentacles retracted and not feeding. Considerable loss of condition occurs during this time. Direct absorption of dissolved organic matter may be important for nutrition. This state of torpor is not complete, respiration and activity is greatly reduced but some movement within the substratum still occurs. Depth of burial is maintained despite surface changes in gravel with water movement.

Life history

Adult characteristics

Parameter	Data
Reproductive type	No information
Reproductive frequency	No information
Fecundity (number of eggs)	No information
Generation time	Insufficient information
Age at maturity	No information found.
Season	Insufficient information
Life span	Insufficient information

Larval characteristics

Parameter	Data
Larval/propagule type	-
Larval/juvenile development	No information
Duration of larval stage	No information
Larval dispersal potential	No information
Larval settlement period	Insufficient information

Life history information

No information has been found in relation to longevity or reproduction. Breeding is presumed to occur between April and September when the population is at the substratum surface. Most holothurians are gonochoristic and are broadcast spawners (although some species brood their larvae). The larvae of some species show planktotrophy, others lecithotrophy, some direct development, others indirect.

Sensitivity reviewHow is sensitivity assessed?

The MarLIN sensitivity assessment approach used below has been superseded by the MarESA (Marine Evidence-based Sensitivity Assessment) approach (see menu). The MarLIN approach was used for assessments from 1999-2010. The MarESA approach reflects the recent conservation imperatives and terminology and is used for sensitivity assessments from 2014 onwards.

