Abra

A Robust genera from the “Molluscs” group, mostly found in Muddy Sand

Muddy Sand

Robust

Molluscs

Genera Characteristics	Recorded Traits	Possible Values
Size in cm	1-10 cm	<1 cm 1-10 cm 11-20 cm >20 cm

Genera Characteristics	Recorded Traits	Possible Values
Motility (Adult)	Short Range Mobility	Adult Short Range Mobility Sedentary “Short Range Mobility”
Motility (Larval)	Long Term Planktonic	Long Term Planktonic Short Term Planktonic Brooded or Laid Eggs “Long Term Planktonic”
Lifespan	3-10 years 1-2 years	>10 years 3-10 years 1-2 years <1 year
Maturity	<1 year	>4 years 3-4 years 1-2 years <1 year
Fecundity	10000 - 1 million	1 - 10 10 - 100 100 - 1000 1000 - 10000 10000 - 1 million > 1 million

Image: Abra © MES

Vulnerability

Abra is a small bivalve mollusc belonging to the Family Semelidae. It reaches a shell length of only 1-2cm & lives in shallow burrows in sands & muddy sands where it filters particulate matter from the water column. It has limited mobility & is vulnerable to the physical disturbances associated with aggregate extraction. It is likely to be able to resurface through relatively thin deposits of sediment mobilised by the dredging process.

Recoverability

Abra is well-adapted for recovery & colonisation. It has a relatively short life-span of about 2-3yrs & reaches sexual maturity in <1yr. It also has high fecundity (15,000-17,000) & high larval/juvenile dispersal potential (>10km). The eggs are fertilised externally & then develop into free swimming veliger larvae that spend about 30 days in the plankton. The juveniles of Abra alba, have also been observed as having a second migratory phase facilitated by “byssus drifting”. The relatively short life-span, high fecundity & larval dispersal suggest that this genus has a high recoverability. Biomass is likely to be restored within 3yrs after initial colonisation.

Abra

Vulnerability

Recoverability

References

Biology

Taxonomy

Distribution

Reproduction