Psammechinus miliaris

Researched ByLizzie TylerData Supplied ByUniversity of Sheffield
Refereed byThis information is not refereed.
Taxonomy
Scientific namePsammechinus miliarisCommon nameGreen sea-urchin
MCS CodeZB193Recent SynonymsNone
PhylumEchinodermataSubphylumEchinozoa
SuperclassClassEchinoidea
SubclassOrderEchinoida
SuborderFamilyPaechinidae
GenusPsammechinusSpeciesmiliaris
Subspecies  
Additional Information
  • Sometimes called the purple tipped sea urchin. Older publications may refer to sea urchins as "burrs" (Hancock, 1957). Strongylocentrotus droebachiensis is also known as the green sea urchin. It is possible for Psammechinus miliaris to be confused with Strongylocentrotus droebachiensis or pale specimens of Paracentrotus lividus.
  • A regular sea urchin with a somewhat flattened test. The colour varies with habitat (Bull, 939; Lindahl & Runnström, 1929; Comely, 1979). Shallow water or littoral individuals are a deep purplish-brown and show no difference between the colour of the test and spines. Those from deeper water tend to be paler in colour, with a light green test and vivid purple spine tips.
  • The tube-feet are arranged in arcs of 3, visible as 3 pairs of pores corresponding with each ambulacral plate on the denuded test.
  • A typical species of bouldered sheltered shores, also found sublittorally in shallow water in sheltered or slightly brackish sites such as sea lochs. Common in the circalittoral on exposed shores in Shetland.
MarLIN would like to thanks Dr Maeve Kelly for her comments and significant additions to the review.
Taxonomy References Howson & Picton, 1997, Hayward & Ryland, 1995b, Hayward et al., 1996, Hancock, 1957, Mortensen, 1927, Massin, 1999(b), Aquascope, 2000(a), Bull, 1939, Lindahl & Runnström, 1929, Comely, 1979
General Biology
Growth formGloboseFeeding methodPredator, Scavenger
Mobility/MovementCrawlerEnvironmental positionEpifaunal
Typical food typesMacroalgae, hydroids, bryozoans, boring sponges, barnacles, mussels, cockles and worms (see Lawrence, 1975).HabitFree living
BioturbatorNot relevantFlexibilityNone (< 10 degrees)
FragilityFragileSizeSmall-medium(3-10cm)
HeightGrowth RateSee additional information
Adult dispersal potential100-1000mDependencyIndependent
SociabilityGregarious
Toxic/Poisonous?No
Additional InformationAbundance
In Scotland Psammechinus miliaris typically occurs in dense, localized populations in sheltered areas of sea lochs on the west coast (Davies, 1989; Holt, 1991). Densities have been recorded of 18 individuals per 100 g dry weight of sea weed (Bedford & Moore, 1985); several /m² in beam trawls in the Wadden Sea (Cranmer, 1985); 34 individuals per m² in a subtidal Laminaria saccharina bed, 182.4 /m² in a shallow bed of Zostera and 28.4 /m² on adjacent mud surfaces (Comely, 1979) and 352 /m² for littoral populations (Kelly, 2000) where individuals in one 0.25 m² quadrat ranged from 3.7 to 24.2 mm horizontal test diameter. Densities of several individuals /m² have been recorded in the German Waddensea (Ursin, 1960). Larsson (1968) found up to 10 individuals /m² in Saltkälle Fjord, Sweden.

Size
Maximum size varies with location. In some places the maximum diameter reached is small (< 20 mm (Jensen, 1969); around 35 mm (Bedford & Moore, 1985; Gage, 1991; Bull, 1939) although others have recorded diameters up to 57.5 mm (Allain, 1978) or up to 50 mm (Massin, 1999b; Aquascope, 2000a). The minimum diameter at maturity recorded (Brattström, 1941 cited in Jensen, 1969) is 6-7 mm but more usually 8-10 mm.

Growth rates
Estimates of growth rate vary. Gage, (1991) tried to relate growth bands found in the calcified plates of the test to seasonal differences in skeletal growth rate. This approach can be used reasonably accurately for estimating age of young urchins. However, when growth slows down, the reduced distance between bands make it difficult to distinguish individual markings. Bull, (1939) recorded growth up to 20 mm test diameter in the first year which then slowed considerably. In the second year mean diameters increased from 20 to 26.2 mm and by the sixth year saw growth of only a further 12 mm or so. Jensen, (1969) found that newly settled urchins (in August) grew 2 mm in the first 2 months and had reached 5.8 mm by the following July. Growth was recorded as maximal in spring and early summer by Gage (1991) and Bedford & Moore (1985) but work by Jensen, (1969) showed no growth between May and October. Growth rates of cultivated Psammechinus miliaris are given by Cook et al. (1998).

