Abra alba

Researched By	Lizzie Tyler	Data Supplied By	University of Sheffield
Refereed by	This information is not refereed.
Taxonomy
Scientific name	Abra alba	Common name	White furrow shell
MCS Code	W2059	Recent Synonyms	None
Phylum	Mollusca	Subphylum
Superclass		Class	Pelecypoda
Subclass		Order	Veneroida
Suborder		Family	Semelidae
Genus	Abra	Species	alba
Subspecies
Additional Information	Abra alba may also be known as the 'white furrow shell' (Rees & Dare, 1993).
Taxonomy References	Howson & Picton, 1997, Tebble, 1976, Hayward & Ryland, 1995b, Rees & Dare, 1993
General Biology
Growth form	Bivalved	Feeding method	Passive suspension feeder, Active suspension feeder, Surface deposit feeder, Sub-surface deposit feeder
Mobility/Movement	Burrower	Environmental position	Infaunal
Typical food types	Phytoplankton, detritus.	Habit	Burrow dwelling
Bioturbator		Flexibility	None (< 10 degrees)
Fragility	Intermediate	Size	Small(1-2cm)
Height	Insufficient information	Growth Rate	0.1 mm/day
Adult dispersal potential	100-1000m	Dependency	Independent
Sociability	Solitary
Toxic/Poisonous?	No
Additional Information	Abundance Although described as solitary animals, adult densities may exceed 1,000 m² in favourable conditions. For instance, in the rich organic muddy harbour sediments in the Ria de la Coruna (NW Spain) Abra alba densities varied from 97 to 2,939 individuals/ m² (Francesch & Lopez-Jamar, 1991). In front of Dunkirk, France, densities can reach 9,000 individuals / m² (Ghertsos et al., 2000). However, abundances typically fluctuate between years owing to variation in recruitment success (juvenile bivalves experience high mortality within the first month after settlement) or adult mortality. High densities of newly settled spat have been reported. For instance, estimated densities of between 16,000 - 22,000 individuals/ m² (collected on 1 mm sieves) were recorded by Jensen (1988) at the time of settlement in the western part of the Limfjord, Denmark. Growth The smallest recorded benthic specimen had a shell length of 0.34 mm (Dauvin & Gentil, 1989). In autumn settled spat growth is insignificant until spring when a maximum growth rate of 0.1 mm/day was reported (Dauvin, 1986). This growth rate applies from spring to autumn (Dauvin, pers. comm.). Nutrition Some bivalves, such as Abra alba, which inhabit muddy low energy environments can switch back and forth from suspension feeding and deposit feeding, depending upon the conditions of the environment (Dame, 1996). While suspension feeding, the inhalant siphon is held a few millimetres above the sediment surface and sucks in suspended particles. For instance, Abra alba significantly reduced the concentration of the flagellate Isochrysis galbana in suspension. Consumption of Isochrysis galbana over four hours was estimated to be 2.7% of the body weight (Rosenberg, 1993). While deposit feeding, the inhalant siphon is bent over toward the sediment surface, sucking up detritus. However, as the food quality of sediments is often low, deposit feeders either have to process large volumes of sediment through the digestive tract in order to gain a small amount of nutrition, or they sort particles before ingestion and reject the majority of particles as pseudofaeces. As a result the feeding rate is lower with a longer residence time for food in the gut, enabling digestion of the more complex organic compounds common to the benthic environment (Dame, 1996). Biomass and productivity In Kiel Bay, mean annual biomass varied greatly between sites and between years: Biomass (B) =0.1-3 g AFDW m², with a long-term average (ratio) P:B = c 2.2 (Rainer, 1985); B = 0.1-2 g AFDW m² and P:B = 1.7-2.9 from five years of sampling at a location off the French coast (Dauvin, 1986); B = 0.3 g AFDW m² and P:B = 1.4 in the Bristol Channel, England (Warwick & George, 1980).
Biology References	Nott, 1980, Allen, 1983, Dauvin, 1982, Dauvin & Gentil, 1989, Dame, 1996, Rosenberg, 1993, Francesch & Lopez-Jamar, 1991, Jensen, 1988, Rees & Dare, 1993, Rainer, 1985, Dauvin, 1986, Warwick & George, 1980, Ghertsos et al., 2000
Distribution and Habitat
Distribution in Britain & Ireland	Widespread around the British Isles.
Global distribution	Abra alba is distributed from the Norwegian Sea and the Baltic, south to the Iberian Peninsula, into the Mediterranean and Black Seas, and south along the coast of Africa to Senegal.