Physical pressures

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	Intolerance	Recoverability	Sensitivity	Evidence / Confidence
Substratum loss [Show more] Substratum loss Benchmark. All of the substratum occupied by the species or biotope under consideration is removed. A single event is assumed for sensitivity assessment. Once the activity or event has stopped (or between regular events) suitable substratum remains or is deposited. Species or community recovery assumes that the substratum within the habitat preferences of the original species or community is present. Further details Evidence Neopentadactyla mixta lives within gravel or maerl substrata. Loss of this substrata would result in the loss of the population.	High	No information	High	Low
Smothering [Show more] Smothering Benchmark. All of the population of a species or an area of a biotope is smothered by sediment to a depth of 5 cm above the substratum for one month. Impermeable materials, such as concrete, oil, or tar, are likely to have a greater effect. Further details. Evidence Although not a fast mover, Neopentadactyla mixta is a quite large active burrower. Smothering by five cm of sediment should not cause too many problems and the sea cucumber will probably be able to burrow back up to the surface. Individuals of this species spend much of the winter buried up to 60 cm deep in aerobic sediment. During this winter period, a torpid stage is entered with respiration and activity greatly reduced. Given sufficient aeration, this species can tolerate long periods without feeding. This may indicate that even if the surface cannot be regained immediately then the species could tolerate a period of smothering. There will be an energetic cost and feeding will be curtailed. Smothering for a month will be less problematic if it occurs during the period when the sea cucumbers are buried within the substratum.	Low	No information	Moderate	Moderate
Increase in suspended sediment [Show more] Increase in suspended sediment Benchmark. An arbitrary short-term, acute change in background suspended sediment concentration e.g., a change of 100 mg/l for one month. The resultant light attenuation effects are addressed under turbidity, and the effects of rapid settling out of suspended sediment are addressed under smothering. Further details Evidence A slight increase in siltation may benefit this species through greater availability of food particles. Larger increases in siltation will cause feeding to stop, the tentacles to be retracted and withdrawal into the substratum. Although the species can tolerate long periods (up to 8 months) without feeding within the substratum, considerable loss of condition occurs during this time. Prevention of feeding for a whole year through increased siltation will probably cause death.	High	No information	High	Low
Decrease in suspended sediment [Show more] Decrease in suspended sediment Benchmark. An arbitrary short-term, acute change in background suspended sediment concentration e.g., a change of 100 mg/l for one month. The resultant light attenuation effects are addressed under turbidity, and the effects of rapid settling out of suspended sediment are addressed under smothering. Further details Evidence			No information
Desiccation [Show more] Desiccation A normally subtidal, demersal or pelagic species including intertidal migratory or under-boulder species is continuously exposed to air and sunshine for one hour. A normally intertidal species or community is exposed to a change in desiccation equivalent to a change in position of one vertical biological zone on the shore, e.g., from upper eulittoral to the mid eulittoral or from sublittoral fringe to lower eulittoral for a period of one year. Further details. Evidence The species only occurs subtidally (below 15 m). The tube feet and tentacles provide surfaces through which water could easily be lost. Exposure to desiccating influences for an hour will probably cause death.	High	No information	High	Low
Increase in emergence regime [Show more] Increase in emergence regime Benchmark. A one hour change in the time covered or not covered by the sea for a period of one year. Further details Evidence The species only occurs subtidally (below 15 m) and is not subject to emergence. Emergence for an hour will probably cause death.	High	No information	High	Low
Decrease in emergence regime [Show more] Decrease in emergence regime Benchmark. A one hour change in the time covered or not covered by the sea for a period of one year. Further details Evidence			No information
Increase in water flow rate [Show more] Increase in water flow rate A change of two categories in water flow rate (view glossary) for 1 year, for example, from moderately strong (1-3 knots) to very weak (negligible). Further details Evidence The gravel sea cucumber is a passive suspension feeder and requires a reasonable flow of water to provide sufficient food particles. The tentacular crown is held up in the water column in order to feed. Strong water flow causes the tentacles to be displaced and bent. This can only be tolerated up to a point and beyond that sea cucumber retracts its tentacles and withdraws into substratum. This would prevent feeding. Although the species can tolerate long periods (up to 8 months) without feeding within the substratum, considerable loss of condition occurs during this time. Prevention of feeding for a whole year will probably cause death.	High	No information	High	Moderate
Decrease in water flow rate [Show more] Decrease in water flow rate A change of two categories in water flow rate (view glossary) for 1 year, for example, from moderately strong (1-3 knots) to very weak (negligible). Further details Evidence			No information
Increase in temperature [Show more] Increase in temperature A short-term, acute change in temperature; e.g., a 5°C change in the temperature range for three consecutive days. This definition includes ‘short-term’ thermal discharges. A long-term, chronic change in temperature; e.g. a 2°C change in the temperature range for a year. This definition includes ‘long term’ thermal discharges. For intertidal species or communities, the range of temperatures includes the air temperature regime for that species or community. Further details Evidence The British Isles falls in the middle of the geographic range of this species. Small chronic changes in temperature will probably have little effect. Short acute changes in temperature may cause death. Temperature changes will have less effect when the population is buried within the substratum and respiration and metabolism are greatly reduced.	Intermediate	No information	High	Low
Decrease in temperature [Show more] Decrease in temperature A short-term, acute change in temperature; e.g., a 5°C change in the temperature range for three consecutive days. This definition includes ‘short-term’ thermal discharges. A long-term, chronic change in temperature; e.g. a 2°C change in the temperature range for a year. This definition includes ‘long term’ thermal discharges. For intertidal species or communities, the range of temperatures includes the air temperature regime for that species or community. Further details Evidence			No information
Increase in turbidity [Show more] Increase in turbidity A short-term, acute change; e.g., two categories of the water clarity scale (see glossary) for one month, such as from medium to extreme turbidity. A long-term, chronic change; e.g., one category of the water clarity scale (see glossary) for one year, such as from low to medium turbidity. Further details Evidence The species has no requirement for photosynthesis and probably only has very limited facility for visual perception. Changes in turbidity will probably have no effect.	Tolerant	No information	Not sensitive	Low
Decrease in turbidity [Show more] Decrease in turbidity A short-term, acute change; e.g., two categories of the water clarity scale (see glossary) for one month, such as from medium to extreme turbidity. A long-term, chronic change; e.g., one category of the water clarity scale (see glossary) for one year, such as from low to medium turbidity. Further details Evidence			No information
Increase in wave exposure [Show more] Increase in wave exposure A change of two ranks on the wave exposure scale (view glossary) e.g., from Exposed to Extremely exposed for a period of one year. Further details Evidence The gravel sea cucumber is a passive suspension feeder and requires a reasonable water movement to provide sufficient food particles. The tentacular crown is held up in the water column in order to feed. Strong wave action causes the tentacles to be displaced and bent. This can only be tolerated up to a point and beyond that sea cucumber retracts its tentacles and withdraws into substratum. This would prevent feeding. Although the species can tolerate long periods (up to 8 months) without feeding within the substratum, considerable loss of condition occurs during this time. Prevention of feeding for a whole year will probably cause death. One large storm on the west coast of Ireland was noted to cause Neopentadactyla mixta to withdraw into the sediment and remain there for ten days (Smith and Keegan, 1984).	High	No information	High	Moderate
Decrease in wave exposure [Show more] Decrease in wave exposure A change of two ranks on the wave exposure scale (view glossary) e.g., from Exposed to Extremely exposed for a period of one year. Further details Evidence			No information
Noise [Show more] Noise Underwater noise levels e.g., the regular passing of a 30-metre trawler at 100 metres or a working cutter-suction transfer dredge at 100 metres for one month during important feeding or breeding periods. Atmospheric noise levels e.g., the regular passing of a Boeing 737 passenger jet 300 metres overhead for one month during important feeding or breeding periods. Further details Evidence Slight vibrations within the immediate substratum will provoke total withdrawal. This will prevent feeding. If the vibration is continuous, habituation may occur. If the noise occurs during the period of torpor then the species will be tolerant.	Low	No information	Moderate	Moderate
Visual presence [Show more] Visual presence Benchmark. The continuous presence for one month of moving objects not naturally found in the marine environment (e.g., boats, machinery, and humans) within the visual envelope of the species or community under consideration. Further details Evidence The species probably only has very limited facility for visual perception.	Tolerant	Not relevant	Not sensitive	High
Abrasion & physical disturbance [Show more] Abrasion & physical disturbance Benchmark. Force equivalent to a standard scallop dredge landing on or being dragged across the organism. A single event is assumed for assessment. This factor includes mechanical interference, crushing, physical blows against, or rubbing and erosion of the organism or habitat of interest. Where trampling is relevant, the evidence and trampling intensity will be reported in the rationale. Further details. Evidence The gravel sea cucumber is highly flexible and has a tough skin but the tentacles are more likely to be damaged by abrasion. Neopentadactyla mixta lives infaunally and hence may avoid physical disturbance caused by a passing dredge, particularly when deeply buried in a state of torpor. Echinoderms are well known for their regenerative abilities. However, no information regarding recruitment or recovery was found.	Intermediate	No information	High	Low
Displacement [Show more] Displacement Benchmark. Removal of the organism from the substratum and displacement from its original position onto a suitable substratum. A single event is assumed for assessment. Further details Evidence The species is an active burrower within the substratum. Displacement will probably have little effect and individuals would be able to re-burrow.	Tolerant	Not relevant	Not sensitive	Low