Locomotion
Locomotion is mediated by the movable spines attached to the test. The echinoid test is very brittle and easily damaged by impact. The spines which are articulated at the base and controlled by muscles may provide some cushioning to impact but overall the flexibility is negligible.

Feeding
Psammechinus miliaris has been recorded as feeding on a wide variety of species (see Lawrence, 1975). Loose lying brown macro algae, particularly Laminaria saccharina, is probably the main nutrient source although softer green algae such as Ulva lactuca may be important when small (Cook et al., 1998). The green sea urchin also feeds on epifauna such as hydroids, barnacles, small bivalves, boring sponges (e.g. Cliona), worms (e.g. Polydora) (Hancock, 1957; Lawrence, 1975).

Parasites
Psammechinus miliaris (as well as other echinoderms) frequently harbours a polychaete, Flabelligera affinis amongst the spines.

Use
The gonads of Psammechinus miliaris, but more commonly Paracentrotus lividus, are eaten as a delicacy in Mediterranean countries. As wild populations of Psammechinus miliaris typically have small gonads, its potential for aquaculture is being investigated (Kelly et al., 1998).
Biology References Gage, 1991, Cranmer, 1985, Bedford & Moore, 1985, Cook et al., 1998, Hancock, 1957, Mortensen, 1927, Jensen, 1969, Massin, 1999(b), Aquascope, 2000(a), Bull, 1939, Kelly et al., 1998, Boolootian, 1966, Kelly & Cook, 2001, Kelly, 2000, Lindahl & Runnström, 1929, Comely, 1979, Lawrence, 1975, Davies, 1989, Holt, 1991, Ursin, 1960, Larsson, 1968, Hinegardner, 1969, Hayward & Ryland, 1990, Julie Bremner, unpub data
Distribution and Habitat
Distribution in Britain & IrelandAll British and Irish coasts. Evenly distributed in the southern North Sea but scarce in northern North Sea.
Global distributionFrom Trondheim Fjord in northern Norway, inner Danish waters from the Skaw into the western Baltic, Iceland, British Isles, south to the Atlantic coast of Morocco and the Azores. Not in Greenland, the Mediterranean or Atlantic coasts of America.
Biogeographic rangeNot researchedDepth range0 - 100 m
MigratoryNon-migratory / Resident
Distribution Additional InformationThe species is found in the intertidal and subtidal, occasionally as deep as 100 m (Mortensen, 1927) but more commonly between 16 -70 m (Cranmer, 1985) or 0 -10 m in Scottish sea lochs (Kelly, 2000).

On the west coast of Scotland Psammechinus miliaris typically occurs in dense, localized populations in sheltered areas of sea lochs (Davies, 1989; Holt, 1991). Its distribution frequently coincides with that of the brown seaweed %Laminaria saccharina%, with Psammechinus miliaris occurring on the fronds as well as on rock surfaces below the fronds. Some populations are exposed to air at low spring tides, and are found attached to the underside of rocks, boulders and seaweed, or shallowly buried under gravel on the foreshore (Kelly, 2000). Individuals from the intertidal and subtidal habitats on the west coast of Sweden were termed 'Z' and 'S' forms by Lindahl & Runnström (1929). The 'Z' form lived in the 'seaweed region' and were larger and darker than the 'S' forms found at greater depths.

Comely (1979) described a population of Psammechinus miliaris from a %Zostera marina% bed in a stable salinity, shallow inlet in Loch Sween, Scotland. Theses individuals were found on the bottom mud and attached to the rhizomes of Zostera at depths of 1 -2 m. Very young individuals have been found in the holdfasts of Laminaria from the Clyde Sea, Scotland (Moore, 1971).

Psammechinus miliaris is often found on man-made surfaces such as bridge supports and wrecks.