Biogeographic range	Not researched	Depth range	0 - 60 m
Migratory	Non-migratory / Resident
Distribution Additional Information	Dispersal of post-larval bivalve molluscs The mechanism termed 'byssus drifting' has been observed in 20 species of bivalve molluscs, including Abra alba, and is the mechanism by which post-larval bivalves enter a second pelagic migratory stage. Young bivalves secrete a single, long thread which increases the drag acting upon them and enables them to be carried along on the current. The drag increase was found to be a factor of 3 to 30 times greater on byssus secreting bivalves than on inactive ones (Sigurdsson et al., 1976). Abra alba were observed to re-enter the water column after larval settlement in the Bay of Seine (Olivier et al., 1996). The post-larvae and juveniles of Abra alba were most abundant at flood tide velocities . Furthermore, Abra alba were found to vary significantly in their vertical distribution in the water column, the species decreasing in abundance with distance from the sea bed. It was noted that Abra alba juveniles can regulate their vertical position in the flow, to some extent, by opening their valves to different extents (Olivier et al., 1996).
Substratum preferences	Muddy gravel, Sandy mud, Mud, Muddy sand	Physiographic preferences	Open coast, Offshore seabed, Strait / sound, Sealoch, Enclosed coast / Embayment
Biological zone	Sublittoral Fringe, Upper Infralittoral, Lower Infralittoral, Upper Circalittoral, Lower Circalittoral	Wave exposure	Sheltered
Tidal stream strength/Water flow	Weak (<1 kn)	Salinity	Full (30-40 psu)
Habitat Additional Information
AMBI Group (Borja et al., 2000)	III
Distribution References	Tebble, 1976, Sigurdsson et al., 1976, JNCC, 1999, NBN, 2002, Picton & Costello, 1998, Olivier et al., 1996
Reproduction/Life History
Reproductive type	Gonochoristic	Developmental mechanism	Planktotrophic
Reproductive Season	February to Autumn	Reproductive Location	Water column
Reproductive frequency	Annual protracted	Regeneration potential	No
Life span	1-2 years	Age at reproductive maturity
Generation time	<1 year	Fecundity	17000
Egg/propagule size	60 µm diameter	Fertilization type	External
Larvae/Juveniles Larval/Juvenile dispersal potential >10km Larval settlement period Insufficient information Duration of larval stage 1-2 months
Additional Information	Gametogenesis Dewarumez (1979) and Nott (1980) described the anatomy of the gonads of Abra alba and changes in the gonad condition during the reproductive cycle. Fertilization and metamorphosis The sexes are separate and may be distinguished microscopically by dissection. Nott (1980) estimated the number of eggs produced from an average sized animal 11 mm in length to be between 15,000 - 17,000 of 60 µm diameter. Gametes are shed within the shell cavity and swept out through the exhalent siphon by pumping, so that fertilization occurs externally. The eggs develop into free-swimming trochophore and then veliger larvae. The larval stage is planktonic, and in Abra alba, lasts about a month (Dauvin & Gentil, 1989). Larvae are subject to very high mortality. At metamorphosis, the larvae settle out of the plankton and the bivalve spends its remaining life as a member of the benthos (Dame, 1996). Recruitment Recruitment varies between localities. In a population of Abra alba from the Irish Sea, proliferation of the gonads commenced in March and the animals reached maturity between June and September. The exact time at which maturity was attained depended upon the size of the individual, but it seemed that only individuals with a minimum shell length of between 7-9 mm reproduced (Nott, 1980). Normally, there two distinct spawning periods in summer and autumn, and according to the season of settlement, individuals differ in terms of growth and potential life span. Although peak recruitment usually occurs in summer (Dauvin & Gentil, 1989). In Kiel Bay a recruitment peak occurred in August, sometimes with a second peak between December and February (Rainer, 1985). Autumn settled individuals from the Bay of Morlaix, France, initially showed no significant growth; they were not collected on a 1 mm mesh sieve until April, 5 to 7 months after settlement. Such individuals were expected to have a maximum life span of 21 months and could produce two spawnings. In contrast, veliger larvae that settled during the summer grew very rapidly and were collected on a 1 mm mesh sieve just one month after settlement. They lived about one year and spawned only once (Dauvin & Gentil, 1989). Dauvin & Gentil (1989) observed three recruitment periods (February-March, April-June and August-October) in response to trophic conditions following the Amoco Cadiz oil spill in the Bay of Morlaix, France (see sensitivity, nutrients). The additional recruitment period was considered to be an adaptive response over the normal pattern of twice yearly recruitment. Two peaks (in July and September-October) were noted in the Limfjord (Jensen, 1988), with spat densities in excess of 20,000 m² recorded (see general biology). Warwick & George (1980) inferred that settlement in Swansea Bay, Wales, occurred over a period of months between July and November.
Reproduction References	Nott, 1980, Dauvin & Gentil, 1989, Dame, 1996, Jensen, 1988, Rees & Dare, 1993, Rainer, 1985, Warwick & George, 1980, Dewarumez, 1979