Chemical pressures

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	Intolerance	Recoverability	Sensitivity	Evidence / Confidence
Synthetic compound contamination [Show more] Synthetic compound contamination Sensitivity is assessed against the available evidence for the effects of contaminants on the species (or closely related species at low confidence) or community of interest. For example: evidence of mass mortality of a population of the species or community of interest (either short or long term) in response to a contaminant will be ranked as high sensitivity; evidence of reduced abundance, or extent of a population of the species or community of interest (either short or long term) in response to a contaminant will be ranked as intermediate sensitivity; evidence of sub-lethal effects or reduced reproductive potential of a population of the species or community of interest will be assessed as low sensitivity. The evidence used is stated in the rationale. Where the assessment can be based on a known activity then this is stated. The tolerance to contaminants of species of interest will be included in the rationale when available; together with relevant supporting material. Further details. Evidence Insufficient information	No information	No information	No information	Not relevant
Heavy metal contamination [Show more] Heavy metal contamination Evidence Insufficient information	No information	No information	No information	Not relevant
Hydrocarbon contamination [Show more] Hydrocarbon contamination Evidence Insufficient information	No information	No information	No information	Not relevant
Radionuclide contamination [Show more] Radionuclide contamination Evidence Insufficient information	No information	No information	No information	Not relevant
Changes in nutrient levels [Show more] Changes in nutrient levels Evidence Insufficient information	No information	No information	No information	Not relevant
Increase in salinity [Show more] Increase in salinity A short-term, acute change; e.g., a change of two categories from the MNCR salinity scale for one week (view glossary) such as from full to reduced. A long-term, chronic change; e.g., a change of one category from the MNCR salinity scale for one year (view glossary) such as from reduced to low. Further details. Evidence Hypo and hypersaline water causes tentacle retraction (Smith, 1983). Neopentadactyla mixta lives in fully saline conditions. Reductions in salinity would cause the sea cucumber to stop feeding. A short term reduction in salinity will probably not be serious, particularly if it occurs during a period of torpor within the substratum. Long term reductions in salinity are more problematic Although the species can tolerate long periods (up to 8 months) without feeding within the substratum, considerable loss of condition occurs during this time. Prevention of feeding for a whole year will probably cause death.	High	No information	High	Moderate
Decrease in salinity [Show more] Decrease in salinity A short-term, acute change; e.g., a change of two categories from the MNCR salinity scale for one week (view glossary) such as from full to reduced. A long-term, chronic change; e.g., a change of one category from the MNCR salinity scale for one year (view glossary) such as from reduced to low. Further details. Evidence			No information
Changes in oxygenation [Show more] Changes in oxygenation Benchmark. Exposure to a dissolved oxygen concentration of 2 mg/l for one week. Further details. Evidence The species can survive with very low oxygen consumption when buried in the substratum during periods of torpor	Low	No information	Moderate	Moderate