Substratum preferencesLarge to very large boulders, Cobbles, Muddy gravel, Mixed, Artificial (e.g. metal/wood/concrete), Under boulders, Bedrock, Small boulders, Gravel / shingle, Muddy sand, Algae, RockpoolsPhysiographic preferencesOpen coast, Offshore seabed, Strait / sound, Sealoch, Ria / Voe
Biological zoneMid Eulittoral, Lower Eulittoral, Sublittoral Fringe, Upper Infralittoral, Lower Infralittoral, Upper Circalittoral, Lower CircalittoralWave exposureModerately Exposed, Sheltered, Very Sheltered, Extremely Sheltered
Tidal stream strength/Water flowModerately Strong (1-3 kn)SalinityVariable (18-40 psu), Full (30-40 psu)
Habitat Additional Information
AMBI Group (Borja et al., 2000)I 
Distribution References Hayward & Ryland, 1995b, Hayward et al., 1996, Gage, 1991, Cranmer, 1985, Bedford & Moore, 1985, Allain, 1978, Hancock, 1957, Rasmussen, 1973, Mortensen, 1927, Jensen, 1969, Bruce et al., 1963, MBA, 1957, Massin, 1999(b), Aquascope, 2000(a), Kelly & Cook, 2001, Lindahl & Runnström, 1929, Comely, 1979, Davies, 1989, Holt, 1991, Ursin, 1960, Larsson, 1968, Leighton, 1995, Moore, 1971, Hayward & Ryland, 1990, Julie Bremner, unpub data
Reproduction/Life History
Reproductive typeGonochoristic Developmental mechanismPlanktotrophic
Reproductive SeasonJune to August in UKReproductive LocationWater column
Reproductive frequencyAnnual protracted Regeneration potentialNo
Life span6-10 yearsAge at reproductive maturity1 year
Generation time1 yearFecundity2500000
Egg/propagule sizeFertilization typeExternal
Larvae/Juveniles
Larval/Juvenile dispersal potential>10kmLarval settlement periodInsufficient information
Duration of larval stage1-2 months  
Additional InformationLongevity
Gage (1991) tried to use skeletal growth bands to estimate the age of urchins but this approach proved to be difficult to use in older specimens due to the proximity of growth lines during slow growth in older individuals. Bull, (1939) estimated longevity to be up to 6 years, Jensen, (1969) up to 8 years and Alain, (1978) up to 10 or 12 years.
Maturity
Elmhirst (1922; cited in Gage, 1991) noted that maturity was reached in the first year after settlement. In contrast, Jensen (1969) found 75 % of one year old urchins to have immature gonads in their first summer. However, Kelly (2001) noted that one year old Psammechinus miliaris produced viable gametes and were able to breed in the laboratory.
Spawning (see additional images)
Individuals have been recorded as having ripe gonads from as early as February to as late as November (Orton, 1923; Sukarno et al., 1979; Mortensen, 1927). Actual breeding occurs in spring and early summer (Mortensen, 1927; Sukarno et al., 1979; Kelly, 2000). Psammechinus miliaris is a broadcast spawner (Massin, 1999(b)). The spawning period has been reported to be June to August in the Clyde Sea area (Elmhirst, 1922); June to October near Bergen, Norway (Runnström, 1925; cited in Lindahl & Runnström, 1929); June to October and May to October in West Norway and Denmark (Jensen, 1969); and July and August on the west coast of Scotland (Comely, 1979). Psammechinus miliaris from two typical habitats (littoral and sublittoral) on the west coast of Scotland had a defined annual cycle of gametogenesis with a single spawning period (Kelly, 2000) and gonad indices that peaked in June and July.
Fecundity
Estimates of fecundity suggest that females produce between 80,000 and 2,500,000 eggs in a single spawning event (Dr Maeve Kelly, unpublished observations). Breeding probably occurs over a couple of months (Kelly, 2000) but whether individuals breed for this entire period or whether this duration is for a whole population is uncertain. Orton (1923) suggests there is no evidence for collective spawning.
Reproduction References Gage, 1991, Orton, 1923, Allain, 1978, Sukarno et al., 1979, Mortensen, 1927, Jensen, 1969, Massin, 1999(b), Bull, 1939, MacBride, 1903, Boolootian, 1966, Kelly & Cook, 2001, Kelly, 2001, Kelly, 2000, Lindahl & Runnström, 1929, Comely, 1979, Kelly et al., 2000, Hinegardner, 1969, Leighton, 1995, Elmhirst, 1922, Cook et al., 2000, Pantazis, 2000, Julie Bremner, unpub data