Biological pressures

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	Intolerance	Recoverability	Sensitivity	Evidence / Confidence
Introduction of microbial pathogens/parasites [Show more] Introduction of microbial pathogens/parasites Benchmark. Sensitivity can only be assessed relative to a known, named disease, likely to cause partial loss of a species population or community. Further details. Evidence Insufficient information	No information	No information	No information	Not relevant
Introduction of non-native species [Show more] Introduction of non-native species Sensitivity assessed against the likely effect of the introduction of alien or non-native species in Britain or Ireland. Further details. Evidence Insufficient information	No information	No information	No information	Not relevant
Extraction of this species [Show more] Extraction of this species Benchmark. Extraction removes 50% of the species or community from the area under consideration. Sensitivity will be assessed as 'intermediate'. The habitat remains intact or recovers rapidly. Any effects of the extraction process on the habitat itself are addressed under other factors, e.g. displacement, abrasion and physical disturbance, and substratum loss. Further details. Evidence It is highly unlikely that this species would be extracted for any reason.	Not relevant	Not relevant	Not relevant	Low
Extraction of other species [Show more] Extraction of other species Benchmark. A species that is a required host or prey for the species under consideration (and assuming that no alternative host exists) or a keystone species in a biotope is removed. Any effects of the extraction process on the habitat itself are addressed under other factors, e.g. displacement, abrasion and physical disturbance, and substratum loss. Further details. Evidence Neopentadactyla mixta frequently lives in maerl beds. The algal nodules form a ideal, coarse, mobile substratum that the sea cucumber can burrow through. Maerl beds are exploited commercially. The effects of maerl removal may be less when the sea cucumber is in a state of torpor buried deep within the substratum.	Intermediate	No information	High	Moderate

Additional information

Importance review

Policy/legislation

- no data -

Status

National (GB) importance	-	Global red list (IUCN) category	-
National (GB) importance -
Global red list (IUCN) category -

Non-native

Parameter	Data
Native	-
Origin	-
Date Arrived	-

Importance information

In suitable coarse, mobile gravel substrata the gravel sea cucumber can reach such high densities that it virtually excludes all other macrofauna. It is possible that Neopentadactyla mixta provides the only food source for the temporarily ectoparasitic gastropod Melanella alba.

Bibliography

Hansen, B. & McKenzie, J.D., 1991. A taxonomic review of northern Atlantic species Thyonidiinae and Semperiellinae (Echinodermata: Holothuroidea: Dendrochirotida). Zoological Journal of the Linnean Society, 103, 101-127.
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.]
Keegan, B.F., 1974. The macro fauna of maerl substrates on the west coast of Ireland. Cahiers de Biologie Marine, XV, 513-530.
Konnecker, G. & Keegan, B.F., 1973. In situ behavioural studies on echinoderm aggregations. Helgolander Wissenschaftliche Meeresuntersuchungen, 24, 157-162.
Mortensen, T.H., 1927. Handbook of the echinoderms of the British Isles. London: Humphrey Milford, Oxford University Press.
Smith T.B. & Keegan, B.F., 1985. Seasonal torpor in Neopentadactyla mixta (Ostergren) (Holothuroidea: Dendrochirotida). In Echinodermata. Proceedings of the Fifth International Echinoderm Conference. Galway, 24-29 September 1984. (B.F. Keegan & B.D.S O'Connor, pp. 459-464. Rotterdam: A.A. Balkema.
Smith, T.B., 1983. Tentacular ultrastructure and feeding behaviour of Neopentadactyla mixta (Holothuroidea: Dendrochirota). Journal of the Marine Biological Association of the United Kingdom, 63, 301-311.
Smith, T.B., 1984. Ultrastructure and function of the proboscis in Melanella alba (Gastropoda: Eulimidae). Journal of the Marine Biological Association of the United Kingdom, 64, 503-512.

Datasets

Centre for Environmental Data and Recording, 2018. Ulster Museum Marine Surveys of Northern Ireland Coastal Waters. Occurrence dataset https://www.nmni.com/CEDaR/CEDaR-Centre-for-Environmental-Data-and-Recording.aspx accessed via NBNAtlas.org on 2018-09-25.
Manx Biological Recording Partnership, 2022. Isle of Man historical wildlife records 1990 to 1994. Occurrence dataset:https://doi.org/10.15468/aru16v accessed via GBIF.org on 2024-09-27.
NBN (National Biodiversity Network) Atlas. Available from: https://www.nbnatlas.org.
OBIS (Ocean Biodiversity Information System), 2025. Global map of species distribution using gridded data. Available from: Ocean Biogeographic Information System. www.iobis.org. Accessed: 2025-05-23

Citation

This review can be cited as:

Jackson, A. 2008. Neopentadactyla mixta Gravel sea cucumber. In Tyler-Walters H. and Hiscock K. Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 23-05-2025]. Available from: https://www.marlin.ac.uk/species/detail/1317

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Last Updated: 17/04/2008

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